Advanced Dynamics: ncloth & Hair - Animating Mitosis

Transcription

1 Advanced Dynamics: ncloth & Hair - Animating Mitosis This tutorial is designed to introduce several approaches to animating various cellular process using advanced dynamic systems such as ncloth and Hair. Maya dynamics are powerful but take some time and practice to master. This is mostly because their implementation in Maya is less than intuitive. To understand why the various dynamic systems are set up the way they are you have to understand that historically, as each version of Maya has been introduced, additions have been made to the dynamics. Particles and rigid body dynamics have only changed a little since Maya version 4. Maya 8 introduced the nucleus dynamic system which is used for ncloth and is rumored to replace the current particle system in upcoming versions of the software. Nucleus is a more advanced particle system that, unlike the standard Maya particle dynamics, has a built-in system for calculating particle interactions (the nucleus solver), which is like a much more advanced and accurate version of Maya particle springs. Currently the Hair solver is self-contained and not attached to nucleus however it is very similar to the nucleus ncloth system. Maya Hair was introduced before ncloth. ncloth and hair are extremely powerful and surprisingly easy to use depending on the requirements of the scene. You can create some interesting dynamic movements to the objects in your scene without the need for complex expressions and connections. ncloth and hair systems can also interact with Maya particles and fluids as well, layering such dynamic simulations is sometimes required to create the very specific and unusual dynamic motions we frequently see in scientific animations. It s important to understand however that although the nucleus system is powerful it is not always appropriate for every situation and in some cases old-fashioned particles and soft-body surfaces and curves may serve you better. When approaching a particular problem you should consider and test all options and not assume that just because a technique or tool is newer that its going to be more appropriate for all situations. Creating and ncloth Cell To introduce the ncloth system we ll create a very simple animation that simulates the rounding up of the edges of a cell during prophase. ncloth is not just for simulating clothing. By adjusting settings on the ncloth object and the nucleus solver you can simulate anything from water balloons to chain mail. One of the nicest aspects of this system is that, when using lower resolution objects, the interaction is fast enough that you can experiment with combinations of settings while the simulation is running and fine tune them to get the type of motion you want. You ll start this exercise by creating a very simple cell with polygons and then convert it to an ncloth object. 1. Start a new scene in Maya by going to File -> New Scene. Maya. 2. Switch to the polygon menu set and create a polygon cube. Create>Polygon Smoothing a cube creates a nice polygon sphere with out poles at the top and bottom. 1

2 Primitive>Cube. Make sure the cube only has 1 subdivision in height, width and depth. 3. Set the cube s scale to 8 units in X, Y, and Z. 4. Smooth the cube. Mesh>Smooth. Select the polysmoothface1 node in the channel box and set the divisions to 4. This creates a nice polygon sphere. The advantage this method has over a conventional polygon sphere is that the smooth cube does not have poles at the top and bottom. The poles that are created when you make a standard poly sphere can sometimes pinch or distort when using the sphere for dynamic simulations. 5. Name the cube cell and delete history on it. 6. Set the Y scale of the cube to 1 to squash it down. To create the fried egg look of the cell you ll use the Artisan paint brush tools. 7. Select the cell and choose Mesh>Sculpt Geometry> Options. In the options set the operation to pull. Set the max displacement to.3 and the opacity to Set the Reference Vector to Y axis. 9. You can set the size of the brush by holding it over the cell model. Drag the mouse while holding the B key. Once you have a brush radius you like, brush across the surface of the cell to raise a small bump. 10. Switch the bush operation to smooth to even out any lumps. 11. To create an uneven look to the shape of the cell you can add a lattice deformer and adjust the points on the lattice. To keep the lattice smooth, turn off the local check box on the ffd1 tab in the lattice s attribute editor. 12. When you re happy with the shape of the cell, delete history on it to commit the lattice changes. The settings for the sculpt geometry tool. The sculpt geometry tool is used to create the lump at the center of the cell. The beauty of ncloth is that any polygon object can be converted into ncloth with no additional preparation. Higher resolution meshes will take longer to calculate however you can always test out your settings on a lower resolution mesh and then apply those A lattice deformer is used to shape the cell. 2

