Implement SkHighContrastFilter

This is a color filter to apply several contrast adjustments for users
with low vision, including inverting the colors (in either RGB or HSL
space), applying gamma correction, converting to grayscale, and increasing
the contrast.

BUG=skia:6235

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: Icb8f3e290932d8bcd9387fb1f39dd20767e15cf6
Reviewed-on: https://skia-review.googlesource.com/7460
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>

1 files changed, 81 insertions, 0 deletions

diff --git a/include/effects/SkHighContrastFilter.h b/include/effects/SkHighContrastFilter.h
new file mode 100644
index 0000000000..2ac37e7975
--- /dev/null
+++ b/include/effects/SkHighContrastFilter.h
@@ -0,0 +1,81 @@
+/*
+* Copyright 2017 Google Inc.
+*
+* Use of this source code is governed by a BSD-style license that can be
+* found in the LICENSE file.
+*/
+
+#ifndef SkHighContrastFilter_DEFINED
+#define SkHighContrastFilter_DEFINED
+
+#include "SkColorFilter.h"
+#include "SkPaint.h"
+
+/**
+ *  Configuration struct for SkHighContrastFilter.
+ *
+ *  Provides transformations to improve contrast for users with low vision.
+ */
+struct SkHighContrastConfig {
+    enum class InvertStyle {
+        kNoInvert,
+        kInvertBrightness,
+        kInvertLightness,
+    };
+
+    SkHighContrastConfig() {
+        fGrayscale = false;
+        fInvertStyle = InvertStyle::kNoInvert;
+        fContrast = 0.0f;
+    }
+
+    SkHighContrastConfig(bool grayscale,
+                         InvertStyle invertStyle,
+                         SkScalar contrast)
+        : fGrayscale(grayscale),
+          fInvertStyle(invertStyle),
+          fContrast(contrast) {}
+
+    // Returns true if all of the fields are set within the valid range.
+    bool isValid() const {
+        return fInvertStyle >= InvertStyle::kNoInvert &&
+            fInvertStyle <= InvertStyle::kInvertLightness &&
+            fContrast >= -1.0 &&
+            fContrast <= 1.0;
+    }
+
+    // If true, the color will be converted to grayscale.
+    bool fGrayscale;
+
+    // Whether to invert brightness, lightness, or neither.
+    InvertStyle fInvertStyle;
+
+    // After grayscale and inverting, the contrast can be adjusted linearly.
+    // The valid range is -1.0 through 1.0, where 0.0 is no adjustment.
+    SkScalar  fContrast;
+};
+
+/**
+ *  Color filter that provides transformations to improve contrast
+ *  for users with low vision.
+ *
+ *  Applies the following transformations in this order. Each of these
+ *  can be configured using SkHighContrastConfig.
+ *
+ *    - Conversion to grayscale
+ *    - Color inversion (either in RGB or HSL space)
+ *    - Increasing the resulting contrast.
+ *
+ * Calling SkHighContrastFilter::Make will return nullptr if the config is
+ * not valid, e.g. if you try to call it with a contrast outside the range of
+ * -1.0 to 1.0.
+ */
+class SK_API SkHighContrastFilter {
+public:
+    // Returns the filter, or nullptr if the config is invalid.
+    static sk_sp<SkColorFilter> Make(const SkHighContrastConfig& config);
+
+    SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
+};
+
+#endif