Programmable Piplelines
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The document discusses the concept of programmable pipelines in graphics, focusing on elements such as vertex and fragment shaders along with shading languages like GLSL and Cg. It outlines the processes involved in rendering, including vertex transformations, lighting calculations using the Phong model, and the rasterization of geometric entities into fragments. Additionally, it highlights the development of shading technologies and the importance of programmable shaders in replacing fixed processing functions.
Programmable Piplelines
- 2. Objectives • Introduce programmable pipelines • Vertex shaders • Fragment shaders • Introduce shading languages • Needed to describe shaders • RenderMan 2
- 3. Introduction • Recent major advance in real time graphics is programmable pipeline • First introduced by NVIDIA GForce 3 • Supported by high-end commodity cards • NVIDIA, ATI, 3D Labs • Software Support • Direct X 8 , 9, 10 • OpenGL Extensions • OpenGL Shading Language (GLSL) • Cg 3
- 4. Background • Two components • Vertex programs (shaders) • Fragment programs (shaders) • Requires detailed understanding of two seemingly contradictory apporachs • OpenGL pipeline • Real time • RenderMan ideas • offline 4
- 5. Black Box View 5 Geometry Processor Frame Buffer Fragment Processor CPU vertices vertices fragments Rasterizer fragments
- 6. Geometric Calculations • Geometric data: set of vertices + type • Can come from program, evaluator, display list • type: point, line, polygon • Vertex data can be • (x,y,z,w) coordinates of a vertex (glVertex) • Normal vector • Texture Coordinates • RGBA color • Other data: color indices, edge flags • Additional user-defined data in GLSL 6
- 7. Per-Vertex Operations • Vertex locations are transformed by the model-view matrix into eye coordinates • Normals must be transformed with the inverse transpose of the model-view matrix so that v·n=v’ ·n’ in both spaces • Assumes there is no scaling • May have to use autonormalization • Textures coordinates are generated if autotexture enabled and the texture matrix is applied 7
- 8. Lighting Calculations • Consider a per-vertex basis Phong model • Phong model requires computation of r and v at every vertex 8 I =kd Id l · n + ks Is (v · r )α + ka Ia
- 9. Calculating the Reflection Term angle of incidence = angle of reflection cos θi = cos θr or r·n = l·n r, n, and l are coplanar r = αl + βn normalize 1 = r·r = n·n = l·l solving: r = 2(l · n)n-l 9
- 10. OpenGL Lighting • Modified Phong model • Halfway vector • Global ambient term • Specified in standard • Supported by hardware 10
- 11. Halfway Vector Blinn proposed replacing v·r by n·h where h = (l+v)/|l + v| (l+v)/2 is halfway between l and v If n, l, and v are coplanar: ψ = φ/2 Must then adjust exponent so that (n·h)e’ ≈ (r.v)e 11
- 12. Primitive Assembly •Vertices are next assembled into objects • Polygons • Line Segements • Points •Transformation by projection matrix •Clipping • Against user defined planes • View volume, x=±w, y=±w, z=±w • Polygon clipping can create new vertices •Perspective Division •Viewport mapping 12
- 13. Rasterization • Geometric entities are rasterized into fragments • Each fragment corresponds to a point on an integer grid: a displayed pixel • Hence each fragment is a potential pixel • Each fragment has • A color • Possibly a depth value • Texture coordinates 13
- 14. Fragment Operations • Texture generation • Fog • Antialiasing • Scissoring • Alpha test • Blending • Dithering • Logical Operation • Masking 14
- 15. Vertex Processor • Takes in vertices • Position attribute • Possibly color • OpenGL state • Produces • Position in clip coordinates • Vertex color 15
- 16. Fragment Processor • Takes in output of rasterizer (fragments) • Vertex values have been interpolated over primitive by rasterizer • Outputs a fragment • Color • Texture • Fragments still go through fragment tests • Hidden-surface removal • alpha 16
- 17. Programmable Shaders • Replace fixed function vertex and fragment processing by programmable processors called shaders • Can replace either or both • If we use a programmable shader we must do all required functions of the fixed function processor 17
- 18. Development • RenderMan Shading Language • Offline rendering • Hardware Shading Languages • UNC, Stanford • NVIDIA • OpenGL Vertex Program Extension • OpenGL Shading Language • Cg • OpenGL • Microsoft HLSL 18
- 19. RenderMan • Developed by Pixar • S. Upstill, The RenderMan Companion, Addison-Wesley, 1989. • Model 19 Modeler Renderer interface file (RIB)
- 20. Modeling vs Rendering • Modeler outputs geometric model plus information for the renderer • Specifications of camera • Materials • Lights • May have different kinds of renderers • Ray tracer • Radiosity • How do we specify a shader? 20
- 21. Shading Trees •Shaders such as the Phong model can be written as algebraic expressions •But expressions can be described by trees •Need now operators such as dot and cross products and new data types such as matrices and vectors •Environmental variables are part of state 21 I =kd Id l · n + ks Is (v · r )s + ka Ia
- 23. Phong Model 23