use coverage modes instead of blend modes

Bug: skia:
Change-Id: Ib3aa0137644358173ea4087693f33dbc2118c6d2
Reviewed-on: https://skia-review.googlesource.com/99661
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/include/core/SkCoverageMode.h b/include/core/SkCoverageMode.h
new file mode 100644
index 0000000000..96863be000
--- /dev/null
+++ b/include/core/SkCoverageMode.h
@@ -0,0 +1,30 @@
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkCoverageMode_DEFINED
+#define SkCoverageMode_DEFINED
+
+#include "SkTypes.h"
+
+/**
+ *  Describes geometric operations (ala SkRegion::Op) that can be applied to coverage bytes.
+ *  These can be thought of as variants of porter-duff (SkBlendMode) modes, but only applied
+ *  to the alpha channel.
+ *
+ *  See SkMaskFilter for ways to use these when combining two different masks.
+ */
+enum class SkCoverageMode {
+    kUnion,             // A ∪ B    A+B-A*B
+    kIntersect,         // A ∪ B    A*B
+    kDifference,        // A - B    A*(1-B)
+    kReverseDifference, // B - A    B*(1-A)
+    kXor,               // A ⊕ B    A+B-2*A*B
+
+    kLast = kXor
+};
+
+#endif