Initial implementation of a SkColorSpace_A2B xform

src/core/SkColorLookUpTable.cpp

Index: src/core/SkColorLookUpTable.cpp
diff --git a/src/core/SkColorLookUpTable.cpp b/src/core/SkColorLookUpTable.cpp
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+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkColorLookUpTable.h"
+#include "SkFloatingPoint.h"
+
+void SkColorLookUpTable::interp3D(float dst[3], float src[3]) const {
+    // Call the src components x, y, and z.
+    const uint8_t maxX = fGridPoints[0] - 1;
+    const uint8_t maxY = fGridPoints[1] - 1;
+    const uint8_t maxZ = fGridPoints[2] - 1;
+
+    // An approximate index into each of the three dimensions of the table.
+    const float x = src[0] * maxX;
+    const float y = src[1] * maxY;
+    const float z = src[2] * maxZ;
+
+    // This gives us the low index for our interpolation.
+    int ix = sk_float_floor2int(x);
+    int iy = sk_float_floor2int(y);
+    int iz = sk_float_floor2int(z);
+
+    // Make sure the low index is not also the max index.
+    ix = (maxX == ix) ? ix - 1 : ix;
+    iy = (maxY == iy) ? iy - 1 : iy;
+    iz = (maxZ == iz) ? iz - 1 : iz;
+
+    // Weighting factors for the interpolation.
+    const float diffX = x - ix;
+    const float diffY = y - iy;
+    const float diffZ = z - iz;
+
+    // Constants to help us navigate the 3D table.
+    // Ex: Assume x = a, y = b, z = c.
+    //     table[a * n001 + b * n010 + c * n100] logically equals table[a][b][c].
+    const int n000 = 0;
+    const int n001 = 3 * fGridPoints[1] * fGridPoints[2];
+    const int n010 = 3 * fGridPoints[2];
+    const int n011 = n001 + n010;
+    const int n100 = 3;
+    const int n101 = n100 + n001;
+    const int n110 = n100 + n010;
+    const int n111 = n110 + n001;
+
+    // Base ptr into the table.
+    const float* ptr = &(table()[ix*n001 + iy*n010 + iz*n100]);
+
+    // The code below performs a tetrahedral interpolation for each of the three
+    // dst components.  Once the tetrahedron containing the interpolation point is
+    // identified, the interpolation is a weighted sum of grid values at the
+    // vertices of the tetrahedron.  The claim is that tetrahedral interpolation
+    // provides a more accurate color conversion.
+    // blogs.mathworks.com/steve/2006/11/24/tetrahedral-interpolation-for-colorspace-conversion/
+    //
+    // I have one test image, and visually I can't tell the difference between
+    // tetrahedral and trilinear interpolation.  In terms of computation, the
+    // tetrahedral code requires more branches but less computation.  The
+    // SampleICC library provides an option for the client to choose either
+    // tetrahedral or trilinear.
+    for (int i = 0; i < 3; i++) {
+        if (diffZ < diffY) {
+            if (diffZ < diffX) {
+                dst[i] = (ptr[n000] + diffZ * (ptr[n110] - ptr[n010]) +
+                                      diffY * (ptr[n010] - ptr[n000]) +
+                                      diffX * (ptr[n111] - ptr[n110]));
+            } else if (diffY < diffX) {
+                dst[i] = (ptr[n000] + diffZ * (ptr[n111] - ptr[n011]) +
+                                      diffY * (ptr[n011] - ptr[n001]) +
+                                      diffX * (ptr[n001] - ptr[n000]));
+            } else {
+                dst[i] = (ptr[n000] + diffZ * (ptr[n111] - ptr[n011]) +
+                                      diffY * (ptr[n010] - ptr[n000]) +
+                                      diffX * (ptr[n011] - ptr[n010]));
+            }
+        } else {
+            if (diffZ < diffX) {
+                dst[i] = (ptr[n000] + diffZ * (ptr[n101] - ptr[n001]) +
+                                      diffY * (ptr[n111] - ptr[n101]) +
+                                      diffX * (ptr[n001] - ptr[n000]));
+            } else if (diffY < diffX) {
+                dst[i] = (ptr[n000] + diffZ * (ptr[n100] - ptr[n000]) +
+                                      diffY * (ptr[n111] - ptr[n101]) +
+                                      diffX * (ptr[n101] - ptr[n100]));
+            } else {
+                dst[i] = (ptr[n000] + diffZ * (ptr[n100] - ptr[n000]) +
+                                      diffY * (ptr[n110] - ptr[n100]) +
+                                      diffX * (ptr[n111] - ptr[n110]));
+            }
+        }
+
+        // Increment the table ptr in order to handle the next component.
+        // Note that this is the how table is designed: all of nXXX
+        // variables are multiples of 3 because there are 3 output
+        // components.
+        ptr++;
+    }
+}