3 settings to a higher resolution version later on. 13. Select the cell and switch to the ncloth menu set. Choose ncloth>create ncloth. The default settings should work just fine. 14. Save the scene as cell_v01.ma Passive ncloth Objects Passive objects are non cloth objects that can interact with ncloth objects. They are very easy to set up. 1. Rewind and play the animation. Immediately you ll see why you need a passive object. The cell starts to fall. The nucleus solver has a gravity field on by default. 2. Create a polygon plane, position it just below, but not intersecting the cell. The plane can have any number of subdivisions, even just a single subdivision in width and length. Since this is a very simple scene you can increase performance by keeping the geometry simple. 3. Select the plane and choose ncloth>create Passive. Again the default options are fine. 4. Play the animation, you ll see the cell drop down and collide with the plane. A simple polygon plane set as a passive object keeps the cell from falling in space. The yellow volume around the plane indicates the collision thickness ncloth properties can be painted onto a surface as a texture or directly on the vertices of an object using the artisan tools. If you want to make one part of the surface, such as the nucleus of the cell, less deformable than other parts of the cell, select the ncloth object and choose Edit ncloth>paint Vertex Properties. The menu has a list of the properties you can paint onto the ncloth object. This works similar to painting goal weights on a soft body object. The main difference between painting a texture and a per-vertex map is that the per-vertex map does not require UV coordinates to be assigned to the object. When working with collision objects you have several options for balancing the speed and accuracy of the simulation. If you select the plane and open its attribute editor to the nrigidshape1 node, under Quality 3

4 settings you ll see the collision flag option. This can be set to Face, Vertex, or Edge. The face setting is the most accurate and slowest to calculate. To get an idea of how collisions are calculated, scroll up and set the solver display to Collision Thickness. The plane turns yellow if the collisions flag is set to face. In fact if you switch to the side view, you ll see that a yellow collision volume has surrounded the plane, the size of which is determined by the Thickness slider. Set the collision flag to vertex and you ll see spheres drawn around the vertices pf the plane. Set the collision flag to Edge and you ll see a collision volume surrounds the triangulated edges of the face. At the moment, if you play the simulation while changing the collision flag from face to vertex to edge, you ll notice that the cell seems a little wobblier when vertex is chosen over face. If you turn off the trapped check flag the cell falls through the plane when the collision flag is set to vertex, it becomes draped over the triangulated edge of the plane when its set to edge, and it continues to collide with the plane properly when set to face. When Trapped Check is on, the solver tries to push any vertices of colliding objects over to one side to prevent interpenetration problems. 5. Set the collision flag to face. Keep trapped check on. 6. Switch to a side view, with the plane selected and the Solver Display set to collisions, adjust the thickness slider so that the collision with the plane is a little more accurate. A setting of.018 should work fine. 7. You ll notice that the cell still hovers above the plane, this is because the cell ncloth object also has a thickness setting. You can select the cell, open the nclothshape1 tab in the attribute editor and display the cell s thickness as well. Set the thickness slider to If you d like to speed up the calculation a little you can set the cell s collision flag to vertex. 9. Turn off the visibility of the collision volumes for the cell and the plane and save the scene as cell_v02.ma The collision attributes in the rigidshape1 node Collisions on the cell have been set to vertex, collisions on the plane have been set to face. 4

5 Initial State At the start of the animation you ll want the cell to be relatively motionless as it sits on the plane. Currently it drops down and then collides with the plane, which is somewhat odd. To prevent this from happening you ll create an initial starting state. 1. Set the timeline length to Play the animation from the beginning and let it run until the cell collides with the plane and then settles down (about 100 frames). 3. Select the cell and choose Edit ncloth>initial State, Set from Current. When you rewind and play the animation, the cell starts resting on the plane. If you decide you want to remove or reset this later on, you can always choose Edit ncloth>initial State>Clear Initial State. Pressure At this point you re ready to animate the rounding of the cell. This will be accomplished in an extremely simple manner. You ll edit the settings on the ncloth shape, nucleus will then apply these settings and the animation will occur automatically as nucleus interpolates between the initial state and the applied settings. 1. Select the ncloth shape. Open the attribute editor for the nclothshape1 tab. 2. Play the animation. As the animation plays, raise the pressure slider. You ll see the cell actually inflate. If you set the slider to 1 it will actually leap off the plane and bounce. Not very cellular but very entertaining. 3. Raise the pressure damp to 1; this will tone down the inflation effect for a more controllable animation. 4. Rewind and play the animation with these new settings. There are two ways to calculate pressure. The current method is set to manual, which is very simple the pressure slider and pressure damping sliders are the only two controls. These can be keyframed to tune the effect. If you set the pressure at 0 and create a keyframe, then play the animation to frame 100, set pressure to 1 and set another keyframe, you ll get a very nice rounding of the cell, it might be all you need for the animation. If you set the out tangent on the first keyframe to flat, the animation will look smooth and elegant. The second method is the volume-tracking model. The settings on this model offer more precise control as this model calculates the Increasing the value of the pressure slider causes the cell to inflate The rounding up of the cell is accomplished by increasing the pressure settings The volume-tracking model simulates air being pumped into the volume. 5

