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perf(vr): SSS resolution-independent sample count #2219

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Merged
alandtse merged 1 commit into from
Apr 28, 2026
Merged

perf(vr): SSS resolution-independent sample count #2219

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5 changes: 5 additions & 0 deletions features/Screen-Space Shadows/Shaders/ScreenSpaceShadows/RaymarchCS.hlsl
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Original file line number Diff line number Diff line change
Expand Up @@ -50,7 +50,12 @@ cbuffer PerFrame : register(b1)

parameters.DynamicRes = DynamicRes;

#if defined(VR)
// Disabled in VR: depth bias causes subtle shadow shifting at stereo seams on camera motion.
parameters.UsePrecisionOffset = false;
#else
parameters.UsePrecisionOffset = true;
#endif

WriteScreenSpaceShadow(parameters, groupID, groupThreadID);
}
27 changes: 16 additions & 11 deletions features/Screen-Space Shadows/Shaders/ScreenSpaceShadows/bend_sss_gpu.hlsli
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Expand Up @@ -226,12 +226,6 @@ void WriteScreenSpaceShadow(DispatchParameters inParameters, int3 inGroupID, int
// Interpolation should only occur on the minor axis of the ray - major axis coordinates should be at pixel centers
half2 read_xy = floor(pixel_xy);

read_xy *= inParameters.DynamicRes;

#if defined(VR)
read_xy *= half2(0.5, 1.0);
#endif

half minor_axis = x_axis_major ? pixel_xy.y : pixel_xy.x;

// If a pixel has been detected as an edge, then optionally (inParameters.IgnoreEdgePixels) don't include it in the shadow
Expand All @@ -249,23 +243,33 @@ void WriteScreenSpaceShadow(DispatchParameters inParameters, int3 inGroupID, int

// HLSL enforces that a pixel offset is a compile-time constant, which isn't strictly required (and can sometimes be a bit faster)
// So this fallback will use a manual uv offset instead
half2 coord = read_xy * inParameters.InvDepthTextureSize;
half2 coord_with_offset = (read_xy + offset_xy) * inParameters.InvDepthTextureSize;
// Apply DynamicRes after offset_xy addition so the bilinear neighbour samples exactly 1 texel away.
half2 coord = read_xy * inParameters.InvDepthTextureSize * inParameters.DynamicRes;
half2 coord_with_offset = (read_xy + offset_xy) * inParameters.InvDepthTextureSize * inParameters.DynamicRes;

#if defined(VR)
// VR side-by-side: halve x to map stereo pixel coords to texture UV.
coord *= half2(0.5, 1.0);
coord_with_offset *= half2(0.5, 1.0);

# if defined(RIGHT)
// Right eye: valid UV range is [0.5, 1.0]
// Right eye: valid UV range is [0.5*DynRes.x, DynRes.x]
bool coord_out_of_eye = coord.x < 0.5 * inParameters.DynamicRes.x;
bool coord_offset_out_of_eye = coord_with_offset.x < 0.5 * inParameters.DynamicRes.x;
# else
// Left eye: valid UV range is [0.0, 0.5)
// Left eye: valid UV range is [0.0, 0.5*DynRes.x)
bool coord_out_of_eye = coord.x >= 0.5 * inParameters.DynamicRes.x;
bool coord_offset_out_of_eye = coord_with_offset.x >= 0.5 * inParameters.DynamicRes.x;
# endif

// Clamp cross-eye depth reads to FarDepthValue (1.0) so rays near the SBS center
// seam see no occluder at the boundary. Shadow weakens by ~1 pixel at the seam but
// stays temporally stable across camera movement.
depths.x = coord_out_of_eye ? 1.0 : inParameters.DepthTexture.SampleLevel(inParameters.PointBorderSampler, coord, 0);
depths.y = coord_offset_out_of_eye ? 1.0 : inParameters.DepthTexture.SampleLevel(inParameters.PointBorderSampler, coord_with_offset, 0);

// HMD mask: depth==0 is outside the visible lens area. Remap to FarDepthValue so
// mask pixels do not cast false shadows.
depths.x = lerp(depths.x, 1.0, (float)(depths.x == 0)); // Stencil area
depths.y = lerp(depths.y, 1.0, (float)(depths.y == 0)); // Stencil area
#else
Expand All @@ -274,7 +278,8 @@ void WriteScreenSpaceShadow(DispatchParameters inParameters, int3 inGroupID, int
#endif

// Depth thresholds (bilinear/shadow thickness) are based on a fractional ratio of the difference between sampled depth and the far clip depth
depth_thickness_scale[i] = abs(inParameters.FarDepthValue - depths.x);
static const half kDepthThicknessFloor = 1e-4h; // Prevents division by zero in depth_scale when depth is at the far clip plane
depth_thickness_scale[i] = max(abs(inParameters.FarDepthValue - depths.x), kDepthThicknessFloor);

// If depth variance is more than a specific threshold, then just use point filtering
bool use_point_filter = abs(depths.x - depths.y) > depth_thickness_scale[i] * inParameters.BilinearThreshold;
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11 changes: 7 additions & 4 deletions src/Features/ScreenSpaceShadows.cpp
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Expand Up @@ -79,11 +79,14 @@ void ScreenSpaceShadows::ClearShaderCache()

uint ScreenSpaceShadows::GetScaledSampleCount()
{
float2 renderSize = Util::ConvertToDynamic(globals::state->screenSize);
if (globals::game::isVR) {
// In VR, SAMPLE_COUNT is a pixel-space ray length that is FOV-driven, not resolution-driven.
// Resolution-scaling produced 2-8x excess samples at VR resolutions with no quality benefit.
// WAVE_SIZE (64) alignment is required for correct Bend READ_COUNT computation.
return bendSettings.SampleCount * 64;
}

// In VR, renderSize covers both eyes side-by-side; raymarch dispatches per-eye
if (globals::game::isVR)
renderSize.x /= 2.0f;
float2 renderSize = Util::ConvertToDynamic(globals::state->screenSize);

// Scale sample count based on both dimensions relative to 1920x1080 reference
float2 referenceRes = { 1920.0f, 1080.0f };
Expand Down
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