Slightly improve cubic chop precision in stroke tessellation shaders
Also adds GrGLSLVertexGeoBuilder::appendRawFunctionDefinition(). This
allows us to define functions in vertex and geometry shaders whose names
don't get mangled.
Bug: skia:10419
Change-Id: I90319b54bcbbb7000c7809cb6ce8d1969a3bd8c5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/327456
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
diff --git a/src/gpu/glsl/GrGLSLVertexGeoBuilder.h b/src/gpu/glsl/GrGLSLVertexGeoBuilder.h
index e5f6630..882f22a 100644
--- a/src/gpu/glsl/GrGLSLVertexGeoBuilder.h
+++ b/src/gpu/glsl/GrGLSLVertexGeoBuilder.h
@@ -15,6 +15,14 @@
* geometry for the rasterizer.
*/
class GrGLSLVertexGeoBuilder : public GrGLSLShaderBuilder {
+public:
+ // Copies the given text verbatim to the function definitions section. Does not mangle the name.
+ // 'functionDefinition' should be a fully valid SkSL function, complete with return type, name,
+ // arguments, braces, and a body.
+ void insertFunction(const char* functionDefinition) {
+ this->functions().append(functionDefinition);
+ }
+
protected:
GrGLSLVertexGeoBuilder(GrGLSLProgramBuilder* program) : INHERITED(program) {}
diff --git a/src/gpu/tessellate/GrStrokeTessellateShader.cpp b/src/gpu/tessellate/GrStrokeTessellateShader.cpp
index e026342..dab581a 100644
--- a/src/gpu/tessellate/GrStrokeTessellateShader.cpp
+++ b/src/gpu/tessellate/GrStrokeTessellateShader.cpp
@@ -76,6 +76,15 @@
v->declareGlobal(GrShaderVar("vsTans01", kFloat4_GrSLType, TypeModifier::Out));
v->declareGlobal(GrShaderVar("vsTans23", kFloat4_GrSLType, TypeModifier::Out));
v->declareGlobal(GrShaderVar("vsPrevJoinTangent", kFloat2_GrSLType, TypeModifier::Out));
+
+ // Unlike mix(), this does not return b when t==1. But it otherwise seems to get better
+ // precision than "a*(1 - t) + b*t" for things like chopping cubics on exact cusp points.
+ // The responsibility falls on the caller to ensure t != 1 before calling.
+ v->insertFunction(R"(
+ float4 unchecked_mix(float4 a, float4 b, float4 t) {
+ return fma(b - a, t, a);
+ })");
+
v->codeAppendf(R"(
// Unpack the control points.
float4x2 P = float4x2(inputPts01, inputPts23);
@@ -206,13 +215,13 @@
}
// Chop the curve at chopT[0] and chopT[1].
- float4 ab = mix(P[0].xyxy, P[1].xyxy, chopT.sstt);
- float4 bc = mix(P[1].xyxy, P[2].xyxy, chopT.sstt);
- float4 cd = mix(P[2].xyxy, P[3].xyxy, chopT.sstt);
- float4 abc = mix(ab, bc, chopT.sstt);
- float4 bcd = mix(bc, cd, chopT.sstt);
- float4 abcd = mix(abc, bcd, chopT.sstt);
- float4 middle = mix(abc, bcd, chopT.ttss);
+ float4 ab = unchecked_mix(P[0].xyxy, P[1].xyxy, chopT.sstt);
+ float4 bc = unchecked_mix(P[1].xyxy, P[2].xyxy, chopT.sstt);
+ float4 cd = unchecked_mix(P[2].xyxy, P[3].xyxy, chopT.sstt);
+ float4 abc = unchecked_mix(ab, bc, chopT.sstt);
+ float4 bcd = unchecked_mix(bc, cd, chopT.sstt);
+ float4 abcd = unchecked_mix(abc, bcd, chopT.sstt);
+ float4 middle = unchecked_mix(abc, bcd, chopT.ttss);
// Find tangents at the chop points if an inner tangent wasn't specified.
if (innerTangents[0] == float2(0)) {
@@ -522,6 +531,13 @@
uniform vec4 sk_RTAdjust;
+ // Unlike mix(), this does not return b when t==1. But it otherwise seems to get better
+ // precision than "a*(1 - t) + b*t" for things like chopping cubics on exact cusp points.
+ // We override this result anyway when t==1 so it shouldn't be a problem.
+ vec2 unchecked_mix(vec2 a, vec2 b, float t) {
+ return fma(b - a, vec2(t), a);
+ }
+
void main() {
// Our patch is composed of exactly "numTotalCombinedSegments + 1" stroke-width edges that
// run orthogonal to the curve and make a strip of "numTotalCombinedSegments" quads.
@@ -666,12 +682,12 @@
float T = max(parametricT, radialT);
// Evaluate the cubic at T. Use De Casteljau's for its accuracy and stability.
- vec2 ab = mix(P[0], P[1], T);
- vec2 bc = mix(P[1], P[2], T);
- vec2 cd = mix(P[2], P[3], T);
- vec2 abc = mix(ab, bc, T);
- vec2 bcd = mix(bc, cd, T);
- vec2 position = mix(abc, bcd, T);
+ vec2 ab = unchecked_mix(P[0], P[1], T);
+ vec2 bc = unchecked_mix(P[1], P[2], T);
+ vec2 cd = unchecked_mix(P[2], P[3], T);
+ vec2 abc = unchecked_mix(ab, bc, T);
+ vec2 bcd = unchecked_mix(bc, cd, T);
+ vec2 position = unchecked_mix(abc, bcd, T);
// If we went with T=parametricT, then update the tangent. Otherwise leave it at the radial
// tangent found previously. (In the event that parametricT == radialT, we keep the radial
