Add SkDefaultXform as a catch-all to handle color conversions

src/core/SkColorSpaceXform.cpp

Index: src/core/SkColorSpaceXform.cpp
diff --git a/src/core/SkColorSpaceXform.cpp b/src/core/SkColorSpaceXform.cpp
index 473e715353f3337b644388b406b3e6a87902d58a..216e9931ba0a8c913fd472702ca70daf1f9a3c29 100644
--- a/src/core/SkColorSpaceXform.cpp
+++ b/src/core/SkColorSpaceXform.cpp
@@ -9,8 +9,8 @@
 #include "SkColorSpace_Base.h"
 #include "SkColorSpaceXform.h"
 
-bool compute_gamut_xform(SkMatrix44* srcToDst, const SkMatrix44& srcToXYZ,
-                         const SkMatrix44& dstToXYZ) {
+static inline bool compute_gamut_xform(SkMatrix44* srcToDst, const SkMatrix44& srcToXYZ,
+                                       const SkMatrix44& dstToXYZ) {
     if (!dstToXYZ.invert(srcToDst)) {
         return false;
     }
@@ -22,15 +22,21 @@ bool compute_gamut_xform(SkMatrix44* srcToDst, const SkMatrix44& srcToXYZ,
 std::unique_ptr<SkColorSpaceXform> SkColorSpaceXform::New(const sk_sp<SkColorSpace>& srcSpace,
                                                           const sk_sp<SkColorSpace>& dstSpace) {
     if (!srcSpace || !dstSpace) {
+        // Invalid input
         return nullptr;
     }
 
-    if (as_CSB(srcSpace)->gammas()->isValues() && as_CSB(dstSpace)->gammas()->isValues()) {
-        SkMatrix44 srcToDst(SkMatrix44::kUninitialized_Constructor);
-        if (!compute_gamut_xform(&srcToDst, srcSpace->xyz(), dstSpace->xyz())) {
-            return nullptr;
-        }
+    if (as_CSB(srcSpace)->colorLUT() || as_CSB(dstSpace)->colorLUT()) {
+        // Unimplemented
+        return nullptr;
+    }
 
+    SkMatrix44 srcToDst(SkMatrix44::kUninitialized_Constructor);
+    if (!compute_gamut_xform(&srcToDst, srcSpace->xyz(), dstSpace->xyz())) {
+        return nullptr;
+    }
+
+    if (as_CSB(srcSpace)->gammas()->isValues() && as_CSB(dstSpace)->gammas()->isValues()) {
         float srcGammas[3];
         float dstGammas[3];
         srcGammas[0] = as_CSB(srcSpace)->gammas()->fRed.fValue;
@@ -44,21 +50,17 @@ std::unique_ptr<SkColorSpaceXform> SkColorSpaceXform::New(const sk_sp<SkColorSpa
                 new SkGammaByValueXform(srcGammas, srcToDst, dstGammas));
     }
 
-    // Unimplemeted
-    return nullptr;
+    return std::unique_ptr<SkColorSpaceXform>(
+            new SkDefaultXform(as_CSB(srcSpace)->gammas(), srcToDst, as_CSB(dstSpace)->gammas()));
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
-SkGammaByValueXform::SkGammaByValueXform(float srcGammas[3], const SkMatrix44& srcToDst,
-                                         float dstGammas[3])
-    : fSrcToDst(srcToDst)
-{
-    memcpy(fSrcGammas, srcGammas, 3 * sizeof(float));
-    memcpy(fDstGammas, dstGammas, 3 * sizeof(float));
+static inline float byte_to_float(uint8_t v) {
+    return ((float) v) * (1.0f / 255.0f);
 }
 
-static uint8_t clamp_float_to_byte(float v) {
+static inline uint8_t clamp_float_to_byte(float v) {
     v = v * 255.0f;
     if (v > 255.0f) {
         return 255;
@@ -69,12 +71,22 @@ static uint8_t clamp_float_to_byte(float v) {
     }
 }
 
