[FFmpeg-devel] ffv1enc: question about "Cannot allocate worst case packet size, the encoding could fail"

[Jerome Martinez]

Hi,

Testing FFmpeg FFV1 encoder on big frames (more than 4K: 4300x3956), I 
have a warning for each frame encoded (so a lot of warnings!):
[ffv1 @ 0000024a6bcfe880] Cannot allocate worst case packet size, the 
encoding could fail

Checking avcodec/ffv1enc.c, it is due to the following lines:
     int64_t maxsize =   AV_INPUT_BUFFER_MIN_SIZE
                       + avctx->width*avctx->height*37LL*4;
[...]
    if (maxsize > INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE - 32) {
         av_log(avctx, AV_LOG_WARNING, "Cannot allocate worst case 
packet size, the encoding could fail\n");
         maxsize = INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE - 32;
     }

The value of maxsize is 2517614784, more than INT_MAX, sure, but I don't 
understand:
- why does FFmpeg need to allocate 148 times the size of a frame?
- why is having a number > INT_MAX an issue? modern machines are 64-bit 
and have 8+ GB of RAM, and in practice I currently saw no encoding 
failure on thousands of frames.

Additionally I didn't get why maxsize is reduced ("only" need of 12 
times the size of the frame) in case of FFV1 version > 3 (experimental 
right now):
     if (f->version > 3)
         maxsize = AV_INPUT_BUFFER_MIN_SIZE + 
avctx->width*avctx->height*3LL*4;

maxsize is used for calling ff_alloc_packet2(), which accepts 64-bit 
numbers.
    if ((ret = ff_alloc_packet2(avctx, pkt, maxsize, 0)) < 0)
         return ret;

Is it possible to reduce this "37x4" multiplier without risk and/or 
remove maxsize reduction in case of sizeof(size_t) == 8 and/or show the 
warning only once?

Issue can be reproduced with:
./ffmpeg.exe -f lavfi -i testsrc2=size=4300x3956 -t 0.040 -pix_fmt 
rgba64 rgba64.dpx
./ffmpeg.exe -i rgba64.dpx -c:v ffv1 rgba64.mkv

Jérôme