Refactored SkColorSpace and added in a Lab PCS GM

src/core/SkColorSpaceXformPriv.cpp

Index: src/core/SkColorSpaceXformPriv.cpp
diff --git a/src/core/SkColorSpaceXformPriv.cpp b/src/core/SkColorSpaceXformPriv.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..ac3e7f7eb252e612c30ab4ae4566cfa67fb6d8bb
--- /dev/null
+++ b/src/core/SkColorSpaceXformPriv.cpp
@@ -0,0 +1,326 @@
+/*
+ * 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 "SkColorSpaceXformPriv.h"
+#include "SkColorSpaceXform.h"
+
+void interp_3d_clut(float dst[3], float src[3], const SkColorLookUpTable* colorLUT) {

[msarett, 2016/10/05 19:05:52]

Please delete this file and then put the implementations themselves in the .h
file.  Sorry I was unclear, that's what I meant by sharing these but keeping
them inline.

[raftias, 2016/10/06 14:43:04]

Done.

+    // Call the src components x, y, and z.
+    uint8_t maxX = colorLUT->fGridPoints[0] - 1;
+    uint8_t maxY = colorLUT->fGridPoints[1] - 1;
+    uint8_t maxZ = colorLUT->fGridPoints[2] - 1;
+
+    // An approximate index into each of the three dimensions of the table.
+    float x = src[0] * maxX;
+    float y = src[1] * maxY;
+    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.
+    float diffX = x - ix;
+    float diffY = y - iy;
+    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 * colorLUT->fGridPoints[1] * colorLUT->fGridPoints[2];
+    const int n010 = 3 * colorLUT->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 = &(colorLUT->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++;
+    }
+}
+
+float sk_linear_from_2dot2[256] = {
+        0.000000000000000000f, 0.000005077051900662f, 0.000023328004666099f, 0.000056921765712193f,
+        0.000107187362341244f, 0.000175123977503027f, 0.000261543754548491f, 0.000367136269815943f,
+        0.000492503787191433f, 0.000638182842167022f, 0.000804658499513058f, 0.000992374304074325f,
+        0.001201739522438400f, 0.001433134589671860f, 0.001686915316789280f, 0.001963416213396470f,
+        0.002262953160706430f, 0.002585825596234170f, 0.002932318323938360f, 0.003302703032003640f,
+        0.003697239578900130f, 0.004116177093282750f, 0.004559754922526020f, 0.005028203456855540f,
+        0.005521744850239660f, 0.006040593654849810f, 0.006584957382581690f, 0.007155037004573030f,
+        0.007751027397660610f, 0.008373117745148580f, 0.009021491898012130f, 0.009696328701658230f,
+        0.010397802292555300f, 0.011126082368383200f, 0.011881334434813700f, 0.012663720031582100f,
+        0.013473396940142600f, 0.014310519374884100f, 0.015175238159625200f, 0.016067700890886900f,
+        0.016988052089250000f, 0.017936433339950200f, 0.018912983423721500f, 0.019917838438785700f,
+        0.020951131914781100f, 0.022012994919336500f, 0.023103556157921400f, 0.024222942067534200f,
+        0.025371276904734600f, 0.026548682828472900f, 0.027755279978126000f, 0.028991186547107800f,
+        0.030256518852388700f, 0.031551391400226400f, 0.032875916948383800f, 0.034230206565082000f,
+        0.035614369684918800f, 0.037028514161960200f, 0.038472746320194600f, 0.039947171001525600f,
+        0.041451891611462500f, 0.042987010162657100f, 0.044552627316421400f, 0.046148842422351000f,
+        0.047775753556170600f, 0.049433457555908000f, 0.051122050056493400f, 0.052841625522879000f,
+        0.054592277281760300f, 0.056374097551979800f, 0.058187177473685400f, 0.060031607136313200f,
+        0.061907475605455800f, 0.063814870948677200f, 0.065753880260330100f, 0.067724589685424300f,
+        0.069727084442598800f, 0.071761448846239100f, 0.073827766327784600f, 0.075926119456264800f,
+        0.078056589958101900f, 0.080219258736215100f, 0.082414205888459200f, 0.084641510725429500f,
+        0.086901251787660300f, 0.089193506862247800f, 0.091518352998919500f, 0.093875866525577800f,
+        0.096266123063339700f, 0.098689197541094500f, 0.101145164209600000f, 0.103634096655137000f,
+        0.106156067812744000f, 0.108711149979039000f, 0.111299414824660000f, 0.113920933406333000f,
+        0.116575776178572000f, 0.119264013005047000f, 0.121985713169619000f, 0.124740945387051000f,
+        0.127529777813422000f, 0.130352278056244000f, 0.133208513184300000f, 0.136098549737202000f,
