Use texture size to determine precision of texture coord varyings

src/gpu/GrCoordTransform.cpp

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "GrCoordTransform.h"
+#include "GrContext.h"
+#include "GrDrawTargetCaps.h"
+#include "GrGpu.h"
+
+void GrCoordTransform::reset(GrCoordSet sourceCoords, const SkMatrix& m, const GrTexture* texture) {
+    SkASSERT(texture);
+    SkASSERT(!fInProcessor);
+
+    fSourceCoords = sourceCoords;
+    fMatrix = m;
+    fReverseY = kBottomLeft_GrSurfaceOrigin == texture->origin();
+
+    // Always start at kDefault. Then if precisions differ we see if the precision needs to be
+    // increased. Our rule is that we want at least 4 subpixel values in the representation for
+    // coords between 0 to 1. Note that this still might not be enough when drawing with repeat
+    // or mirror-repeat modes but that case can be arbitrarily bad. 
+    fPrecision = GrShaderVar::kDefault_Precision;
+    if (texture->getContext()) {
+        const GrDrawTargetCaps* caps = texture->getContext()->getGpu()->caps();
+        if (caps->floatPrecisionVaries()) {
+            int maxD = SkTMax(texture->width(), texture->height());
+            const GrDrawTargetCaps::PrecisionInfo* info;
+            info = &caps->getFloatShaderPrecisionInfo(kFragment_GrShaderType, fPrecision);
+            do {
+                SkASSERT(info->supported());
+                // Make sure there is at least 2 bits of subpixel precision in the range of
+                // texture coords from 0.5 to 1.0.
+                if ((2 << info->fBits) / maxD > 4) {
+                    break;
+                }
+                if (GrShaderVar::kHigh_Precision == fPrecision) {
+                    break;
+                }
+                GrShaderVar::Precision nextP = static_cast<GrShaderVar::Precision>(fPrecision + 1);
+                info = &caps->getFloatShaderPrecisionInfo(kFragment_GrShaderType, nextP);
+                if (!info->supported()) {
+                    break;
+                }
+                fPrecision = nextP;
+            } while (true);
+        }
+    }
+}
+
+void GrCoordTransform::reset(GrCoordSet sourceCoords,
+                             const SkMatrix& m,
+                             GrShaderVar::Precision precision) {
+    SkASSERT(!fInProcessor);
+    fSourceCoords = sourceCoords;
+    fMatrix = m;
+    fReverseY = false;
+    fPrecision = precision;
+}