Tutorial 9: Using Alpha Channel Mattes

Transcription

1 Tutorial 9: Using Alpha Channel Mattes Completion time 15 minutes The Alpha Matte is a unique and versatile feature. It allows you to define areas of an image to "drop out", and use the image in material definitions so that when rendered the drop out areas appear transparent. Because of this ability to drop out a background color, Renderize Live provides you with the ability to take 2D "clip art" image bitmaps and include them as "objects" in your renderings. Clip art images of people, cars, trees, etc., can be used to populate a scene and increase realism without having to model every detail. In this tutorial you will learn: What is an alpha matte How alpha matte materials appear during rendering How to enable and disable the matte drop-out for materials How to define an alpha matte for an image How to save an image to disk with its alpha matte How to integrate 2D clip art images as objects in a view Alpha Matte Applications First we'll load a new project. Restart Renderize Live. Load project "tutor9.eye". Load the view "view1" into the View Well. Select the button to compute shadows. Set "Test:1" and select the button on the Render Toolbox. Examples of the two types of Alpha Matte uses are seen in this rendering: partially transparent materials, and the use of 2D clip art in a view. Both uses give the impression that the view contains much more detail than the simple models indicate. In both images, an "alpha channel matte" has been defined to indicate areas of the image that should render transparent. The image of the tree is actually on a black background: this background has been set as transparent so that only the tree appears. Similarly, the wicker image that you see on the pyramid exists with a brown base hue between the strands of wicker, and this background has been set as transparent. In addition to Tutorial 9: Using Alpha Channel Mattes 115

2 rendering part of the image completely transparent, shadows are calculated from only the visible portions, providing intriguingly detailed shadows as a result. An alpha matte serves to define those areas of an image that you wish to make transparent. An image's alpha matte is saved as a part of the image, but whether or not that alpha matte is used to create a transparent effect is controlled in the Material Designer. To appreciate the power of the Matte drop out, let's turn it off for the pyramid. Drag and drop the material "wicker" into the Edit Well. Find the "Drop Out Matte Color" check box and disable it. Re-save the material "wicker" by clicking "Save" and confirming "Replace." Note that the material icon updates. Minimize the Material Designer. Select the button on the Render Toolbox. Now the object renders as a solid. Once a Matte is saved with an image it can always be switched on or off using the "Drop Out Matte Color" button. Creating the Alpha Matte Alpha Matte information is defined in the Image Designer, and saved to disk as part of the image. Drag and Drop the image "wicker1" into the Edit Well. Select the button at the top of the Image Designer. Creating an alpha matte involves nothing more than selecting a color from the image and assigning all pixels with that color to the matte. The alpha matte may also be set to apply to a range of colors around the selected value via the "Delta" dial. Let's re-define the alpha matte in the wicker so the wicker, not the base color, is dropped out. Hold down the Shift key on the keyboard, and drag the cursor over the loaded image while depressing the left mouse button. Notice the Color Well in the bottom of the Image Designer: it reports the color value of the pixel at the cursor location. Make sure not to press and hold the left mouse button until the cursor is actually in the image or nothing will happen. With the cursor over the wicker portion of the image (yellowish area) stop selecting. Drag and drop the selected color from this Color Well on the lower left corner of the Image Designer to the Background Color Well above it. 116 Tutorial 9: Using Alpha Channel Mattes

3 Select on the "Create Matte" button. We have now set an alpha matte! When the cursor returns we can view our new alpha matte. Select on the "Show Matte" button. When the Show Matte button is depressed, those areas of the image that are defined as part of the alpha matte (and will render transparent when the Matte "Drop Out" button is selected in the Material Designer) are displayed in black. Those areas of the image that will appear are displayed in white. Note that some wicker portions were not included in the alpha matte: there is a break in the black cross pattern. Let's see why this occurred. Select on the "Show Matte" button again to deselect it and bring back the original image. Notice that the wicker image gives the impression of a weave due to a drop shadow. The shadow area is a different color value than the wicker area. Because we didn't make allowances for this difference, the shadow portion of the image was excluded from the alpha matte. To include it we must set a greater delta value. The delta value, set by the "Delta" dial, instructs the matte to include color values above and below the precise value that appears in the "Background Color" Well. Set the "Delta" dial to.15. Select "Create Matte". Select "Show Matte". Now we have included all portions of the wicker in the alpha matte. Save the image and confirm "Replace". You must always remember to save your image after creating an alpha matte. No outward change will be evident, but the re-saved image will now contain alpha information. Important Note: Images with alpha information must be saved to disk as a 32-bit non compressed TGA file in order to retain the alpha channel matte information that you have defined. We recommend that you save the image to this file format as soon as you have defined the alpha channel matte to your satisfaction. Let's render to see our new alpha at work. Minimize the Image Designer. Maximize the Material Designer and select the "Drop Out Matte Color" option to enable it. Save and confirm "Replace". Close the Material Designer. Select the button on the Render Toolbox. It's important to note when mapping a 3-D object with alpha matte images, the object becomes partially see-through. You may therefore want the backside of the object to render. Setting the object to render with backfaced polygons is the best way to accomplish this. The pyramid is being rendered with backface polygons. Let's see what happens if this is turned off. Tutorial 9: Using Alpha Channel Mattes 117

