CS 354 Blending, Compositing, Anti-aliasing Mark Kilgard University of Texas February 14, 2012

A Simplified Graphics Pipeline Application Vertex batching & assembly Triangle assembly Triangle clipping Triangle rasterization Fragment shading Depth testing Color update/blending Application- OpenGL API boundary Framebuffer NDC to window space Depth buffer Re-examine framebuffer color update…

A few more steps expanded Application Vertex batching & assembly Lighting View frustum clipping Triangle rasterization Fragment shading Depth testing Color update/blending Application- OpenGL API boundary Framebuffer NDC to window space Depth buffer Vertex transformation User defined clipping Back face culling Perspective divide Triangle assembly Texture coordinate generation

Blending Enabled vs. Disabled pixel color fragment color blend operation pixel color fragment color glDisable ( GL_BLEND ) glEnable ( GL_BLEND )

Why blending? compositing window systems volumetric effects; explosions medical imaging compositing complex art work

Conventional Blend Operation source color destination factor destination color source factor × × + clamp [0,1] pixel color fragment color

Conventional Blend Operation source color destination factor destination color source factor × × + clamp [0,1] pixel color fragment color × × × + + + × × × clamp [0,1] clamp [0,1] clamp [0,1] modulate, add, and clamp operations are vector on RGBA components

Conventional Blend Operation source color destination factor destination color source factor × × + clamp [0,1] pixel color fragment color glBlendFunc ( srcFunc , dstFunc )

Blend Function Parameters where s = min(A s ,1-A d ) (s,s,s,s) GL_SRC_ALPHA_SATURATE (1-A c ,1-A c ,1-A c ,1-A c ) GL_ONE_MINUS_CONSTANT_ALPHA (A c ,A c ,A c ,A c ) GL_CONSTANT_ALPHA (1-R c ,1-G c ,1-B c ,1-A c ) GL_ONE_MINUS_CONSTANT_COLOR (R c ,G c ,B c ,A c ) GL_CONSTANT_COLOR (1-A d ,1-A d ,1-A d ,1-A d ) GL_ONE_MINUS_DST_ALPHA (A d ,A d ,A d ,A d ) GL_DST_ALPHA (1-A s ,1-A s ,1-A s ,1-A s ) GL_ONE_MINUS_SRC_ALPHA (A s ,A s ,A s ,A s ) GL_SRC_ALPHA (1-R d ,1-G d ,1-B d ,1-A d ) GL_ONE_MINUS_DST_COLOR (R d ,G d ,B d ,A d ) GL_DST_COLOR (1-R s ,1-G s ,1-B s ,1-A s ) GL_ONE_MINUS_SRC_COLOR (R s ,G s ,B s ,A s ) GL_SRC_COLOR (1,1,1,1) GL_ONE (0,0,0,0) GL_ZERO ( f r , f g , f b , f a ) Parameter

Blend Color for Factors source color destination factor destination color source factor × × + clamp [0,1] pixel color fragment color glBlendColor (r,g,b,a) blend color

Min/Max Blend Operation source color destination color min or max clamp [0,1] pixel color fragment color glBlendEquation ( GL_MIN ) glBlendEquation ( GL_MAX )

Properly Ordered Compositing vs. Incorrectly Ordered

Properly Ordered Compositing vs. Incorrectly Ordered

Porter-Duff Composite Operators

Porter & Duff Modes Porter & Duff blend modes 1 1 0 0 1 1 0 1 1 0 1 0 Y 0 0 0 Clear 1 0 0 Xor 0 1 Bc Dst-atop 1 1 Ac Src-atop 1 0 0 Dst-out 0 0 0 Src-out 0 0 Bc Dst-In 0 1 Ac Src-In 1 1 Bc Dst-Over 1 1 Ac Src-Over 1 1 Bc Dst 0 1 Ac Src Z X f(Ac,Bc) Operation

Porter & Duff Modes Expanded Uncorrelated blend mode expansion of Porter & Duff blend modes 1 0 1 1 0 0 0 1 1 1 0 0 Z 1 1 0 0 1 1 0 1 1 0 1 0 Y 0 0 0 Clear Aca×(1-Ba)+(1-Aa)×Bca 0 0 Xor (1-Ba)×Aca+Aa×Bca 1 Bc Dst-atop Aca×Ba+(1-Aa)×Bca 1 Ac Src-atop (1-Aa)×Bca 0 0 Dst-out (1-Ba)×Aca 0 0 Src-out Bca×Aa 0 Bc Dst-In Aca×Ba 1 Ac Src-In Bca+(1-Ba)×Aca 1 Bc Dst-Over Aca+(1-Aa)×Bca 1 Ac Src-Over Bca 1 Bc Dst Aca 1 Ac Src Blend mode X f(Ac,Bc) Operation

Porter & Duff for glBlendFunc GL_ONE_MINS_DST_ALPHA GL_ONE_MINUS_DST_ALPHA GL_DST_ALPHA GL_ZERO GL_ONE_MINUS_DST_ALPHA GL_ZERO GL_DST_ALPHA GL_ONE_MINUS_DST_ALPHA GL_ONE GL_ZERO GL_ONE GL_ZERO srcFactor Aca×(1-Ba)+(1-Aa)×Bca (1-Ba)×Aca+Aa×Bca Aca×Ba+(1-Aa)×Bca (1-Aa)×Bca (1-Ba)×Aca Bca×Aa Aca×Ba Bca+(1-Ba)×Aca Aca+(1-Aa)×Bca Bca Aca 0 Blend mode GL_ONE_MINUS_SRC_ALPHA GL_DST_ALPHA GL_ONE_MINUS_SRC_ALPHA GL_ONE_MINUS_SRC_ALPHA GL_ZERO GL_SRC_ALPHA GL_ZERO GL_ONE GL_ONE_MINUS_SRC_ALPHA GL_ONE GL_ZERO GL_ZERO dstFactor Clear Xor Dst-atop Src-atop Dst-out Src-out Dst-In Src-In Dst-Over Src-Over Dst Src Operation

Aliased Jagged artifacts

Multi-sample 8x Smoother appearance

Multi-sample Coverage Positions 4x jittered 1x (aliased) 8x jittered 4x orthogonal