6 volume of the cloth combined with the inflow and outflow of air. The pump rate value determines the rate at which air is added within the volume. Positive values continue to pump air into the volume, negative values suck the air out. The start pressure value sets the initial pressure of the air inside the volume at the start of the animation. The Air Tightness value determines the permeability of the ncloth object. Lower settings allow the air to escape the volume. The incompressibility setting refers to the air within the volume. A lower value means the air is more compressible which slows down the inflation effect of the cell. Whichever method you choose depends on what you intend for the final animation. The Volume-Tracking model will continue to inflate the cell until its almost perfectly round which may work well if you want the animation to show the transition from prophase to prometaphase. 5. Set the pressure method to volume tracking model, set the pressure damping to 1, the start pressure to 0, the pump rate to 1, the Air Tightness to 1, and the incompressibility to Rewind the animation and play to about frame 100. Stop the animation and set a keyframe on pump rate. 7. Play the animation to 150. Set the pump rate to 0 and set another keyframe. 8. Open the Graph Editor; select the nclothshape1 node in the outliner so that you can see the keyframes set on the pump rate. 9. Flatten the in and out tangents of the keyframes. 10. Play the animation and save it if as cell_v03.ma if you re happy with the animation. 11. If you find that the cell tends to move about on the plane after it has been inflated you can scroll up to the surface properties section of the nclothshape1 tab and increase the friction and mass. Set the Friction to 1 and the Mass to You can also try removing the keyframes from the pump rate, set it to 1 and set the air tightness to.95 so that some air is lost from the volume as it reaches its maximum inflation level. The pump rate of the pressure can be keyframed. The cell is completely inflated. Presets can help you get started creating a look for the simulation. Maya 2008 has a large number of presets, Maya 8.5 does not. 6

7 ncloth Presets As you can see from taking a look at all the settings on the nclothshape1 node, there are many different attributes which work together to create the quality of the ncloth simulation. The pressure settings are just a small section. You can achieve a very high level of control by adjusting the settings, keyframing them or creating expressions, you can also create & assign textures to control the surface properties of the ncloth so that some parts have more mass than others or wrinkles, etc. It takes some time and experimentation to master all of these settings. Many Maya nodes come with a number of presets, which can help you quickly create an initial quality for the node. From there you can adjust the settings until you get what you would like. The Presets button on the upper right of the Attribute Editor will contain a list of presets (if there s an asterisk next to presets then the presets are loaded). Maya 8.5 does not ship with ncloth presets, Maya 2008 has a large number of presets from silk to rubber to water. You can also save your presets; they will be available for any Maya session as long as your preferences are loaded. Presets can also be blended together to create original ncloth qualities. ncloth presets can be downloaded form the Autodesk AREA in the "Bonus Tools" section (all the way at the bottom): Create an ncloth Cache As with all dynamic simulations, it s a good idea to create a cache file once you are happy with the simulation. Cache files can easily be deleted or replaced so you don t have to wait until everything is perfect before creating one. 1. Select the cell object and choose ncache>create new ncache>options. 2. In the options you can choose a location for the file as well as the name. You can also decide to make a single file or a file for each frame. There may not be much of a difference however if you decided you wanted to create an interesting cloth effect that goes backwards you can try making a cache file for each frame and then use a file renaming program to reverse the order of the cache file names. 3. You can set the frame range for the cache, for now leave it as the time slider. 4. Create the cache. As the cache is created Maya will play through the scene file. When its finished you ll see that the scene plays much faster. Any changes you make to the ncloth surface or nucleus will not be be seen in the simulation until you delete or replace the cache. Dynamic Curves Maya s dynamic hair system can be used for much more than character hairstyles. The most useful property of the system for the purpose of creating scientific animations is the ability to turn a regular NURBS curve in to a dynamic curve. This is similar to creating a softbody curve however, like cloth, the interactions and collisions between curves are much more realistic and easier to set up. Like ncloth, once you convert a curve into a dynamic curve, you can adjust its quality by changing the parameters on the hair node, this can be done while the animation is playing allowing you to experiment with the settings until you get something you like. In this section you ll see how dynamic curves can be used to create chromatin loops hanging off of the arm of a chromosome. You ll also see how deformers can be used to manipulate a cached hair system. 1. You ll start by creating a curve to represent the position of the chromosome arm. Start a new scene n Maya. Switch to the side view and turn on grid snapping. 7

8 2. Select the Create EP curve tool. Click once at the origin and then again 10 units to the right. 3. Switch to the Surfaces Menu set. Rebuild the curve so that it has 12 spans (Edit Curves>Rebuild Curve). Rename the curve armcurve. 4. Select the Pencil Curve Tool from the Create menu and draw one long looping curve starting from the origin and moving down towards the end and back to the origin. 5. Hit enter when you ve finished drawing the curve. 6. Switch to the perspective view, rightclick over the curve and choose control vertices. Use the move tool to randomly position the CVs away from the center line so that the loops have some dimension. You can do this quickly without fussing over precision. 7. Select the looping curve and rebuild it so that it has 100 spans. This will smooth the edits that you have made to the curve. 8. Save the scene as chromatin_v01.ma. The Options for curve rebuild Create a Dynamic Curve The looping curve will be converted into a hair and the dynamics will be adjusted so that it has a nice turbulent motion. 1. Switch to the Dynamics menu set, choose the curve and select Hair>Make Selected Curves Dynamic. 2. Play the animation. You ll see the original curve motionless and a duplicate that is moving. In the Outliner you ll see a hair system node, a follicle group and the output curve group. A Long looping curve is drawn around the original curve using the Pencil Curve Tool. The curve has been duplicated. The follicle curve is similar to a duplicate goal curve that you create when making a softbody curve. The follicle has attributes related to the hair curve and can also be used as an attractor to the hair curve. The hair system node contains many of the attributes you ll use to adjust the settings on the hair. 3. Expand the follicle node and select the follicleshape1 node, open the attribute editor. Set the Point Lock drop down The move tool is used to add dimension to the loops 8

