avformat/apngdec: account for blend and dispose operations.

When the dimensions are the entire frame ones, and the dispose operation
is to reset to background, or the new frame overwrites the new one, then
consider the frame as a key one.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/apng.h

+/*
+ * APNG common header
+ * Copyright (c) 2014 Benoit Fouet
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * APNG common header
+ */
+
+#ifndef AVFORMAT_APNG_H
+#define AVFORMAT_APNG_H
+
+enum {
+   APNG_DISPOSE_OP_NONE       = 0,
+   APNG_DISPOSE_OP_BACKGROUND = 1,
+   APNG_DISPOSE_OP_PREVIOUS   = 2,
+};
+
+enum {
+    APNG_BLEND_OP_SOURCE = 0,
+    APNG_BLEND_OP_OVER   = 1,
+};
+
+#endif /* AVFORMAT_APNG_H */

libavformat/apngdec.c

@@ -26,6 +26,7 @@
  * @see http://www.w3.org/TR/PNG
  */
 
+#include "apng.h"
 #include "avformat.h"
 #include "avio_internal.h"
 #include "internal.h"
@@ -298,7 +299,10 @@ static int decode_fctl_chunk(AVFormatContext *s, APNGDemuxContext *ctx, AVPacket
             return AVERROR_INVALIDDATA;
         ctx->is_key_frame = 0;
     } else {
-        ctx->is_key_frame = 1;
+        if (sequence_number == 0 && dispose_op == APNG_DISPOSE_OP_PREVIOUS)
+            dispose_op = APNG_DISPOSE_OP_BACKGROUND;
+        ctx->is_key_frame = dispose_op == APNG_DISPOSE_OP_BACKGROUND ||
+                            blend_op   == APNG_BLEND_OP_SOURCE;
     }
 
     return 0;