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+SkGammaByValueXform::SkGammaByValueXform(float srcGammas[3], const SkMatrix44& srcToDst,
+                                         float dstGammas[3])
+    : fSrcToDst(srcToDst)
+{
+    memcpy(fSrcGammas, srcGammas, 3 * sizeof(float));
+    memcpy(fDstGammas, dstGammas, 3 * sizeof(float));
+}
+
 void SkGammaByValueXform::xform_RGBA_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const {
     while (len-- > 0) {
         float srcFloats[3];
-        srcFloats[0] = ((*src >>  0) & 0xFF) * (1.0f / 255.0f);
-        srcFloats[1] = ((*src >>  8) & 0xFF) * (1.0f / 255.0f);
-        srcFloats[2] = ((*src >> 16) & 0xFF) * (1.0f / 255.0f);
+        srcFloats[0] = byte_to_float((*src >>  0) & 0xFF);
+        srcFloats[1] = byte_to_float((*src >>  8) & 0xFF);
+        srcFloats[2] = byte_to_float((*src >> 16) & 0xFF);
 
         // Convert to linear.
         srcFloats[0] = pow(srcFloats[0], fSrcGammas[0]);
@@ -107,3 +119,153 @@ void SkGammaByValueXform::xform_RGBA_8888(uint32_t* dst, const uint32_t* src, ui
         src++;
     }
 }
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+// Interpolating lookup in a variably sized table.
+static inline float interp_lut(uint8_t byte, float* table, size_t tableSize) {
+    float index = byte_to_float(byte) * (tableSize - 1);
+    float diff = index - sk_float_floor2int(index);
+    return table[(int) sk_float_floor2int(index)] * (1.0f - diff) +
+            table[(int) sk_float_ceil2int(index)] * diff;
+}
+
+// Inverse table lookup.  Ex: what index corresponds to the input value?  This will
+// have strange results when the table is non-increasing.  But any sane gamma
+// function will be increasing.
+// FIXME (msarett):
+// This is a placeholder implementation for inverting table gammas.  First, I need to
+// verify if there are actually destination profiles that require this functionality.
+// Next, there are certainly faster and more robust approaches to solving this problem.
+// The LUT based approach in QCMS would be a good place to start.
+static inline float interp_lut_inv(float input, float* table, size_t tableSize) {
+    if (input <= table[0]) {
+        return table[0];
+    } else if (input >= table[tableSize - 1]) {
+        return 1.0f;
+    }
+
+    for (uint32_t i = 1; i < tableSize; i++) {
+        if (table[i] >= input) {
+            // We are guaranteed that input is greater than table[i - 1].
+            float diff = input - table[i - 1];
+            float distance = table[i] - table[i - 1];
+            float index = (i - 1) + diff / distance;
+            return index / (tableSize - 1);
+        }
+    }
+
+    // Should be unreachable, since we'll return before the loop if input is
+    // larger than the last entry.
+    SkASSERT(false);
+    return 0.0f;
+}
+
+SkDefaultXform::SkDefaultXform(const sk_sp<SkGammas>& srcGammas, const SkMatrix44& srcToDst,
+                               const sk_sp<SkGammas>& dstGammas)
+    : fSrcGammas(srcGammas)
+    , fSrcToDst(srcToDst)
+    , fDstGammas(dstGammas)
+{}
+
+void SkDefaultXform::xform_RGBA_8888(uint32_t* dst, const uint32_t* src, uint32_t len) const {
+    while (len-- > 0) {
+        // Convert to linear.
+        // FIXME (msarett):
+        // Rather than support three different strategies of transforming gamma, QCMS
+        // builds a 256 entry float lookup table from the gamma info.  This handles
+        // the gamma transform and the conversion from bytes to floats.  This may
+        // be simpler and faster than our current approach.
+        float srcFloats[3];
+        for (int i = 0; i < 3; i++) {
+            const SkGammaCurve& gamma = (*fSrcGammas)[i];
+            uint8_t byte = (*src >> (8 * i)) & 0xFF;
+            if (gamma.isValue()) {
+                srcFloats[i] = pow(byte_to_float(byte), gamma.fValue);
+            } else if (gamma.isTable()) {
+                srcFloats[i] = interp_lut(byte, gamma.fTable.get(), gamma.fTableSize);
+            } else {
+                SkASSERT(gamma.isParametric());
+                float component = byte_to_float(byte);