+        0.139022453734703000f, 0.141980290685736000f, 0.144972125597231000f, 0.147998022982685000f,
+        0.151058046870511000f, 0.154152260812165000f, 0.157280727890073000f, 0.160443510725344000f,
+        0.163640671485290000f, 0.166872271890766000f, 0.170138373223312000f, 0.173439036332135000f,
+        0.176774321640903000f, 0.180144289154390000f, 0.183548998464951000f, 0.186988508758844000f,
+        0.190462878822409000f, 0.193972167048093000f, 0.197516431440340000f, 0.201095729621346000f,
+        0.204710118836677000f, 0.208359655960767000f, 0.212044397502288000f, 0.215764399609395000f,
+        0.219519718074868000f, 0.223310408341127000f, 0.227136525505149000f, 0.230998124323267000f,
+        0.234895259215880000f, 0.238827984272048000f, 0.242796353254002000f, 0.246800419601550000f,
+        0.250840236436400000f, 0.254915856566385000f, 0.259027332489606000f, 0.263174716398492000f,
+        0.267358060183772000f, 0.271577415438375000f, 0.275832833461245000f, 0.280124365261085000f,
+        0.284452061560024000f, 0.288815972797219000f, 0.293216149132375000f, 0.297652640449211000f,
+        0.302125496358853000f, 0.306634766203158000f, 0.311180499057984000f, 0.315762743736397000f,
+        0.320381548791810000f, 0.325036962521076000f, 0.329729032967515000f, 0.334457807923889000f,
+        0.339223334935327000f, 0.344025661302187000f, 0.348864834082879000f, 0.353740900096629000f,
+        0.358653905926199000f, 0.363603897920553000f, 0.368590922197487000f, 0.373615024646202000f,
+        0.378676250929840000f, 0.383774646487975000f, 0.388910256539059000f, 0.394083126082829000f,
+        0.399293299902674000f, 0.404540822567962000f, 0.409825738436323000f, 0.415148091655907000f,
+        0.420507926167587000f, 0.425905285707146000f, 0.431340213807410000f, 0.436812753800359000f,
+        0.442322948819202000f, 0.447870841800410000f, 0.453456475485731000f, 0.459079892424160000f,
+        0.464741134973889000f, 0.470440245304218000f, 0.476177265397440000f, 0.481952237050698000f,
+        0.487765201877811000f, 0.493616201311074000f, 0.499505276603030000f, 0.505432468828216000f,
+        0.511397818884880000f, 0.517401367496673000f, 0.523443155214325000f, 0.529523222417277000f,
+        0.535641609315311000f, 0.541798355950137000f, 0.547993502196972000f, 0.554227087766085000f,
+        0.560499152204328000f, 0.566809734896638000f, 0.573158875067523000f, 0.579546611782525000f,
+        0.585972983949661000f, 0.592438030320847000f, 0.598941789493296000f, 0.605484299910907000f,
+        0.612065599865624000f, 0.618685727498780000f, 0.625344720802427000f, 0.632042617620641000f,
+        0.638779455650817000f, 0.645555272444935000f, 0.652370105410821000f, 0.659223991813387000f,
+        0.666116968775851000f, 0.673049073280942000f, 0.680020342172095000f, 0.687030812154625000f,
+        0.694080519796882000f, 0.701169501531402000f, 0.708297793656032000f, 0.715465432335048000f,
+        0.722672453600255000f, 0.729918893352071000f, 0.737204787360605000f, 0.744530171266715000f,
+        0.751895080583051000f, 0.759299550695091000f, 0.766743616862161000f, 0.774227314218442000f,
+        0.781750677773962000f, 0.789313742415586000f, 0.796916542907978000f, 0.804559113894567000f,
+        0.812241489898490000f, 0.819963705323528000f, 0.827725794455034000f, 0.835527791460841000f,
+        0.843369730392169000f, 0.851251645184515000f, 0.859173569658532000f, 0.867135537520905000f,
+        0.875137582365205000f, 0.883179737672745000f, 0.891262036813419000f, 0.899384513046529000f,
+        0.907547199521614000f, 0.915750129279253000f, 0.923993335251873000f, 0.932276850264543000f,
+        0.940600707035753000f, 0.948964938178195000f, 0.957369576199527000f, 0.965814653503130000f,
+        0.974300202388861000f, 0.982826255053791000f, 0.991392843592940000f, 1.000000000000000000f,
+};
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+void build_table_linear_from_gamma(float* outTable, float exponent) {
+    for (float x = 0.0f; x <= 1.0f; x += (1.0f/255.0f)) {
+        *outTable++ = powf(x, exponent);
+    }
+}
+
+float interp_lut(float input, const float* table, int tableSize) {
+    float index = input * (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;
+}
+
+void build_table_linear_from_gamma(float* outTable, const float* inTable,
