--- /dev/null
+/*
+ * Copyright 2017 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 "sk_tool_utils.h"
+
+// Hue, Saturation, Color, and Luminosity blend modes are oddballs.
+// They nominally convert their inputs to unpremul, then to HSL, then
+// mix-and-match H,S,and L from Src and Dst, then convert back, then premul.
+//
+// Each mode's name describes the Src H,S,L components to keep, taking the
+// others from Dst, where Color == Hue + Saturation. Color and Luminosity
+// are each other's complements; Hue and Saturation have no complement.
+//
+// We have had many inconsistent implementations of these modes.
+// This GM tries to demonstrate unambigously how they should work.
+//
+// TODO: double- and triple-check expected colors
+// TODO: how does gamma-correction factor into this?
+
+DEF_SIMPLE_GM(hsl, canvas, 600, 100) {
+ SkPaint label;
+ sk_tool_utils::set_portable_typeface(&label);
+ label.setAntiAlias(true);
+
+ const char* comment = "HSL blend modes are correct when you see no circles in the squares.";
+ canvas->drawText(comment, strlen(comment), 10,10, label);
+
+ // Just to keep things reaaaal simple, we'll only use opaque colors.
+ SkPaint bg, fg;
+ bg.setColor(0xff00ff00); // Fully-saturated bright green, H = 120°, S = 100%, L = 50%.
+ fg.setColor(0xff7f3f7f); // Partly-saturated dim magenta, H = 300°, S = ~33%, L = ~37%.
+
+ struct {
+ SkBlendMode mode;
+ SkColor expected;
+ } tests[] = {
+ { SkBlendMode::kSrc, fg.getColor() },
+ { SkBlendMode::kDst, bg.getColor() },
+ { SkBlendMode::kHue, 0xffff00ff }, // bright magenta, H = 300°, S = 100%, L = 50%
+ { SkBlendMode::kSaturation, 0xff55aa55 }, // a duller green, H = 120°, S = ~33%, L = 50%
+ { SkBlendMode::kColor, 0xffaa55aa }, // a lighter fg, H = 300°, S = ~33%, L = 50%
+ { SkBlendMode::kLuminosity, 0xff00bd00 }, // a darker bg, H = 120°, S = 100%, L = ~37%
+ };
+ for (auto test : tests) {
+ canvas->drawRect({20,20,80,80}, bg);
+
+ fg.setBlendMode(test.mode);
+ canvas->drawRect({20,20,80,80}, fg);
+
+ SkPaint expected;
+ expected.setColor(test.expected);
+ canvas->drawCircle(50,50, 20, expected);
+
+ const char* name = SkBlendMode_Name(test.mode);
+ canvas->drawText(name, strlen(name), 20,90, label);
+
+ canvas->translate(100,0);
+ }
+}