Graphics Pipeline. Rendering Operations: Clipping, Rasterization, & Hidden Surface Removal

Transcription

1 Graphics Pipeline Rendering Operations: Clipping, Rasterization, & Hidden Surface Removal Adapted from 3D Computer Graphics by Alan Watt Third Edition, Pearson Education Limited,

2 Graphics Pipeline 3D scene mapped as a two-dimensional surface. Find the color of the pixel. Two process Geometric CPU intensive Algorithmic Rendering Video Card - GPU 3D Space 2D Space 2

3 Graphics Pipeline Object Space Local Coordinates - geometric Object Definition - geometric World Space World Coordinate - geometric Compose Scene - geometric Define View - geometric Define Lighting - geometric Radiosity & Photon Mapping rendering View Space View volume - geometric Cull - geometric Clip to View Volume geometric/rendering 3D Screen Space Rasterization rendering Hidden surface removal - rendering Shading rendering Local Space Modeling Transformation World Space View Transformation View Space Projection Transformation 3D Screen Space Display or Image Space 3

4 Rendering Operations 1. Clipping to the View Volume Determining what portion of a polygon that crosses the View Volume should be rendered. 2. Rasterization Finding the set of pixels in which a polygon projects. 3. Hidden Surface Removal Determines which polygons will not be seen. 4. Shading Interpolating the value of the pixels within a polygon. 4

5 Clipping to the View Volume Cull then Clip. Geometric Operation Cull to View Volume View Space View Coordinates Determine if polygons are outside or inside the View Volume Determine if polygons straddle the View Volume Discard as many polygons as possible 5

6 Projection Transformation Geometric Operation Transforming scene geometry from View Space into 3D Screen Space. View Space - Perspective Cube Projection Transformation Clip rectangle 3D Screen Space - Orthographic Transformed Cube 6

7 Clipping to the View Volume Each polygon is evaluated against the clipping boundary of the View Volume and clipped accordingly. F1 Inside View Space - Perspective S1 Clip boundary F1 S2 I1 Polygon Cube Outside F2 Clip rectangle Polygon View Transformation F1 Case 1 Case 2 3D Screen Space - Orthographic F1 F4 I2 F1 F4 I1 S3 F3 I1 S4 Transformed I1 Cube I2 Bottom Clip Left Clip Right Clip Top Clip Case 3 Case 4 Case 5 Figure 1 Sutherland-Hodgman clipper Figure 2 7

8 3D Screen Space Geometric Operations Point projection onto Screen Space Geometric Transformation Figure 1 - Geometric Transformation a) View Space b) 3d Screen Space Object Space World Space View Space 3D Screen Space Display or Image Space Figure 2 Projection of object s vertices into 3d Screen Space 8

9 Rasterization Rasterizing edges Generate a linear set of pixels with no gaps Raster Grid Figure 1a 1b Projection of vertices edges into 3D Screen Space. Raster Grid 9

10 Rasterization 3D Screen Space Rasterization Find pixel coordinates of interior points Scan line Procedure Filling the polygon Assign a value based on shading scheme Raster Grid y Object Space World Space View Space 3D Screen Space Display or Image Space scan lines Scan line Scan line plane z 3D Screen Space x Figure 1 - Raster Grid / Scan Line Figure 2 Scan Line Plane 10

11 Rasterization 3D Screen Space Rasterization Find pixel coordinates of interior points Scan line Procedure Filling the polygon Assign a value based on shading scheme Raster Grid y Object Space World Space View Space 3D Screen Space Display or Image Space scan lines Scan line Scan line plane z 3D Screen Space x Figure 1 - Raster Grid / Scan Line Figure 2 Scan Line Plane 11

12 Hidden Surface Removal (HSR) Hidden Surface Removal (HSR) Hidden Surface Determination Visible Surface Determination Hidden Line Removal (HLR). Determining which surface are visible. 12

13 Hidden Surface Removal Geometric Operations Tessellation Converting all geometry to triangles. Culling View Volume - surfaces outside of the view volume are culled. Backface - Orientation of Surface Normal to the Camera. Geometric Transformation Converting Geometry from View Space to Orthographic 3D Screen Space. Patch Poly Three sided Polygon 13

14 Hidden Surface Removal Geometric Operations Tessellation Converting all geometry to triangles. Culling View Volume - surfaces outside of the view volume are culled. Backface - Orientation of Surface Normal to the Camera. Geometric Transformation Converting Geometry from View Space to Orthographic 3D Screen Space. Back Face Culling Culling to the View Volume 14

15 Hidden Surface Removal Geometric Operations Tessellation Converting all geometry to triangles. Culling View Volume - surfaces outside of the view volume are culled. Backface - Orientation of Surface Normal to the Camera. Geometric Transformation Converting Geometry from View Space to Orthographic 3D Screen Space. Transformed Cube Cube View Space - Perspective 3D Screen Space - Orthographic 15

16 Hidden Surface Removal Z-Buffer the depth of a generated pixel z coordinate is stored in a buffer (z-buffer). Two-dimensional X-Y array Excerpted from Wikipedia, 16

17 Hidden Surface Removal (HSR) Polygon-by-Polygon or Painter s Algorithm Scanline Renderer 17

18 Hidden Surface Removal (HSR) Polygon-by-Polygon or Painter s Algorithm Simple to implement No upper limit on scene complexity Overlapping of polygons are difficult to render. Inefficient Hidden Line Removal - Z-buffer memory intensive Wikipedia, 18

19 Hidden Surface Removal (HSR) Scan Line /w Z-buffer Most commonly used Polygons rendered row by row. Polygons sorted based on Z-depth. HLR - Z-buffer, scan-line z- buffer, spanning scan line algorithm. Scan line order efficient for anti-aliasing. Not as memory intensive Limits complexity of scene y Scan line z 3D Screen Space Scan Line Rendering x Excerpted from Wikipedia: 19

20 Hidden Surface Removal Z-buffer algorithm Advantage Simplicity Fetch individual polygons in any order No upward limit of polygons (scene complexity) Disadvantage Memory Dedicated Z-Buffer memory Unseen polygons are rendered Z-depth precision Scanline algorithm Advantages Machine efficient Needs limited memory Disadvantages Limited scene complexity Scan line De facto standard - Scanline Z-buffer Scan-lined algorithm intensive Screen Space Z-Buffered memory intensive Object Space Scan Line Rendering y 3D Screen Space z x 20

21 Graphics Pipeline Object Space Local Coordinates - geometric Object Definition - geometric World Space World Coordinate - geometric Compose Scene - geometric Define View - geometric Define Lighting - geometric Radiosity & Photon Mapping rendering View Space View volume - geometric Cull - geometric Clip to 3D geometric/rendering 3D Screen Space Rasterization rendering Hidden surface removal - rendering Shading rendering Local Space Modeling Transformation World Space View Transformation View Space Projection Transformation 3D Screen Space Display or Image Space 21

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