+                                          int inTableSize) {
+    if (256 == inTableSize) {
+        memcpy(outTable, inTable, sizeof(float) * 256);
+        return;
+    }
+
+    for (float x = 0.0f; x <= 1.0f; x += (1.0f/255.0f)) {
+        *outTable++ = interp_lut(x, inTable, inTableSize);
+    }
+}
+
+void build_table_linear_from_gamma(float* outTable, float g, float a, float b, float c,
+                                   float d, float e, float f) {
+    // Y = (aX + b)^g + c  for X >= d
+    // Y = eX + f          otherwise
+    for (float x = 0.0f; x <= 1.0f; x += (1.0f/255.0f)) {
+        if (x >= d) {
+            *outTable++ = powf(a * x + b, g) + c;
+        } else {
+            *outTable++ = e * x + f;
+        }
+    }
+}
+
+// Expand range from 0-1 to 0-255, then convert.
+uint8_t clamp_normalized_float_to_byte(float v) {
+    // The ordering of the logic is a little strange here in order
+    // to make sure we convert NaNs to 0.
+    v = v * 255.0f;
+    if (v >= 254.5f) {
+        return 255;
+    } else if (v >= 0.5f) {
+        return (uint8_t) (v + 0.5f);
+    } else {
+        return 0;
+    }
+}
+
+const int kDstGammaTableSize =
+        SkColorSpaceXform_Base<kTable_SrcGamma, kTable_DstGamma, kNone_ColorSpaceMatch>
+        ::kDstGammaTableSize;
+
+void build_table_linear_to_gamma(uint8_t* outTable, float exponent) {
+    float toGammaExp = 1.0f / exponent;
+
+    for (int i = 0; i < kDstGammaTableSize; i++) {
+        float x = ((float) i) * (1.0f / ((float) (kDstGammaTableSize - 1)));
+        outTable[i] = clamp_normalized_float_to_byte(powf(x, toGammaExp));
+    }
+}
+
+// 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.
+float inverse_interp_lut(float input, const float* table, int tableSize) {
+    if (input <= table[0]) {
+        return table[0];
+    } else if (input >= table[tableSize - 1]) {
+        return 1.0f;
+    }
+
+    for (int 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;
+}
+
+
+void build_table_linear_to_gamma(uint8_t* outTable, const float* inTable, int inTableSize) {
+    for (int i = 0; i < kDstGammaTableSize; i++) {
+        float x = ((float) i) * (1.0f / ((float) (kDstGammaTableSize - 1)));
+        float y = inverse_interp_lut(x, inTable, inTableSize);
+        outTable[i] = clamp_normalized_float_to_byte(y);
+    }
+}
+
+float inverse_parametric(float x, float g, float a, float b, float c, float d, float e, float f) {
+    // We need to take the inverse of the following piecewise function.
+    // Y = (aX + b)^g + c  for X >= d
+    // Y = eX + f          otherwise
+
+    // Assume that the gamma function is continuous, or this won't make much sense anyway.
+    // Plug in |d| to the first equation to calculate the new piecewise interval.
+    // Then simply use the inverse of the original functions.
+    float interval = e * d + f;
+    if (x < interval) {
+        // X = (Y - F) / E
+        if (0.0f == e) {
+            // The gamma curve for this segment is constant, so the inverse is undefined.
+            // Since this is the lower segment, guess zero.
+            return 0.0f;
+        }
+
+        return (x - f) / e;
+    }
+
+    // X = ((Y - C)^(1 / G) - B) / A
+    if (0.0f == a || 0.0f == g) {
+        // The gamma curve for this segment is constant, so the inverse is undefined.
+        // Since this is the upper segment, guess one.
+        return 1.0f;
+    }
+
+    return (powf(x - c, 1.0f / g) - b) / a;
+}
+
+void build_table_linear_to_gamma(uint8_t* outTable, float g, float a,
+                                 float b, float c, float d, float e, float f) {
+    for (int i = 0; i < kDstGammaTableSize; i++) {
+        float x = ((float) i) * (1.0f / ((float) (kDstGammaTableSize - 1)));
+        float y = inverse_parametric(x, g, a, b, c, d, e, f);
+        outTable[i] = clamp_normalized_float_to_byte(y);
+    }
+}
+
+const GammaFns<float> kToLinear = {
+    sk_linear_from_srgb,
+    sk_linear_from_2dot2,
+    &build_table_linear_from_gamma,
+    &build_table_linear_from_gamma,
+    &build_table_linear_from_gamma,
+};
+
+const GammaFns<uint8_t> kFromLinear = {
+    nullptr,
+    nullptr,
+    &build_table_linear_to_gamma,
+    &build_table_linear_to_gamma,
+    &build_table_linear_to_gamma,
+};
+