9 menu to No attach. Set the timeline length to 200. Play the animation, you ll see the hair curve drop and fly off into space. 4. Rewind the animation and set the Point lock to base. When you play the animation only one end of the curve is attached, the other is free to fly off depending on the dynamics. If you set the point lock to tip then the opposite end will be attached. The curve s U direction determines which end is the tip and which is the base. Set the Point lock back to BothEnds. You can use the flip option to reverse the tip and base. 5. Select the hairsystemshape1 tab in the Attribute editor. In the Forces section set the gravity to 0 so that the hair no longer drops. 6. Set the Stiffness to 0. The stiffness scale adds a springy quality to the curve which can exaggerate the dynamics. You want a nice fluid motion to the hair so you can lower this to 0. In some cases you can use the Stiffness scale to fine-tune the stiffness along the length of the curve. This scale curve works only works when stiffness is at a nonzero value. 7. Set the drag value to.2. When you play the animation the motion of the hair has pretty much ceased. This is because there are no forces acting upon the hair. 8. Expand the Turbulence tab. Play the animation and slowly raise the turbulence, you ll see the hair now starts to move. Set the intensity to.08 and the frequency to 1. Set the speed to Set the Start Curve Attract value to.027. The start curve is the original curve shape as represented by the follicle curve. Increasing this value cause the hair to conform to the shape of the follicle curve. Raise the Turbulence intensity to Select the follicle1 node. Play the animation and raise the sample density value. This increases the detail in the hair curve. Set this value to Adjust these settings until you have a nice turbulent motion that does not cause the hair to move too far from the original position. Select the Follicle node and hide it so that you can clearly see the motion of the hair curve. Creating a dynamic curve creates several sets of nodes. When the Point lock attribute is set to Base only one end of the hair curve is attached to the Follicle. 9

10 12. Save the scene as chromatin_loops_v02.ma Create a Hair Cache When you create a hair cache the dynamics of the curve are stored in a file or a series of files. Not only does this ensure that the animation will be correct when its time to render, it also allows you to add deformers to the hair curve. You ll take advantage of this so that the hair and the original chromosome curve can be animated together. 1. Select the hair curve and choose Hair>Create Cache. You can use the options to determine the frame range of the cache, you can leave this set to Time Slider. 2. The animation will play through and the hair dynamics will be written to disk. Playing the animation should now be somewhat faster, you can also scrub through the timeline and the hair will update correctly. 3. Remember that if you change settings on the hairsystems node or the follicle node you won t see a change unless the cache is deleted. Increasing the sample density adds detail to the hair dynamics. Deform the Curves As long as the cache is available and properly linked, the dynamic curve can be animated using deformers. 1. Rewind the animation, Select the hair curve and the original chromosome curve and group them. 2. Select the group, switch to the side view, switch to the animation menu set. 3. Turn on grid snapping and select the joint tool. Create a chain of 10 joints each one unit long that follows the armcurve created at the start of the lesson. 4. Select the Group and the joint chain and choose skin>smooth bind. 5. Try rotating some of the joints and play the animation, the looping curve should still move even after being deformed by the joints. The dynamic hair output curve and the original curve are grouped. A chain of 10 joints is added along the center line. Thanks to the cache, the dynamic curve will animate even while it is being deformed by joints. 10

