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// Given a cube map (passed as a 2D array), builds CDFs of two distributions:
// 1. 1D texture with marginal densities, telling us the likelihood of selecting a particular row,
// 2. 2D texture with conditional densities, which correspond to the PDF of the texel given its row.
// Ref: PBRT v3, 13.6.7 "Piecewise-Constant 2D Distributions".
// Note that we use the equiareal sphere-to-square mapping instead of the latitude-longitude one.
#include "ShaderLibrary/Common.hlsl"
#include "ShaderLibrary/ImageBasedLighting.hlsl"
#include "../../ShaderVariables.hlsl"
/* --- Input --- */
#define TEXTURE_HEIGHT 256 // Equiareal texture map: cos(theta) = 1.0 - 2.0 * v
#define TEXTURE_WIDTH 2 * TEXTURE_HEIGHT // Equiareal texture map: phi = TWO_PI * (1.0 - u)
TEXTURECUBE(envMap); // Input cubemap
/* --- Output --- */
RWTexture2D<float> marginalRowDensities; // [(TEXTURE_HEIGHT + 1) x 1] (+ 1 for the image integral)
RWTexture2D<float> conditionalDensities; // [TEXTURE_WIDTH x TEXTURE_HEIGHT]
/* --- Implementation --- */
// Creates an access pattern which avoids shared memory bank conflicts.
#define NUM_BANKS 32
#define SHARED_MEM(x) ((x) + (x) / NUM_BANKS)
// Performs a block-level parallel scan.
// Ref: GPU Gems 3, Chapter 39: "Parallel Prefix Sum (Scan) with CUDA".
#define PARALLEL_SCAN(i, n, temp, sum) \
{ \
uint offset; \
\
/* Execute the up-sweep phase. */ \
for (offset = 1; offset <= n / 2; offset *= 2) \
{ \
GroupMemoryBarrierWithGroupSync(); \
\
/*** a1 = (2 * i + 1) * offset - 1 */ \
uint a1 = Mad24(Mad24(2, i, 1), offset, -1); \
uint a2 = a1 + offset; \
\
if (a2 < n) \
{ \
temp[SHARED_MEM(a2)] += temp[SHARED_MEM(a1)]; \
} \
} \
\
GroupMemoryBarrierWithGroupSync(); \
\
/* Prevent NaNs arising from the division of 0 by 0. */ \
sum = max(temp[SHARED_MEM(n - 1)], FLT_EPS); \
\
GroupMemoryBarrierWithGroupSync(); \
\
/* The exclusive scan requires the last element to be 0. */ \
if (i == 0) \
{ \
temp[SHARED_MEM(n - 1)] = 0.0; \
} \
\
/* Execute the down-sweep phase. */ \
for (offset = n / 2; offset > 0; offset /= 2) \
{ \
GroupMemoryBarrierWithGroupSync(); \
\
/*** a1 = (2 * i + 1) * offset - 1 */ \
uint a1 = Mad24(Mad24(2, i, 1), offset, -1); \
uint a2 = a1 + offset; \
\
if (a2 < n) \
{ \
float t1 = temp[SHARED_MEM(a1)]; \
temp[SHARED_MEM(a1)] = temp[SHARED_MEM(a2)]; \
temp[SHARED_MEM(a2)] += t1; \
} \
} \
\
GroupMemoryBarrierWithGroupSync(); \
}
#pragma kernel ComputeConditionalDensities
groupshared float rowVals[SHARED_MEM(TEXTURE_WIDTH)];
[numthreads(TEXTURE_WIDTH / 2, 1, 1)]
void ComputeConditionalDensities(uint3 groupId : SV_GroupID,
uint3 groupThreadId : SV_GroupThreadID)
{
// There are TEXTURE_HEIGHT thread groups processing 2 texels per thread.
const uint n = TEXTURE_WIDTH;
const uint i = groupThreadId.x;
const uint j = groupId.x;
const uint i1 = i;
const uint i2 = i + n / 2;
float w = TEXTURE_WIDTH;
float h = TEXTURE_HEIGHT;
float u1 = i1 / w + 0.5 / w;
float u2 = i2 / w + 0.5 / w;
float v = j / h + 0.5 / h;
float3 L1 = ConvertEquiarealToCubemap(u1, v);
float3 L2 = ConvertEquiarealToCubemap(u2, v);
float3 c1 = SAMPLE_TEXTURECUBE_LOD(envMap, s_linear_clamp_sampler, L1, 0).rgb;
float3 c2 = SAMPLE_TEXTURECUBE_LOD(envMap, s_linear_clamp_sampler, L2, 0).rgb;
// Compute the integral of the step function (row values).
// TODO: process 4 texels per thread, and manually unroll.
rowVals[SHARED_MEM(i1)] = c1.r + c1.g + c1.b;
rowVals[SHARED_MEM(i2)] = c2.r + c2.g + c2.b;
float rowValSum;
PARALLEL_SCAN(i, n, rowVals, rowValSum)
// Compute the CDF. Note: the value at (i = n) is implicitly 1.
conditionalDensities[uint2(i1, j)] = rowVals[SHARED_MEM(i1)] / rowValSum;
conditionalDensities[uint2(i2, j)] = rowVals[SHARED_MEM(i2)] / rowValSum;
if (i == 0)
{
float rowIntegralValue = rowValSum / n;
marginalRowDensities[uint2(j, 0)] = rowIntegralValue;
}
}
#pragma kernel ComputeMarginalRowDensities
groupshared float rowInts[SHARED_MEM(TEXTURE_HEIGHT)];
[numthreads(TEXTURE_HEIGHT / 2, 1, 1)]
void ComputeMarginalRowDensities(uint3 groupThreadId : SV_GroupThreadID)
{
// There is only one thread group processing 2 texels per thread.
const uint n = TEXTURE_HEIGHT;
const uint i = groupThreadId.x;
const uint i1 = i;
const uint i2 = i + n / 2;
// Compute the integral of the step function (row integrals).
// TODO: process 4 texels per thread, and manually unroll.
rowInts[SHARED_MEM(i1)] = marginalRowDensities[uint2(i1, 0)];
rowInts[SHARED_MEM(i2)] = marginalRowDensities[uint2(i2, 0)];
float rowIntSum;
PARALLEL_SCAN(i, n, rowInts, rowIntSum)
// Compute the CDF. Note: the value at (i = n) is implicitly 1.
marginalRowDensities[uint2(i1, 0)] = rowInts[SHARED_MEM(i1)] / rowIntSum;
marginalRowDensities[uint2(i2, 0)] = rowInts[SHARED_MEM(i2)] / rowIntSum;
if (i == 0)
{
float imgIntegralValue = rowIntSum / n;
marginalRowDensities[uint2(n, 0)] = imgIntegralValue;
}
}