Cubemap Filtering with CubeMapGen. Thorsten Scheuermann John Isidoro 3D Application Research Group

Transcription

1 Cubemap Filtering with CubeMapGen Thorsten Scheuermann John Isidoro 3D Application Research Group

2 Outline >CubeMapGen tool >Angular extent filtering >Edge fixup methods >Pre-filtered per-pixel glossy reflections >Examples 2

3 Cubemap Uses >Cube mapping has become ubiquitous >Good for any directional lookup table > Diffuse > Specular (a.k.a. environment map) > Glossy Specular >Dynamic cubemaps 3

4 Cubemap Filtering Issues > Cube map is defined over spherical domain > Standard mipmapping tools process each face independently > No filtering across faces > Fail to take the solid angle of cube map texels into account > Can introduce artifacts and discontinuities at face seams > Graphics hardware generally does not support Bi/Trilinear cubemap filtering across cube faces > Lots of game developers complained about the seam issues Example of seam artifacts on lower miplevels 4

5 CubeMapGen Tool > CubeMapGen is a tool for cubemap filtering and mip-chain generation > To be released as standalone tool, command-line tool, HDRShop plugin, library, source code > Available on 5

6 CubeMapGen Overview >CubeMapGen addresses the two main filtering issues that remain troublesome for cube map users > Missing bi/trilinear filtering across cube faces in HW > Mipmap generation for each cubemap face in isolation >CubeMapGen helps to addresses these issues with a couple of techniques > Angular extent filtering > Edge averaging/fixup >Pure software solution compatible with all hardware > This is just a cubemap preprocessing step 6

7 CubeMapGen Feature List (1) >Performs all filtering operations in HDR space (32-bit float) >Angular extent filtering allows for filtering across faces > Disc, Cone, Angular Gaussian, and Cosine filtering modes > All texels within angular extent are used when computing average > Optional weighting of texels by their solid angles >Edge and Corner averaging to obscure cube edge seams > Adjustable edge fixup band to help obscure effect of edge averaging 7

8 CubeMapGen Feature List (2) >Adjustable pre-filtering of base miplevel (0-180 degree kernel) > E.g. for blurred reflections >Imports and exports 8/16 bit uint and 16/32bit float pixel formats > Supports.hdr.pfm.bmp.jpg.png.dds.dib > Adjustable Degamma/Gamma and Output Intensity Scaling >Command Line Interface for use in scripting and as a preprocess tool 8

9 CubeMapGen Demo 9

10 Angular Extent Filtering Angular extent Angular extent θ Center tap vector θ Center tap vector > Angular extent filtering processes all taps within a given angle of the 3D center tap vector > Filter extent is not texel size dependant > Instead, filter extent has constant solid angle > e.g. Filter may (and should) encompass a different number of texels for different centertap vectors > Allows for filter kernels that span across cube face edges 10

11 Angular Filter Types Disc Filter Cone Filter Angular Gaussian Cosine Filter 11

12 Disc Filter > Equal weights for all taps within a specified angle of the center tap 12

13 Cone Filter > Linear falloff based on angle from center tap 13

14 Angular Gaussian Filter > Gaussian falloff based on angle from center tap > 3 standard deviations within specified extent angle 14

15 Cosine Filter > Falloff based on cosine of angle from center tap > Most useful to compute diffuse lighting cubemaps > Incorporates the hemisphere cosine term > Use a filter angle of

16 Implementation Details >Filtering performed on CPU >For each output texel: > Determine bounding box regions for angular extent in each face on input cubemap > Process all texels within each bounding box region > Test against threshold determined by extent angle > Weight using solid angle and weights of chosen filter function > Accumulate sum of weighted texels, and sum of weights >Every texel within angular extent is processed > Important for HDR imagery: Single very bright texels can significantly influence the filtered result 16

17 Edge Averaging and Fixup >Idea: make texels on each side of a cubemap edge match to hide artifacts due to hardware not filtering across edges >Apply fixup in region around edge > Fade out using linear or hermite (smoothstep) falloff >Two fixup modes: > Pull: Add weighted edge texel adjustment amount to texels in fixup band > Average: Lerp between original and average edge texel color based on distance from edge >Special support for DXTC compression > Accounts for neighboring block across faces 17

18 Edge Fixup Example (Pull) Intensity Value Cube Face Edge Cube Face Edge.50 Texel Intensity Values Fraction of averaging amount for 4 texel fixup.50 band.25 Fixup region Fixup region 18

19 2x2 Miplevel (Standard) > 2x2 mip-level without edge fixup and no filtering across faces > Strong edge artifacts. 2x2 miplevel is unusable by itself. 19

20 2x2 Miplevel (CubeMapGen) > CubeMapGen s edge fixup ability allows for the 2x2 mip level to be used as a diffuse environment lighting term 20

21 4x4 Miplevel (Standard) > Edge filtering artifacts make the 4x4 miplevel from standard mip filtering algorithms only useful for traditional mipmapping purposes 21

22 4x4 Miplevel (CubeMapGen) > CubeMapGen generates cubemaps with a 4x4 miplevel that can also be used for rough metal shaders 22

23 Other Miplevels (CubeMapGen) 4x4 8x8 16x16 32x32 23

24 Per-pixel Roughness > Different mip levels can be used for shiny surfaces with different surface roughness > Rougher surface more blurry reflection > Use MaxMipLevel texture sampler state to clamp miplevel, not texcubebias( ) > Per-pixel miplevel clamping: > Pack miplevel index into alpha channel of each cubemap mipmap > Use it to determine bias amount for texcubebias( ) and simulate miplevel clamping in shader > Or use texcubelod( ) on PS 3.0 > Example 24

25 Per-pixel Gloss Shader Example float fnummiplevels; // number of mip-levels in cubemap float fblurscale = 4.0; // scale factor for blurriness float4 main(float2 inuv : TEXCOORD0, float3 innormal : TEXCOORD1, float3 inview : TEXCOORD2) : COLOR0 { // Surface roughness stored in alpha of base map float4 cbase = tex2d(tbase, inuv); float froughness = cbase.a; float3 R = reflect (normalize(inview), normalize(innormal)); // Each cubemap stores miplevel index in alpha (scaled by 16/255). // Determine mip-lod levels from 0 to fnummiplevels (minification) float fmiplevelminification = (255.0/16.0) * texcube(tcube, R).a; // Determine mip-lod levels from -fnummiplevels to 0 (magnification) float fmiplevelmagnification = (255.0/16.0) * texcubebias(tcube, float4(r, fnummiplevels-1.0)).a; [ ] 25

26 Per-pixel Gloss Shader Example [ ] float fmiplevel = 0; //choose between magnification and minification range if(fmiplevelminification == 0) { // 0 is the largest (base) miplevel // the whole cubemap is being magnified, compute "logical" miplevel // (which is negative) fmiplevel = fmiplevelmagnification - (fnummiplevels - 1.0); } else { // the cubemap is being minified fmiplevel = fmiplevelminification; } // compute final mip bias to clamp miplevel float fmipbias = max(fglossscale * froughness - fmiplevel, 0.0); float4 crefl = texcubebias(tcube, float4(r, fmipbias)); } return cbase * crefl; 26

27 Acknowledgements >CubeMapGen was thought up and developed by John Isidoro > I m merely presenting his work here today >Lightprobe examples courtesy of Paul Debevec: 27