+                if (component < gamma.fD) {
+                    // Y = E * X + F
+                    srcFloats[i] = gamma.fE * component + gamma.fF;
+                } else {
+                    // Y = (A * X + B)^G + C
+                    srcFloats[i] = pow(gamma.fA * component + gamma.fB, gamma.fG) + gamma.fC;
+                }
+            }
+        }
+
+        // Convert to dst gamut.
+        float dstFloats[3];
+        dstFloats[0] = srcFloats[0] * fSrcToDst.getFloat(0, 0) +
+                       srcFloats[1] * fSrcToDst.getFloat(1, 0) +
+                       srcFloats[2] * fSrcToDst.getFloat(2, 0) + fSrcToDst.getFloat(3, 0);
+        dstFloats[1] = srcFloats[0] * fSrcToDst.getFloat(0, 1) +
+                       srcFloats[1] * fSrcToDst.getFloat(1, 1) +
+                       srcFloats[2] * fSrcToDst.getFloat(2, 1) + fSrcToDst.getFloat(3, 1);
+        dstFloats[2] = srcFloats[0] * fSrcToDst.getFloat(0, 2) +
+                       srcFloats[1] * fSrcToDst.getFloat(1, 2) +
+                       srcFloats[2] * fSrcToDst.getFloat(2, 2) + fSrcToDst.getFloat(3, 2);
+
+        // Convert to dst gamma.
+        // FIXME (msarett):
+        // Rather than support three different strategies of transforming inverse gamma,
+        // QCMS builds a large float lookup table from the gamma info.  Is this faster or
+        // better than our approach?
+        for (int i = 0; i < 3; i++) {
+            const SkGammaCurve& gamma = (*fDstGammas)[i];
+            if (gamma.isValue()) {
+                dstFloats[i] = pow(dstFloats[i], 1.0f / gamma.fValue);
+            } else if (gamma.isTable()) {
+                // FIXME (msarett):
+                // An inverse table lookup is particularly strange and non-optimal.
+                dstFloats[i] = interp_lut_inv(dstFloats[i], gamma.fTable.get(), gamma.fTableSize);
+            } else {
+                SkASSERT(gamma.isParametric());
+                // FIXME (msarett):
+                // This is a placeholder implementation for inverting parametric gammas.
+                // First, I need to verify if there are actually destination profiles that
+                // require this functionality. Next, I need to explore other possibilities
+                // for this implementation.  The LUT based approach in QCMS would be a good
+                // place to start.
+
+                // We need to take the inverse of a piecewise function.  Assume that
+                // the gamma function is continuous, or this won't make much sense
+                // anyway.
+                // Plug in |fD| to the first equation to calculate the new piecewise
+                // interval.  Then simply use the inverse of the original functions.
+                float interval = gamma.fE * gamma.fD + gamma.fF;
+                if (dstFloats[i] < interval) {
+                    // X = (Y - F) / E
+                    if (0.0f == gamma.fE) {
+                        // The gamma curve for this segment is constant, so the inverse
+                        // is undefined.
+                        dstFloats[i] = 0.0f;
+                    } else {
+                        dstFloats[i] = (dstFloats[i] - gamma.fF) / gamma.fE;
+                    }
+                } else {
+                    // X = ((Y - C)^(1 / G) - B) / A
+                    if (0.0f == gamma.fA || 0.0f == gamma.fG) {
+                        // The gamma curve for this segment is constant, so the inverse
+                        // is undefined.
+                        dstFloats[i] = 0.0f;
+                    } else {
+                        dstFloats[i] = (pow(dstFloats[i] - gamma.fC, 1.0f / gamma.fG) - gamma.fB)
+                                       / gamma.fA;
+                    }
+                }
+            }
+        }
+
+        *dst = SkPackARGB32NoCheck(((*src >> 24) & 0xFF),
+                                   clamp_float_to_byte(dstFloats[0]),
+                                   clamp_float_to_byte(dstFloats[1]),
+                                   clamp_float_to_byte(dstFloats[2]));
+
+        dst++;
+        src++;
+    }
+}