DM2336 Programming hardware shaders
Dioselin Gonzalez - 2007 S2
Week 2: Review of
graphics API
Programmable pipeline
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
OpenGL block diagram
Generation
Traversal
Xformation
Rasteriz. & tex.
Display
Display
list
Evaluator
Application
generation
Application
traversal
Per-vertex
operations
& Primitive
assembly
Rasterization
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
Application &
higher level
graphics libraries
OpenGL implementation
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Per-vertex operations
•
•
•
•
Transform vertex coords. (modelview m.)
Transform and renormalize normals
Generate and transform texture coords.
Lighting calculations
Display
list
Per-vertex
operations
Evaluator
Application
generation
Application
traversal
Rasterization
& Primitive
assembly
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Primitive assembly
•
•
•
•
•
Flat shading
Clipping
Projection transformations
Viewport and depth-range operations
Culling
Display
list
Per-vertex
operations
Evaluator
Application
generation
Application
traversal
Rasterization
& Primitive
assembly
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Rasterization
• Convert each primitive into fragments
• Fragment: “transient data structures”
– position (x,y); depth; color; texture
coordinates; coverage; …
Display
list
Per-vertex
operations
Rasterization
Evaluator
Application
generation
Application
traversal
& Primitive
assembly
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Interpolation issues
http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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
p1
P1’
p
p
P2’
L
p2
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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
s/w, t/w
“Denominator” 1/w
– “Numerators”
–
http://www.cse.ohio-state.edu/~hwshen/781/pipeline.ppt
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Perspective-Correct Interpolation
• That fixed it!
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Back to the pipeline
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Rasterization
• Convert each primitive into fragments
• Fragment: “transient data structures”
– position (x,y); depth; color; texture
coordinates; coverage; …
Display
list
Per-vertex
operations
Rasterization
Evaluator
Application
generation
Application
traversal
& Primitive
assembly
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Per fragment operations (I)
• Generate texel for every fragment
• Fog calculations
• Coverage (antialiasing) values application
Display
list
Per-vertex
operations
Evaluator
Application
generation
Application
traversal
Rasterization
& Primitive
assembly
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Per fragment operations (II)
•
•
•
•
Scissoring
Alpha test
Stencil test
Depth-buffer test
• Blending
• Dithering and
logical operation
• Masking
Display
list
Per-vertex
operations
Evaluator
Application
generation
Application
traversal
Rasterization
& Primitive
assembly
Perfragment
operations
Frame
buffer
Video
display
Texture
memory
Pixel
operations
15
DM2336 PHS – Dioselin Gonzalez – 2007 S2
Hello world
Pervertex
Operations
Perfragment
Operations
(I)
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Hello world
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
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
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DM2336 PHS – Dioselin Gonzalez – 2007 S2
Playing with GLSL Demo
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