Transcription
Advanced Shading I: ShadowRasterization Techniques
Shadow Terminologyumbra: light totally blockedpenumbra: light partially blockedoccluder: object blocking light
Shadow Terminologyumbra: light totally blockedpenumbra: light partially blockedoccluder: object blocking lightpoint lights haveno penumbra
Shadow RenderingHard shadows: umbra only easy with ray tracing (shadow rays)Soft shadows: penumbra and umbra very difficult without global illuminationToday: rasterizing hard shadows
Projection ShadowsEasiest case: one object one light shadow cast on flat ground
Projection ShadowsMain idea: render scene twice once as usual once with object shaded black andflattened onto plane
Projection Shadowsprojection of v lies along ray
Projection Shadowsprojection of v lies along ray
Projection Shadowsprojection of v lies along ray
Projection Shadowsprojection of v lies along rayhow to write as affinetransformation?
Projection OperationNeeded:
Projection OperationNeeded:
Projection Shadowsprojection of v lies along rayhow to write as affinetransformation?
Projection Shadowsprojection of v lies along ray
Projection ShadowsPros: easy to codeCons:
Projection ShadowsPros: easy to code, fastCons: only draws shadows on flat surfaces no soft shadows no self-shadows
Projection ShadowsMultiple lights? fake with alpha blendingFinal gotcha:
Shadow MapsRender scene from the light record depth onlywhy useful?
Shadow MapsTo shade a pixel, calculate: 1) pixel tolight distance; 2) shadow map value
Shadow Mapscompare pixel-to-lightdistance to shadow mapif equal, no shadowif greater, in shadow
What’s The Bug? Part I
What’s The Bug? Part IFloating point is very dangerous numerical floating-point error depth buffer has finite precision
What’s The Bug? Part IFloating point is very dangerous numerical floating-point error depth buffer has finite precisionFix by including tolerance (“bias”) in check
What’s The Bug? Part II
What’s The Bug? Part IIShadow map is too coarse shadows are aliased
What’s The Bug? Part IIShadow map is too coarse shadows are aliasedCascaded shadow maps: use higherresolution shadow map near eye
What’s The Bug? Part IIShadow map is too coarse shadows are aliasedCascaded shadow maps: use higherresolution shadow map near eye highest-quality fix complicated (have to partition shadow map)
Percentage Closer FilteringSample several nearby pixels on shadowmap instead of only one pixelSet shadow intensity proportional tonumber of “shadow” votes
Shadow MapsVery common real-time technique used in Unreal Engine, etcPros: works with curved objects works with self-shadowsCons:
Shadow MapsVery common real-time technique used in Unreal Engine, etcPros: works with curved objects works with self-shadowsCons: very prone to aliasing artifacts still no soft shadows
Shadow VolumesConsider single light, single triangle
Shadow VolumesMain idea: build 3D object representing theshadow volumeIf pixel is inside volume, it is in shadow
Shadow VolumesMain idea: build 3D object representing theshadow volumeIf pixel is inside volume, it is in shadow shoot ray from pixel to eye, countnumber of intersections odd in shadow
Shadow VolumesFor more complicated object, look only atsilhouette edges edges connectingfront-facing toback-facingtriangles
Shadow VolumesIncreasingly popular in real-time graphics
Shadow VolumesIncreasingly popular in real-time graphicsPros: high-quality shadows (no aliasing) self-shadows automatically includedCons:
Shadow VolumesIncreasingly popular in real-time graphicsPros: high-quality shadows (no aliasing) self-shadows automatically includedCons: slow and complicated shadows chunky if meshes too coarse still no soft shadows
Shadow map is too coarse shadows are aliased Cascaded shadow maps: use higher- . front-facing to back-facing triangles. Shadow Volumes Increasingly popular in real-time graphics. Shadow Vol
