Realishtime RadiosityAnd Next Shooter Selection

Jeff PoolCOMP 870 Final

December 4, 2008UNC Dept. of Computer Science

Approaches to Radiosity

• CPU matrix solving

• GPU gathering• GPU scattering• GPU scatter/gathering

• Hemicubes• Hemispheres

I tried:

• GPU gathering• Hemicubes – 5 renders per subelement– No form factor calculations, though

• FBOs• Hemispheres• Scatter/Gather

Approach 1 - Hemicubes

• Gathering– 50x50 elements

• Hemicubes– 5 renders per element

• CPU processing– Bandwidth issues

~24 hours

Approach 2 – Hemicubes w/ FBO

• Gathering• Hemicubes• FBO– No sending data

back and forth

~5 minutes

Approach 3 – Coombe, et al.

• Scatter/Gather• Hemispheres• FBO

~1/2 second

Key Points

• ID Buffer• Hemispheres• Progressive refinement– Next shooter selection– (fp MipMap generation)

• Texture packing for residual/full textures– (HW now has MRT)

ID Buffer

• Elements rendered with ID colors for visibility (hard part of form factors)

• Correlated to hemisphere rendered by shooter

Hemispheres

• Only one pass, not 5• Can be some distortion,

so target framebuffer must be large for accurate visibility

ID texture size

Shooting Pass

• Scene rendered into hemisphere by shooter• Each element is rendered as a quad• Per fragment:– Form factor computation– Visibility = Idcolor==hemisphere(projectedPos) – Energy = FF * Reflectance * Visibility

Updating the elements

• Easy: Blend 1 + 1 (src + dst) = additive rendering

• Render to multiple targets – residual AND full• Shooter’s residual framebuffer is simply

cleared

Next Shooter Selection

• Trivial to select element with most energy left– Multiply lowest mipmap level by area

• HOWEVER, two things to consider– Visual impact of color bleeding– Frame-frame coherence

Problem #1

Importance of color over brightness6 shots over 64x64 (~0.3 seconds)

Problem #2

Note the flickering as the light source descends, which can be pretty distracting

Problem #2 (2)

Different Preference Measures

• Sum of components• Max of components• Variance (+mean)• Luminance

SO:

• Radiosity at realtime framerates (1-10 fps)

• Quality tradeoffs for speed:– Size of hemisphere

texture– Subdivision size– Aggregate shooting

• Preferential next shooter selection– Brightness– Color– Coherence