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, 1 insertions, 2 deletions

diff --git a/gm/animatedGif.cpp b/gm/animatedGif.cpp
index 5f4fe71cb0..80bcdd9ee2 100644
--- a/gm/animatedGif.cpp
+++ b/gm/animatedGif.cpp
@@ -53,7 +53,6 @@ private:
 
             SkCodec::Options opts;
             opts.fFrameIndex = frameIndex;
-            opts.fHasPriorFrame = false;
             const int requiredFrame = fFrameInfos[frameIndex].fRequiredFrame;
             if (requiredFrame != SkCodec::kNone) {
                 SkASSERT(requiredFrame >= 0
@@ -62,7 +61,7 @@ private:
                 // For simplicity, do not try to cache old frames
                 if (requiredBitmap.getPixels() &&
                         sk_tool_utils::copy_to(&bm, requiredBitmap.colorType(), requiredBitmap)) {
-                    opts.fHasPriorFrame = true;
+                    opts.fPriorFrame = requiredFrame;
                 }
             }