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Week 2: Review of graphics API

Week 2: Review of graphics API. Programmable pipeline. OpenGL block diagram. Generation. Traversal. Xformation. Rasteriz. & tex. Display. Display list. Per-vertex operations & Primitive assembly. Per- fragment operations. Frame buffer. Video display. Evaluator. Rasterization.

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Week 2: Review of graphics API

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  1. Week 2: Review of graphics API Programmable pipeline

  2. OpenGL block diagram Generation Traversal Xformation Rasteriz. & tex. Display Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations Application & higher level graphics libraries OpenGL implementation

  3. Per-vertex operations • Transform vertex coords. (modelview m.) • Transform and renormalize normals • Generate and transform texture coords. • Lighting calculations Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations

  4. Primitive assembly • Flat shading • Clipping • Projection transformations • Viewport and depth-range operations • Culling Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations

  5. Rasterization • Convert each primitive into fragments • Fragment: “transient data structures” • position (x,y); depth; color; texture coordinates; coverage; … Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations

  6. Interpolation issues http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt

  7. p1 P1’ p p L P2’ p2 Linear Interpolation Issues • Linear interpolation takes place in screen space • Problems can happen when projecting • Texture coordinates • Colors • Positions are okay. • Cannot recover p’s color and texture coordinates from linear Interpolation Lerp(Cp1’,Cp2’) along L != Cp • Need perspective correct interpolation for unprojected values http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt

  8. More explanations • Equal spacing in screen (pixel) space is not the same as in texture space in perspective projection • Perspective foreshortening from Hill courtesy of H. Pfister http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt

  9. Perspective-Correct Texture Coordinate Interpolation • Interpolate (tex_coord/w) over the polygon, then do perspective divide after interpolation • Compute at each vertex after perspective transformation • “Numerators” s/w, t/w • “Denominator” 1/w http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt

  10. Perspective-Correct Texture Coordinate Interpolation • Linearly interpolate 1/w, s/w, and t/w across the polygon • At each pixel • Perform perspective division of interpolated texture coordinates (s/w, t/w) by interpolated 1/w (i.e., numerator over denominator) to get (s, t) http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt

  11. Perspective-Correct Interpolation • That fixed it!

  12. Back to the pipeline

  13. Rasterization • Convert each primitive into fragments • Fragment: “transient data structures” • position (x,y); depth; color; texture coordinates; coverage; … Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations

  14. Per fragment operations (I) • Generate texel for every fragment • Fog calculations • Coverage (antialiasing) values application Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations

  15. Per fragment operations (II) • Scissoring • Alpha test • Stencil test • Depth-buffer test • Blending • Dithering and logical operation • Masking Display list Per-vertex operations & Primitive assembly Per- fragment operations Frame buffer Video display Evaluator Rasterization Application generation Application traversal Texture memory Pixel operations

  16. Hello world Per- vertex Operations Per- fragment Operations (I)

  17. Vertex shader • Is run once every time a vertex position is specified • glVertex, glDrawArrays, … • Must compute gl_Position • May compute gl_ClipVertex or gl_PointSize

  18. Hello world

  19. Fragment shader • Is run once for every fragment produced • Has access to the interpolated value for each varying variable • Color, normal, texture coordinates, arbitrary values • Output goes to the fixed pipeline • gl_FragColor – computed R, G, B, A for the fragment • gl_FragDepth – computed depth value for the fragment • gl_FragData[n] – arbitrary data per fragment, stored in multiple render targets

  20. Playing with GLSL Demo

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