11 6. Undo changes to the joint rotation and save the file as chromatin_loops_v03.ma Use Hair as an IK Spline Curve The joint chain can be controlled using a spline IK, to add another level of dynamic motion you can convert the spline IK curve into a dynamic hair curve. 1. Switch to the animation menu set and choose Skeleton>IK Spline handle tool>options. In the Options, turn on Auto Create Curve and set the spans to Click on the first joint in the chain at the origin and then the last joint to create the IK spline. A third curve (curve4) will be created in the Outliner. 3. Select curve4 and switch to the dynamics menu set. Choose Hair>Make selected curves dynamic. 4. A new hair system will be created named hairsystem2. Select the associated follicle and in the attributes, set the point lock to base. 5. If you play the animation you ll see the new hair curve droop but the joints do not follow. This is because when the curve is converted to a dynamic curve a duplicate is made of the original (the original becomes the follicle curve). To make the joints follow the motion of the dynamic curve you ll need to rearrange some connections in the Hypergraph. 6. Expand the hairsystem2output curves group in the outliner, you ll see a curve named curve5. To reduce confusion, rename this curve dynamicikspline. 7. Select the dynamiciksplineshape node in the Outliner, Ctrl/Command select the IkHandle1 node and open the Hypergraph (Windows>Hypergraph:Connections). 8. You ll see that curveshape4 (which is the follicle for the dynamicikspline curve) is connected to the ikhandle1 node. If you hold your mouse over the arrow it shows that the worldspace attribute of curveshape3 is connected to the incurve attribute of the ikhandle. 9. Select the dynamiciksplineshape node and MMB drag it on top of the ikhandle The IK spline has been converted to a dynamic curve but the joints do not follow its motion. The dynamiciksplineshape node needs to replace the connection between the ikhandle node and the curveshape3 node. Middle Mouse Button drag the dynamiciksplineshape node on top of the ikhadle1 node and choose Other 11

12 node, form the pop up menu, choose Other to open the Connection Editor. 10. Find the World Space attribute on the left of the connection editor and select it. Scroll down and find the In Curve attribute on the right - it will be in italics and grayed out indicating it already has an input connection. Select In Curve, this will update the IK spline so that now the dynamicikspline curve controls the joints. Rewind and play the animation. 11. Select the hairsystem2 node and experiment with the dynamics, try these settings: The World Space attribute on the left needs to be connected to the In Curve attribute on the right Stiffness=0 Drag=.136 Mass=0 Gravity=0 Start Curve Attract = 0 Turbulence Intensity=.091 Frequency =.582 Speed =1.072 In the follicleshape2 tab set the Sample Density to Play the animation and adjust the settings to your liking. Save the scene as chromatin_loops_v04.ma. Add Paint Effects Strokes to the Curves The final step is to add Paint Effects strokes to the curves you need to render. 1. Select and hide all of the nodes except the grouped curves in group1. 2. Open the Visor (Windows>General Editors>Visor). From the Paper folder, select the Confetti brush. 3. Switch to the rendering menu set. 4. Select armcurve in group1. Choose Paint Effects > Curve Utilities > Attach Brush to Curves. The confetti brush will be attached to the main curve; this will be the chromosome arm. 5. Set the following settings in the confetti1 shape node tab of the Attribute Editor: Adding Paint Effects strokes to the curves allows them to be rendered. 12

13 Creation: Tubes Per Step = 100 Segments = 2 Length Min =.163 Length Max =.273 Tube Width 1 =.066 Tube Width 2 =.079 Width Rand =.364 Width Bias =.982 Tube Direction: Along Path Azimuth Min = -1 Azimuth Max = 1 Shading: Set color 1 to a nice purple/blue Under Tube Shading set Color2 to purple Set Hue Rand to 0 Sat Rand to.054 Val Rand to.145 Illumination: Turn on Illuminated and Real Lights Under Shadow Effects set Fake Shadows to 3D cast. Select the StrokeShape1 tab and turn on Use Normal this will keep the stroke from twisting erratically as the curve moves. Do some test renders and adjust the settings until you are satisfied. Apply a similar stroke to the looping curve but lower the tubes per step to 10 and the Global Scale to.18. Set the colors to something that contrasts with the purple and blue of the chromosome arm, try a green/blue. Add a light to the scene. When you like the look of the arm and the loops, create a hair cache for the IK spline curve and render a sequence. Constrain Cloth to Hair It s fairly simple to attach an ncloth object to a hair curve indirectly using a locator and an nconstraint. In this exercise you ll see how you can use this technique to animate the separation of chromosomes during anaphase. This section will go through setting up just part of the animation, completing the animation is simply a matter of expanding the techniques so that both sets of chromosomes are animated. First you ll create the spindles using hair. 13

14 1. Start a new scene in Maya. 2. Create a polygon cube and use the smooth operation and set the divisions to 2 to create the now familiar pole-less rounded cube. Rename the cube roundcube. 3. Scale the roundcube up so that the radius is about 3 units. 4. Switch to the Dynamics menu set. 5. Right-click over the roundcube and choose face, drag a selection over the entire cube to select all of the faces. 6. Choose Hair>Create Hair>Options. In the options choose to create hair at selected points/faces. Set the output to NURBS curves, set the length to Press the Create Hairs button to create the hairs. Hairs now appear attached to the center of each face. 8. Set the length of the timeline to 300 and play the animation, you ll see the hairs drop due to gravity. 9. In the Outliner, select the hairsystem1 node, set Gravity to Set the Drag to 2. Set stiffness to Select the roundcube and set a keyframe on the translatex channel. 12. Play the animation to frame 100, set translate X to 50 and set another key frame. 13. Open the graph editor for the roundcube and flatten the tangents on the animation curve. 14. Play the animation. Model a Simple Chromosome Hairs are attached to the faces of the smoothed cube. The hairs are dragged by the animated cube. The next step is to create a very simple chromosome model and convert it to ncloth. 1. Select the roundcube and hairs and hide them. 2. Create a polygon cube and name it chromosome1. 3. Set the height to 9 and the subdivisions in height to 9 as well. 4. Right-click over the cube and choose face, select the faces at the center section of the cube. 5. Perform a extrusion and pull these faces out a little to form the centromere. 14

