add a new hsl GM

Change-Id: I2c8b9df3e6bf40de4b38966fa8ea671fc68aa45f
Reviewed-on: https://skia-review.googlesource.com/15635
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

gm/hsl.cpp

@@ -0,0 +1,64 @@
+/*
+ * 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);
+    }
+}

gn/gm.gni

@@ -149,6 +149,7 @@ gm_sources = [
   "$_gm/hardstop_gradients.cpp",
   "$_gm/highcontrastfilter.cpp",
   "$_gm/hittestpath.cpp",
+  "$_gm/hsl.cpp",
   "$_gm/image.cpp",
   "$_gm/image_pict.cpp",
   "$_gm/image_shader.cpp",