4 Drag and drop the object "pyramid.dxf" into the Edit Well. Select "Render Backface" to disable it. Save the object and confirm "Replace". Close the Object Designer. Select the button on the Render Toolbox. Notice the difference. Integrating 2D Clip Art Images into a Rendering The alpha matte feature also provides users the unique ability to integrate 2D elements in a 3D environment. The tree in our view is not a 3D object: that would take more effort to model and render than we wish to spend for this project. It's an image of a tree mapped onto a flat object. Rendering photographic clip art images stripped from their background and casting shadows around the remaining image makes for very powerful visual impact, without much effort. Click on if it is not already selected. Drop the object "eucapoly" into the Move Well. Notice that this object is a simple rectangle. The 2D image is mapped onto this rectangle in a straight ortho perspective. Press the button, located on the Object Space area of the Move Toolbox, to rotate the object around its local Z axis. Drag the pointer in the viewport to rotate the object about 90 degrees around its Z axis. Select the button on the Command Bar to display the Render Toolbox. Select the button on the Render Toolbox. Of course you are somewhat limited in the placement of 2D image/objects, making certain that they face both camera and shadowing lights. They are after all 2D images: they have no real depth, so viewing them and lighting them from the side should be avoided. Their use can add enormous amounts of detail without excessive design time. Modeling trees and people, or other objects with intricate detail, can prove exhausting without returning the same effectiveness as a simple 2D plane and photographic image. To encourage the use of 2D images, Renderize Live provides a very simple and automatic way of adding them to a rendering. There is a button in the Matte Operations section of the Image Designer called "Create 3D Object" which, when selected, does the entire process for you. The "Create 3D Object" feature in the Image Designer accomplishes four important actions: It creates an object (2D plane) at the same aspect ratio as the image; It creates a material using the current image as the Image Texture, and enables the "Drop Out Alpha Matte" feature"; 118 Tutorial 9: Using Alpha Channel Mattes

5 It assigns the material to the object; It adds the object to the currently loaded view. Because the ability to create 3D objects out of 2D images is most often used in conjunction with the alpha matte feature, it is included as a part of the Matte Image Operation. Load "view2" into the View Well. Nothing appears in the viewport because no objects have been added to this view yet. Drag and drop the image "tree" into the Edit Well. Select the operation. Notice the button at the bottom of the operations area on the Image Designer - "Create 3D Object". First let's create a Matte for the image's black background. Hold down the Shift key on the keyboard and use the left mouse button to select the black background of the image as the background color. Drag that color from the Color Well below the Image Designer viewport and drop it into the Background Color Well of the Matte command. Set "delta" to about Select "Create Matte". Select "Show Matte" and verify. The tree should appear white on a black background. Save the image and confirm "Replace". Now we will create a 3D object. Select "Create 3D Object". Important: When you create a new object using this command, that object is automatically loaded into the current view. If there is no current view (no view is loaded into the View Well), this command will not function. Close the Image Designer. Look at both the Material Resource Palette and the Objects Resource Palette, and notice the addition of both a material and object with the same name as the image from which they were created, "tree". Also note that the object icon name is highlighted, indicating that it is already loaded in the current view. Now let's center the camera according to the position of the new object. Drop "view2" into the Move Well and press the button to center the camera to the object's position. A large white rectangle containing our tree image appears with a black background. Select on the Command Bar to display the Render Toolbox, then press the button on the Render Toolbox to render the image. As you can see, 2D bitmaps can be quickly and easily integrated into your renderings to increase realism. Tutorial 9: Using Alpha Channel Mattes 119

6 Notes 120 Tutorial 9: Using Alpha Channel Mattes