Centralize decision about whether to do bicubic filtering, and fallbacks to mip, bilerp, or nearest

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 159 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);
 }
+
+//////////////////////////////////////////////////////////////////////////////
+
+bool GrBicubicEffect::ShouldUseBicubic(const SkMatrix& matrix,
+                                       GrTextureParams::FilterMode* filterMode) {
+    if (matrix.isIdentity()) {
+        *filterMode = GrTextureParams::kNone_FilterMode;
+        return false;
+    }
+
+    SkScalar scales[2];
+    if (!matrix.getMinMaxScales(scales) || scales[0] < SK_Scalar1) {
+        // Bicubic doesn't handle arbitrary minimization well, as src texels can be skipped
+        // entirely,
+        *filterMode = GrTextureParams::kMipMap_FilterMode;
+        return false;
+    }
+    // At this point if scales[1] == SK_Scalar1 then the matrix doesn't do any scaling.
+    if (scales[1] == SK_Scalar1) {
+        if (matrix.rectStaysRect() && SkScalarIsInt(matrix.getTranslateX()) &&
+            SkScalarIsInt(matrix.getTranslateY())) {
+            *filterMode = GrTextureParams::kNone_FilterMode;
+        } else {
+            // Use bilerp to handle rotation or fractional translation.
+            *filterMode = GrTextureParams::kBilerp_FilterMode;
+        }
+        return false;
+    }
+    // When we use the bicubic filtering effect each sample is read from the texture using
+    // nearest neighbor sampling.
+    *filterMode = GrTextureParams::kNone_FilterMode;
+    return true;
+}