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perf: reduce register pressure in compute GSplat TileCount projection #8563

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Merged
mvaligursky merged 1 commit into from
Mar 31, 2026
Merged

perf: reduce register pressure in compute GSplat TileCount projection #8563

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55 changes: 26 additions & 29 deletions src/scene/shader-lib/wgsl/chunks/gsplat/compute-gsplat-common.js
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Original file line number Diff line number Diff line change
Expand Up @@ -23,8 +23,7 @@ struct SplatCov2D {
a: f32,
b: f32,
c: f32,
radius: vec2f,
radiusFactor: f32,
viewDepth: f32,
valid: bool,
}

Expand Down Expand Up @@ -68,37 +67,36 @@ fn computeSplatCov(
s.z * vec3f(rot[2])
));

let covA = vec3f(dot(M[0], M[0]), dot(M[0], M[1]), dot(M[0], M[2]));
let covB = vec3f(dot(M[1], M[1]), dot(M[1], M[2]), dot(M[2], M[2]));

let Vrk = mat3x3f(
vec3f(covA.x, covA.y, covA.z),
vec3f(covA.y, covB.x, covB.y),
vec3f(covA.z, covB.y, covB.z)
);

let ortho = isOrtho == 1u;
let v = select(viewCenter.xyz, vec3f(0.0, 0.0, 1.0), ortho);
let vz = select(min(v.z, -nearClip), v.z, ortho);
let J1 = focal / vz;
let J2 = -J1 / vz * v.xy;
let J = mat3x3f(
vec3f(J1, 0.0, J2.x),
vec3f(0.0, J1, J2.y),
vec3f(0.0, 0.0, 0.0)
);

let W = transpose(mat3x3f(
viewMatrix[0].xyz,
viewMatrix[1].xyz,
viewMatrix[2].xyz
));
let TT = W * J;
let cov = transpose(TT) * Vrk * TT;

let a = cov[0][0] + 0.3;
let b = cov[0][1];
let c = cov[1][1] + 0.3;
// Compute TT columns directly without materializing full J and W matrices.
// Original code:
// let J = mat3x3f(vec3f(J1, 0.0, J2.x), vec3f(0.0, J1, J2.y), vec3f(0.0, 0.0, 0.0));
// let W = transpose(mat3x3f(viewMatrix[0].xyz, viewMatrix[1].xyz, viewMatrix[2].xyz));
// let TT = W * J;
let w0 = vec3f(viewMatrix[0].x, viewMatrix[1].x, viewMatrix[2].x);
let w1 = vec3f(viewMatrix[0].y, viewMatrix[1].y, viewMatrix[2].y);
let w2 = vec3f(viewMatrix[0].z, viewMatrix[1].z, viewMatrix[2].z);
let tt0 = J1 * w0 + J2.x * w2;
let tt1 = J1 * w1 + J2.y * w2;

// Fused covariance: cov = TT^T * Vrk * TT = TT^T * (M^T * M) * TT = (M * TT)^T * (M * TT).
// Compute B = M * TT then cov = B^T * B, avoiding the intermediate Vrk (mat3x3f) matrix.
// Original code:
// let covA = vec3f(dot(M[0], M[0]), dot(M[0], M[1]), dot(M[0], M[2]));
// let covB = vec3f(dot(M[1], M[1]), dot(M[1], M[2]), dot(M[2], M[2]));
// let Vrk = mat3x3f(vec3f(covA.x, covA.y, covA.z), ...);
// let cov = transpose(TT) * Vrk * TT;
let b0 = M * tt0;
let b1 = M * tt1;

let a = dot(b0, b0) + 0.3;
let b = dot(b0, b1);
let c = dot(b1, b1) + 0.3;

let det = a * c - b * b;
if (det <= 0.0) {
Expand Down Expand Up @@ -156,8 +154,7 @@ fn computeSplatCov(
result.a = scaledCov.x;
result.b = scaledCov.y;
result.c = scaledCov.z;
result.radius = vec2f(radiusX, radiusY);
result.radiusFactor = radiusFactor;
result.viewDepth = -viewCenter.z;
result.valid = true;
return result;
}
Expand Down
3 changes: 1 addition & 2 deletions src/scene/shader-lib/wgsl/chunks/gsplat/compute-gsplat-local-tile-count.js
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Original file line number Diff line number Diff line change
Expand Up @@ -95,8 +95,7 @@ fn main(@builtin(global_invocation_id) gid: vec3u, @builtin(num_workgroups) numW
projCache[base + 6u] = pack2x16float(vec2f(rgb.z, opacity));
#endif

let viewDepth = -(uniforms.viewMatrix * vec4f(center, 1.0)).z;
projCache[base + 7u] = bitcast<u32>(viewDepth);
projCache[base + 7u] = bitcast<u32>(proj.viewDepth);

let eval = computeSplatTileEval(proj.screen, coeffX, coeffY, coeffXY, half(opacity),
uniforms.viewportWidth, uniforms.viewportHeight,
Expand Down