[FFmpeg-devel] [PATCH] Issue1247 - libavcodec/snow.c:2357: pred_block: Assertion `b_w>1 && b_h>1' failed.

[Chris Stones]

On Thu, Jul 16, 2009 at 1:40 AM, Michael Niedermayer <michaelni at gmx.at>wrote:

>
> It might be better to pad the buffers to avoid such sizes
>


update on my playing with snow, and the resolution 74.

The assertion:
  ffmpeg: libavcodec/snow.c:2357: pred_block: Assertion `b_w>1 && b_h>1'
failed.
    in function pred_block. both b_w and b_h passed as parameters, and not
written to.

    pred_block called in function add_yblock, but not before b_w is
truncated at line 2449:
        }else if(src_x + b_w > w){
        b_w = w - src_x;

    add_yblock function called by function predict_slice_buffered on line
2616.

      for(mb_x=0; mb_x<=mb_w; mb_x++){
        add_yblock(s, 1, sb, old_buffer, dst8, obmc,
                     block_w*mb_x - block_w/2,        // this is parameter
src_x
                     block_w*mb_y - block_w/2,
                     block_w, block_w,            // this is parameter b_w
and bh,
                     w, h,                // this is parameter w
                     w, ref_stride, obmc_stride,
                     mb_x - 1, mb_y - 1,
                     add, 0, plane_index);

    remember... b_w == 1 will cause the assertion to fail.
    and if(src_x + b_w > w) then b_w = w - src_x; ( line 2449 in add_yblock
)

    block_w is plane_index ? block_size/2 : block_size;
    block_size is MB_SIZE >> s->block_max_depth  // (block_max_depth 0, snow
doesnt support CODEC_FLAG_4MV ? )
    MB_SIZE is 16 ()  // #define LOG2_MB_SIZE 4 \n #define MB_SIZE
(1<<LOG2_MB_SIZE)

    So.. my thinking is as follows..
    when encoding/decoding chroma ( plane_index == 1) then block_w == 8.

    when the for loop above gets to m_x == 5, then
    src_x = block_w*mb_x - block_w/2  // == 36
    and int w = p->width; ( plane width, which i believe will be 74/2 ( x
resolution / 2 for chroma ( plane_index 1 ))

   which means, we have no tripped the afore mentioned truncation of b_w.
   src_x(36) + block_w(8) > w(37)

   the truncation..now puts b_w = w - src_x  ( 1 ).

   Please correct me if im wrong, but it looks like a mathematical fact,
that with a resolution of 74xanything
   will trip this assertion.

   It seems like a mathematical fact, that b_w==1 will occur ?????

   I dont see anywhere i can *padd the buffers*

   Im struggleing a little.. I cant think of a way ( other than my first
patch ) of fixing this.
   Could anyone provide a little insight... Im a little lost :-(

   THANKS!!!

   p.s. I dont really NEED a resolution of 74xY, but my O.C.D just wont let
me quit ;)