15 6. Select the faces above and below the centromere, scale these down in X and Z. 7. Perform and smooth operation on the chromosome, set the divisions to Duplicate the chromosome twice for a total of three chromosomes. 9. Unhide the roundcube and hair system 10. Position the chromosomes near the end of three of the hairs. Choose hairs that are closer together so that you can test the collision properties of the ncloth objects. Make sure the chromosomes are on the side of the roundcube opposite from where it travels when the animation is played. 11. Save the scene as anaphase_v01.ma. The chromosome is modeled from a simple cube Convert the Chromosome to ncloth and Add Constraints Multiple objects can all be assigned to the same nucleus node so that their dynamics are shared. ncloth objects cannot be constrained directly to the hair curves, however a locator can be attached to each hair and then each chromosome can be constrained to a locator. 1. Select the three chromosome objects, switch to the ncloth menu and choose ncloth>create ncloth. 2. Select any one of the chromosomes and open its attribute editor. Switch to the nucleus tab and set the gravity to 0 and the air density to If you play the animation now, not much will happen since there are no forces acting upon the ncloth. 4. Rewind the animation and create three locators to act as constraint objects. 5. Select the locator1 and the hair curve closest to chromosome1. Switch to the Animation menu set, choose animate>motion Paths>Attach to Motion Path. 6. By setting the curve as a motion path to the locator you are essentially constraining the locator to the curve. Of course it currently moves along the curve as the animation plays. Select the curve and open the motionpath1 node in the channel box. You ll see the U Value channel is orange indicating that it has a keyframe. Right-click over the U Smoothing the cube twice finishes the model The chromosome is duplicated and placed near the end of some of the curves. 15

16 Value channel and choose Break Connections to remove the keyframe. 7. Set the uvalue channel to 1 so that the locator is at the end of the curve next to the chromosme. 8. Play the animation; you ll see the locator is attached to the curve as it travels through space. 9. Rewind the animation. Right-click over chromosme1 and choose Vertex to switch to vertex selection mode. Carefully select all the vertices of the centromere. 10. Hold the ctrl/command key and select locator1 in the channel box. You want both the locator and the vertices of the centromere to be selected at the same time. 11. Switch to the ncloth menu set and choose nconstraint>transform. This will constrain the selected vertices to the locator. (Note that nconstraints are not the same as the constraints found in the animation menu set, these are specific to ncloth objects). 12. Play the animation, you ll see the chromosme is dragged along by the dynamic curve. 13. Repeat these steps for the other two chromosomes so that they are attached to the other curves. 14. Try experimenting with settings on the ncloth chromosome. Set bend resistance to 70, stretch resistance to 10 and compression resistance to Select all of the follicle nodes in the Outliner and set their sample density to 2. This increases the detail in the hair dynamics by double the number of points on the dynamic version of the curve. A setting of 1 causes the dynamic hair to have the same number of points as the original CV curve. 16. Select the hair node and in the attribute editor, set the turbulence intensity to 1 and the frequency to At this point the animation will be a bit slow so you may need to create a playblast to see how it will look at the proper speed. Notice that the chromosomes will collide and interact if they get close to each other. 18. You can add more chromosomes to the scene if you d like although it will get increasing computational intensive so save frequently. Remember that before A locator is attached to the curve using a motion path. The vertices of the centromere are selected The vertices are constrained to the locator using a transform nconstraint. 16

17 rendering you ll need to make a cache for both the hair system and the ncloth chromosomes. When the animation is played the hair curves drag the chromosomes. ncloth Cell Division There are numerous approaches for creating believable cell division animation in Maya. In this section you ll see some of the possibilities offered by ncloth. The goal of this lesson, as well as the others in this section, is to make you aware of the many choices you have and inspire you to create your own solutions to the problems posed by scientific visualization. In the first part of this lesson you ll use ncloth to animate one side of the cell division. Later on you ll create a cache and duplicate the right side of the cell to create the left side of the cell. By using applying a cache to both halves of the cell you ll only have to create a ncloth simulation for one side of the cell. 1. Create a new scene in Maya. 2. Create a polygon cube and scale it up 10 units in X, Y, and Z. 3. Switch to the polygon menu set, select the cube and perform a smooth operation, leave the divisions at Switch to the front camera. Right-click over the cube and choose Vertex. Drag a selection marquee around all of the vertices on the left side of the origin. The vertices on one side of the cube are scaled down. 17

