@@ -37,7 +37,8 @@ typedef struct PNGDecContext {

     const uint8_t *bytestream;

     const uint8_t *bytestream_start;

     const uint8_t *bytestream_end;

-    AVFrame picture;

+    AVFrame picture1, picture2;

+    AVFrame *current_picture, *last_picture;

     int state;

     int width, height;

@@ -385,10 +386,14 @@ static int decode_frame(AVCodecContext *avctx,

     int buf_size = avpkt->size;

     PNGDecContext * const s = avctx->priv_data;

     AVFrame *picture = data;

-    AVFrame * const p= &s->picture;

+    AVFrame *p;

     uint32_t tag, length;

     int ret, crc;

+    FFSWAP(AVFrame *, s->current_picture, s->last_picture);

+    avctx->coded_frame= s->current_picture;

+    p = s->current_picture;

+

     s->bytestream_start=

     s->bytestream= buf;

     s->bytestream_end= buf + buf_size;

@@ -584,7 +589,24 @@ static int decode_frame(AVCodecContext *avctx,

         }

     }

  exit_loop:

-    *picture= s->picture;

+     /* handle p-frames only if a predecessor frame is available */

+     if(s->last_picture->data[0] != NULL) {

+         if(!(avpkt->flags & PKT_FLAG_KEY)) {

+            int i, j;

+            uint8_t *pd = s->current_picture->data[0];

+            uint8_t *pd_last = s->last_picture->data[0];

+

+            for(j=0; j < s->height; j++) {

+                for(i=0; i < s->width * s->bpp; i++) {

+                    pd[i] += pd_last[i];

+                }

+                pd += s->image_linesize;

+                pd_last += s->image_linesize;

+            }

+        }

+    }

+

+    *picture= *s->current_picture;

     *data_size = sizeof(AVFrame);

     ret = s->bytestream - s->bytestream_start;

@@ -602,8 +624,10 @@ static int decode_frame(AVCodecContext *avctx,

 static av_cold int png_dec_init(AVCodecContext *avctx){

     PNGDecContext *s = avctx->priv_data;

-    avcodec_get_frame_defaults(&s->picture);

-    avctx->coded_frame= &s->picture;

+    s->current_picture = &s->picture1;

+    s->last_picture = &s->picture2;

+    avcodec_get_frame_defaults(&s->picture1);

+    avcodec_get_frame_defaults(&s->picture2);

     dsputil_init(&s->dsp, avctx);

     return 0;