Real-time 3D Graphics. Doug A. Bowman

Transcription

1 Real-time 3D Graphics Doug A. Bowman 1

2 Outline (Very) brief review of 3D image synthesis Real-time 3D graphics for VEs (C) 2008 Doug Bowman, Virginia Tech 2 2

3 Rendering 3D scenes Most 3D graphics is based on scenes composed of objects built of polygons. Goal: from scene specification and viewing specification, produce a 2D image of the scene with proper 3D perspective by determining the color for each pixel on the display 3D teapot animation (C) 2008 Doug Bowman, Virginia Tech 3 3

4 Overview of 3D image synthesis (rendering) 3D model projection 2D projection rasterization Pixel values display Displayed image Includes scene definition, camera properties, lighting characteristics, etc. Process of mapping the desired 3D primitives into a 2D 2D versions of 3D primitives Process of changing primitive definitions into pixel locations/colors Color value for each pixel on the screen, stored in the frame buffer Process of taking information from frame buffer and producing physical representation on a display device (C) 2008 Doug Bowman, Virginia Tech 4 4

5 3D Viewing Example Y Y Z Object model X Z X (C) 2008 Doug Bowman, Virginia Tech 5 5

6 Coordinate systems Right- vs. left-handed World vs. object (object trees, scenegraphs) World vs. camera Screen/display Z Y Righthanded CS X Transformations (mappings) between CSs (C) 2008 Doug Bowman, Virginia Tech 6 6

7 Graphics pipeline Local World View 3D screen Display Object definition Compose scene Define view parameters Perform culling Clip to 3D view volume Hidden surface removal Shading Modeling transformation Viewing transformation Perspective projection Rasterization (C) 2008 Doug Bowman, Virginia Tech 7 7

8 Modeling transformation For each vertex: transform object to world coords. Each M represents a transformation matrix. Where is chair2 relative to room? table M 3 M 4 room M 1 M 2 chair1 chair2 M 0 rug (C) 2008 Doug Bowman, Virginia Tech 8 8

9 Graphics pipeline Local World View 3D screen Display Object definition Compose scene Define view parameters Perform culling Clip to 3D view volume Hidden surface removal Shading Modeling transformation Viewing transformation Perspective projection Rasterization (C) 2008 Doug Bowman, Virginia Tech 9 9

10 Viewing transformation Transform world to camera coords. Where are the objects with respect to the camera? View frustum Viewing transformation animation (C) 2008 Doug Bowman, Virginia Tech 10 10

11 Clipping Process of removing polygons and parts of polygons that fall outside the view frustum Basically, comparing each vertex with plane equations for the six view frustum sides (C) 2008 Doug Bowman, Virginia Tech 11 11

12 Projections Projection: mapping 3D to 2D Center of projection = camera = eye Projection plane = viewplane: 2D plane onto which objects are projected Perspective projection (C) 2008 Doug Bowman, Virginia Tech 12 12

13 Mathematics of a one-point projection P(x, y, z) x projected = dx/z = x/(z/d) y projected = dy/z = y/(z/d) d Projection animation (C) 2008 Doug Bowman, Virginia Tech 13 13

14 Graphics pipeline Local World View 3D screen Display Object definition Compose scene Define view parameters Perform culling Clip to 3D view volume Hidden surface removal Shading Modeling transformation Viewing transformation Perspective projection Rasterization (C) 2008 Doug Bowman, Virginia Tech 14 14

15 Hidden-surface elimination Problem: we only need to display polygons that are visible Example approaches: z-buffer (hardware) BSP tree Spatial subdivision (C) 2008 Doug Bowman, Virginia Tech 15 15

16 Shading We can increase realism by adding shading/lighting, but more complex approaches take longer to calculate constant flat Gouraud (C) 2008 Doug Bowman, Virginia Tech 16 16

17 Rasterization Once vertices have been projected, we only need to fill in the primitives between the vertices Rasterization (sometimes called scan conversion) is the process of determining which pixels are part of a primitive and turning on those pixels. Polygon fill animation (C) 2008 Doug Bowman, Virginia Tech 17 17

18 Graphics pipeline Local World View 3D screen Display Object definition Compose scene Define view parameters Perform culling Clip to 3D view volume Hidden surface removal Shading Modeling transformation Viewing transformation Perspective projection Rasterization (C) 2008 Doug Bowman, Virginia Tech 18 18

19 What is real time? Fast enough that the user can Perceive smooth motion Interact with the environment Often stated in frames per second (fps) number of views of the scene rendered in one second A frame rate of approx fps is the minimum for real-time (C) 2008 Doug Bowman, Virginia Tech 19 19

20 Speed vs. Quality tradeoff More complex models slower rendering We must manage this tradeoff to achieve maximum quality with an acceptable frame rate (C) 2008 Doug Bowman, Virginia Tech 20 20

21 Techniques for real-time 3D graphics Polygonal simplification View culling Occlusion culling Level of detail management Texture substitution Lightmaps (C) 2008 Doug Bowman, Virginia Tech 21 21

22 Polygonal simplification Decrease the number of polygons without sacrificing visual quality (meet polygon budget) triangles 2502 triangles 621 triangles 251 triangles (C) 2008 Doug Bowman, Virginia Tech 22 22

23 Level of detail management LOD: change the complexity of the model/image dynamically to maintain realtime performance Ex: use simplified models when objects are at a great distance (C) 2008 Doug Bowman, Virginia Tech 23 23

24 Level of detail - another example Match characteristics of human eye Use high detail in fovea, low detail in periphery (C) 2008 Doug Bowman, Virginia Tech 24 24

25 (C) 2008 Doug Bowman, Virginia Tech 25 25

26 View culling Not the same as clipping! Easy: remove all objects that are behind the plane of the camera Harder: remove all objects outside the view frustum (C) 2008 Doug Bowman, Virginia Tech 26 26

27 Occlusion culling Hidden-surface elimination techniques: still render all polygons (e.g. z-buffer) Occlusion culling: remove polygons from consideration before rendering (C) 2008 Doug Bowman, Virginia Tech 27 27

28 Texture substitution (imposters) Textures can cause objects to appear much more detailed than they actually are Problem: when the user is close (C) 2008 Doug Bowman, Virginia Tech 28 28

29 Lightmaps Pre-calculate lighting for a scene, then texture lighting onto objects (C) 2008 Doug Bowman, Virginia Tech 29 29

30 Lightmapped VEs (C) 2008 Doug Bowman, Virginia Tech 30 30

31 Other graphics issues for VEs Stereo rendering Off-axis projections Inserting the user into the scenegraph Parallel / distributed rendering (C) 2008 Doug Bowman, Virginia Tech 31 31