18 5. Use the scale too to scale these down roughly 30%. This will cause the cell to be squished out towards the right in the final simulation. 6. Select the cube and perform another smooth operation, set the divisions to Name the cube cell_right 8. A rigid body object will cleave the cell. Create a polygon Pipe (Create>Polygon Primitives>Pipe). 9. Rotate the pipe 90 degrees in Z. 10. Set the X and Z scale to 12, set Scale Y to Set the thickness to.5, subdivisions axis to 36, subdivisions height to 2, and subdivision caps to 1. You can increase the rings around the flat area of the pipe by raising the subdivisions of the caps, however, as you saw in the first part of this chapter, increasing the subdivisions on the passive geometry is not always necessary. 12. Right Click over the pipe and choose vertex. Select all the vertices at the center of the pipe, use the scale tool to scale the center along the Y axis so that the center of the pipe is thinner than the outside edge. This will create a wedgelike profile in the pipe. This may or may not be necessary depending on the final ncloth settings. 13. Delete history on the pipe. Freeze transforms on the pipe (modify>freeze Transformations) 14. Rewind the animation to the start, set a key frame on the pipe s X, Y, and Z scale. 15. Set the time line to frame 100. Scale the pipe down in X, Y, and Z to.05 in X, Y, and Z. 16. Save the file as cellconstrict_v01.ma A polygon pipe will be used to cleave the cell. ncloth Settings As the pipe scales it collapses the ncloth cell. Creating the quality of the cell as it divides involves some experimentation with the ncloth settings. 1. Open the cell_v01.ma file or continue with the scene from the previous section. 2. Select the cell_right object and switch to the ncloth menu set. Choose 18

19 ncloth>create cloth, the default settigs are fine. 3. Select the pipe and choose ncloth>create Passive. 4. Select the cell and switch to the nucleus tab in the Attribute Editor. Set Gravity to Play the animation,. It will be somewhat slow but you ll see the pipe clamp down on the cell. The settings need to be adjusted so that the cell is squished out to the right side. 6. Use these values for the settings, if a setting isn t mentioned you can leave it at the default value. The name of these settings are fairly self-explanatory and the values were determined through experimentation. I found that a high compression and bend resistance allowed the cell to successfully escape from the clamping action of the pipe, use these settings as a starting point to create the type of motion you want: Adjusting the settings of the ncloth node creates a quality to the cell which allows it to be squished out one side of the pipe. Friction = 0 Stretch Resistance = 1.82 Compression resistance = 200 Bend Resistance = 120 Tip : When you have settings you like, or even close to what you like, use the preset button in the upper right of the attribute editor to save your own presets. If you need to replicate the settings in another scene, you can apply them directly from the ncloth attribute editor without having to reopen the original scene. 7. Hide the pipe object and create a playblast of the animation, the cell half should squish out to the right, however it keeps traveling after it resumes it s shape, to stop the cell you can add a drag field. Rewind the animation, select the cell_right object and choose fields>drag. 8. Select the drag field and open its attribute editor, set the Volume shape of the field to Cube. 9. Scale the cube up to 4.5 in X, Y, and Z, move the field to the right of the cell (set the translate X to 7.15). 10. Set the Attenuation to 0 and the magnitude to1. Higher magnitudes can even act as a barrier to the cloth The settings in the nclothshape1 node 19

20 causing in to deform as it presses against the drag field. 11. Create another playblast. The drag field should halt the cell s momentum so that it stops after the division is complete. You can increase the air density in the nucleus settings if you want the cell to appear more like its in a fluid medium. 12. Save the scene as cellconstrict_v02.ma Tip: You can try keyframing the pressure settings to make the cell grow into a spherical shape after it divides, experiment with manual settings and the volume tracking method. The pressure should start increasing after the cell divides and then stop once it has settled. A drag field keeps the cell from drifting after it divides. Using ncaches Once an ncache is created it can be applied to a duplicate of the original object to create the other half of the cell. 1. Select the cell_right object. From the ncloth menu set choose ncache>create New ncache. You can use the options to name the cache, set the directory and the time range. You can choose to either create one file of one file per frame, in this case, choose to create a single file. If a cache already exists, a dialog box will open asking you if you want to replace or rename the existing cache. Make a note of where the cache file is stored, it should create a directory in the project s data folder by default. 2. The scene will play through as it creates the cache, once it is finished rewind and play the scene. It should play at full speed. 3. Rewind the animation and select the cell_right object and duplicate it, name the duplicate cell_left. In the outliner, set the display to show shape nodes, expand each of the nodes for the two halves of the cells and make sure the shape nodes have different names, rename them cell_rightshape and cell_leftshape. 4. Select the cell_left object and choose ncache>attache existing ncache. A dialog box will open up; by default it should open to the data folder by default. Find the directory created by the Rename the shape nodes of the cell to ensure that Maya does not become confused when dynamics are applied. 20

