HQ filtering for tiled/bleed drawBitmap

src/gpu/effects/GrBicubicEffect.cpp

 #include "GrBicubicEffect.h"
 
 #define DS(x) SkDoubleToScalar(x)
 
 const SkScalar GrBicubicEffect::gMitchellCoefficients[16] = {
     DS( 1.0 / 18.0), DS(-9.0 / 18.0), DS( 15.0 / 18.0), DS( -7.0 / 18.0),
     DS(16.0 / 18.0), DS( 0.0 / 18.0), DS(-36.0 / 18.0), DS( 21.0 / 18.0),
     DS( 1.0 / 18.0), DS( 9.0 / 18.0), DS( 27.0 / 18.0), DS(-21.0 / 18.0),
     DS( 0.0 / 18.0), DS( 0.0 / 18.0), DS( -6.0 / 18.0), DS(  7.0 / 18.0),
 };
@@ skipping 55 matching lines @@
         GrGLShaderVar("c3",            kVec4f_GrSLType),
     };
     builder->fsEmitFunction(kVec4f_GrSLType,
                             "cubicBlend",
                             SK_ARRAY_COUNT(gCubicBlendArgs),
                             gCubicBlendArgs,
                             "\tvec4 ts = vec4(1.0, t, t * t, t * t * t);\n"
                             "\tvec4 c = coefficients * ts;\n"
                             "\treturn c.x * c0 + c.y * c1 + c.z * c2 + c.w * c3;\n",
                             &cubicBlendName);
-    builder->fsCodeAppendf("\tvec2 coord = %s - %s * vec2(0.5, 0.5);\n", coords2D.c_str(), imgInc);
-    builder->fsCodeAppendf("\tvec2 f = fract(coord / %s);\n", imgInc);
+    builder->fsCodeAppendf("\tvec2 coord = %s - %s * vec2(0.5);\n", coords2D.c_str(), imgInc);
+    // We unnormalize the coord in order to determine our fractional offset (f) within the texel
+    // We then snap coord to a texel center and renormalize. The snap prevents cases where the
+    // starting coords are near a texel boundary and accumulations of imgInc would cause us to skip/
+    // double hit a texel.
+    builder->fsCodeAppendf("\tcoord /= %s;\n", imgInc);
+    builder->fsCodeAppend("\tvec2 f = fract(coord);\n");
+    builder->fsCodeAppendf("\tcoord = (coord - f + vec2(0.5)) * %s;\n", imgInc);
     for (int y = 0; y < 4; ++y) {
         for (int x = 0; x < 4; ++x) {
             SkString coord;
             coord.printf("coord + %s * vec2(%d, %d)", imgInc, x - 1, y - 1);
             builder->fsCodeAppendf("\tvec4 s%d%d = ", x, y);
             builder->fsAppendTextureLookup(samplers[0], coord.c_str());
             builder->fsCodeAppend(";\n");
         }
         builder->fsCodeAppendf("\tvec4 s%d = %s(%s, f.x, s0%d, s1%d, s2%d, s3%d);\n", y, cubicBlendName.c_str(), coeff, y, y, y, y);
     }
@@ skipping 64 matching lines @@
                                          const GrDrawTargetCaps&,
                                          GrTexture* textures[]) {
     int texIdx = random->nextBool() ? GrEffectUnitTest::kSkiaPMTextureIdx :
                                       GrEffectUnitTest::kAlphaTextureIdx;
     SkScalar coefficients[16];
     for (int i = 0; i < 16; i++) {
         coefficients[i] = random->nextSScalar1();
     }
     return GrBicubicEffect::Create(textures[texIdx], coefficients);
 }