Make SkCodec more flexible about its required frame

SkCodec sets fRequiredFrame to be the earliest possible frame that a
given frame can depend on. e.g.

- Frame A fills the screen, Keep
- Frame B does not cover A, Keep
- Frame C covers B but not A, and is opaque

Frame C can depend on either A or B. SkCodec already reports that C
depends on A. This CL allows a client of SkCodec to use either A or
B to create C.

Also expose the DisposalMethod. Since any frame between A and C can
be used to create C except for DisposePrevious frames, the client
needs to be able to know the disposal method so they do not try to
use such a frame to create C.

Further, the disposal method can be used to give the client a better
idea whether they will continue to need a frame. (e.g. if frame i is
DisposePrevious and depends on i-1, the client may not want to steal
i-1 to create i, since i+1 may also depend on i-1.)

TODO: Share code for decoding prior frames between GIF and WEBP

Change-Id: I91a5ae22ba3d8dfbe0bde833fa67ae3da0d81ed6
Reviewed-on: https://skia-review.googlesource.com/13722
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/codec/SkCodecAnimationPriv.h b/src/codec/SkCodecAnimationPriv.h
new file mode 100644
index 0000000000..233a79b211
--- /dev/null
+++ b/src/codec/SkCodecAnimationPriv.h
@@ -0,0 +1,32 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkCodecAnimationPriv_DEFINED
+#define SkCodecAnimationPriv_DEFINED
+
+namespace SkCodecAnimation {
+    /**
+     * How to blend the current frame.
+     */
+    enum class Blend {
+        /**
+         *  Blend with the prior frame. This is the typical case, supported
+         *  by all animated image types.
+         */
+        kPriorFrame,
+
+        /**
+         *  Do not blend.
+         *
+         *  This frames pixels overwrite previous pixels "blending" with
+         *  the background color of transparent.
+         */
+        kBG,
+    };
+
+}
+#endif // SkCodecAnimationPriv_DEFINED