21 cache. In the dialog box, set the file type to best guess. Select the file ending in.mc. 5. Rewind and play the scene, you ll see both cell halves have the same animation applied. 6. Select the cell_left object and set its Y rotation to 180. Rewind and play the scene, you ll see the two cell halves now move in opposite direction. 7. If you find that one side of the cell pokes through the opposite half as it divides you can move one or the other slightly in X. Try setting the cell_left s translate X value to In this case, moving the left side a little actually improves the look of the division. Tip: Its possible to delete all of the ncloth nodes and fields except for the original geometry and apply the cache to each of the duplicate cell halves, this can speed up playback of the scene, just male sure you have a version of the scene with the ncloth nodes saved in case you want to go back and make changes. A cache can be attached to a similar, non-cloth object to duplicate its motion. Tip: as long as the cache file exists and is properly linked you can add animation and deformers to the cached geometry, if you want to add an additional level of variation to the cell objects you can add a lattice on top of the geometry and adjust the lattice point positions. To add even more dynamic motion you can experiment with making the lattice a softbody object or even create soft body duplicates of the cell geometry. Coat the Geometry with Particles To create the look of a single cell object dividing into two cells, you can use coat each cell half with a single particle object set to the blobby surface render type. This works almost as well as metaballs in other leading 3D packages such as Houdini. 1. Continue with the scene from the previous section, create a particle emitter. Set the emitter type to omni, the rate to 500, and the speed to Select the particle object in the Outliner. Ctrl (Command) select the cell_right object. Switch to the dynamics menu set The duplicate cell with the cache applied adds the other half of the cell to the division. 21

22 and choose particle>goal>options. In the options, set the goal weight to Repeat step 2 for the cell_left object, you want both of the objects to be goals for the cell. 4. Set the particle render type to blobby surface. If you play the animation you ll see the particles get stuck between the two halves of the cell, this is because each goal is exerting an equal force on the particle. 5. Open the attribute editor for the particleshape node. Expand the goal weights and objects section. Click on the creategoalweight0pp button and the creategoalweight1pp. This creates a per particle goal weight for each goal object. Each goal has a designated index number. The cell_right object is goal0 and the cell_left object is goal1 because of the order in which the goals were added to the particle. 6. Scroll down to the per-particle array attributes. Right click over the goalweight1pp field and choose creation expression. 7. In the expression editor, add the following expression: If (particleid % 2 == 0 ) { goalweight0pp=1; goalweight1pp=0; } else { goalweight0pp=0; goalweight1pp=1; } 8. This expression simply states that if the ID number of a particle is even then set the goal weight of goal0 to 1 and the goal weight of goal1 to 0, otherwise set the reverse (so odd numbers go to goal1). The % sign stand for modulus, particleid % 2==0 says that the Id number can be divided by 2 without leaving a remainder, thus it is even. Note that you must use the double equals sign in this expression. A double equals sign is an evaluation, a single equals sign is an assignment. The particles are stuck between the two goal surfaces Attributes to control per-particle weights for each goal can be created in the Goal Weights and Objects section of the particle s attribute editor. The expression divides the particles evenly between the two goals. 9. Click on the Create button in the expression editor to make the expression. Play the 22

23 animation, you should see that both cell halves are coated with particles. 10. Currently the particles continue to be added to the cell halves, you can set a limit on the number of cells generated by the emitter. If you set the Heads Up display to show the polygon count and select the two cell halves, you ll see that the total vertices is Select the particle shape node and in the Attribute Editor under Emission attributes, set the max count to You can raise the emitter s rate to 10,000 so that the cells are coated in particles within the first couple frames of the animation. 11. Select the particle shape. In the Attribute editor click on the Add Attributes for current render type button. Raise the threshold to Select the original cell geometry and hide it or assign a transparent lambert shader so that it does not show up in the render. A Blinn shader is assigned to the particles, when rendered the cells appear to be one object split into two. You may notice that the divided cells are rather lumpy. This is because for each half of the cell only half the vertices are being covered the particles are attracted to the goal based on their particle Id number so some vertices are left open because the corresponding particle is on the opposite half of the cell. The easiest way to remedy this is to perform a smooth operation on each of the cell halves and then raise the max particle count on the particle shape node so that enough particles are created (about particles will need to be created to cover the cells). Then raise the radius and threshold of the blobby surface particles. This will smooth the surface of the cells somewhat but it s not a perfect solution. The reason you would use a single particle object as opposed to two particle objects is that the metaball quality of blobby surfaces can t be shared between two separate particle nodes. You may notice that it takes a few frames at the start of the animation before the cells are coated with particles. You can either cut these out of the final render sequence or play the animation up to the frame where the cells are coated but before the cells start to move. Then set an initial state on the particles (Solvers>Set Initial State for Selected). Tip: Before rendering the scene, make sure you create a particle cache! 23