Add a SkMixerColorFilter GM

Change-Id: I26382a669601f2bebb1f95e56eab2ad3718db4b9
Reviewed-on: https://skia-review.googlesource.com/c/193165
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

gm/mixercolorfilter.cpp

@@ -0,0 +1,112 @@
+/*
+ * Copyright 2019 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+
+#include "SkCanvas.h"
+#include "SkColorFilter.h"
+#include "SkGradientShader.h"
+#include "SkLumaColorFilter.h"
+#include "SkTableColorFilter.h"
+
+namespace {
+
+class MixerCFGM final : public skiagm::GM {
+public:
+    MixerCFGM(const SkSize& tileSize, size_t tileCount)
+        : fTileSize(tileSize)
+        , fTileCount(tileCount) {}
+
+protected:
+    SkString onShortName() override {
+        return SkString("mixerCF");
+    }
+
+    SkISize onISize() override {
+        return SkISize::Make(fTileSize.width()  * 1.2f * fTileCount,
+                             fTileSize.height() * 1.2f * 3);         // 3 rows
+    }
+
+    void onDraw(SkCanvas* canvas) override {
+        SkPaint paint;
+
+        const SkColor gradient_colors[] = { SK_ColorRED, SK_ColorGREEN, SK_ColorBLUE, SK_ColorRED };
+        paint.setShader(SkGradientShader::MakeSweep(fTileSize.width()  / 2,
+                                                    fTileSize.height() / 2,
+                                                    gradient_colors, nullptr,
+                                                    SK_ARRAY_COUNT(gradient_colors)));
+
+        auto cf0 = MakeTintColorFilter(0xff300000, 0xffa00000);  // red tint
+        auto cf1 = MakeTintColorFilter(0xff003000, 0xff00a000);  // green tint
+
+        this->mixRow(canvas, paint, nullptr,     cf1);
+        this->mixRow(canvas, paint,     cf0, nullptr);
+        this->mixRow(canvas, paint,     cf0,     cf1);
+    }
+
+private:
+    const SkSize fTileSize;
+    const size_t fTileCount;
+
+    void mixRow(SkCanvas* canvas, SkPaint& paint,
+                sk_sp<SkColorFilter> cf0, sk_sp<SkColorFilter> cf1) {
+        canvas->translate(0, fTileSize.height() * 0.1f);
+        {
+            SkAutoCanvasRestore arc(canvas, true);
+            for (size_t i = 0; i < fTileCount; ++i) {
+                paint.setColorFilter(
+                    SkColorFilter::MakeMixer(cf0, cf1, static_cast<float>(i) / (fTileCount - 1)));
+                canvas->translate(fTileSize.width() * 0.1f, 0);
+                canvas->drawRect(SkRect::MakeWH(fTileSize.width(), fTileSize.height()), paint);
+                canvas->translate(fTileSize.width() * 1.1f, 0);
+            }
+        }
+        canvas->translate(0, fTileSize.height() * 1.1f);
+    }
+
+    // A tint filter maps colors to a given range (gradient), based on the input luminance:
+    //
+    //   c' = lerp(lo, hi, luma(c))
+    //
+    // TODO: move to public headers/API?
+    //
+    static sk_sp<SkColorFilter> MakeTintColorFilter(SkColor lo, SkColor hi) {
+        const auto r_lo = SkColorGetR(lo),
+                   g_lo = SkColorGetG(lo),
+                   b_lo = SkColorGetB(lo),
+                   a_lo = SkColorGetA(lo),
+                   r_hi = SkColorGetR(hi),
+                   g_hi = SkColorGetG(hi),
+                   b_hi = SkColorGetB(hi),
+                   a_hi = SkColorGetA(hi);
+
+        // We map component-wise:
+        //
+        //   r' = lo.r + (hi.r - lo.r) * luma
+        //   g' = lo.g + (hi.g - lo.g) * luma
+        //   b' = lo.b + (hi.b - lo.b) * luma
+        //   a' = lo.a + (hi.a - lo.a) * luma
+        //
+        // The input luminance is stored in the alpha channel
+        // (and RGB are cleared -- see SkLumaColorFilter). Thus:
+        const SkScalar tint_matrix[] = {
+            0, 0, 0, (r_hi - r_lo) / 255.0f, SkIntToScalar(r_lo),
+            0, 0, 0, (g_hi - g_lo) / 255.0f, SkIntToScalar(g_lo),
+            0, 0, 0, (b_hi - b_lo) / 255.0f, SkIntToScalar(b_lo),
+            0, 0, 0, (a_hi - a_lo) / 255.0f, SkIntToScalar(a_lo),
+        };
+
+        return SkColorFilter::MakeMatrixFilterRowMajor255(tint_matrix)
+                 ->makeComposed(SkLumaColorFilter::Make());
+    }
+
+    using INHERITED = skiagm::GM;
+};
+
+} // namespace
+
+DEF_GM( return new MixerCFGM(SkSize::Make(200, 250), 5); )

gn/gm.gni

@@ -229,6 +229,7 @@ gm_sources = [
   "$_gm/megalooper.cpp",
   "$_gm/mipmap.cpp",
   "$_gm/mixedtextblobs.cpp",
+  "$_gm/mixercolorfilter.cpp",
   "$_gm/modecolorfilters.cpp",
   "$_gm/morphology.cpp",
   "$_gm/multipicturedraw.cpp",