Cannot decode PNG or TIF files with libavcodec.

[sgan]

Summary of the bug:
With libavcodec dlls, I cannot decode .PNG or .TIF files format but it is working fine with .JPEG or .BMP files.
Remark: with ffmpeg.exe, I am able to decode/encode these files (-i input.png -pix_fmt rgb32 output.bmp)
How to reproduce:

I am using latest dll provided by Zeranoe FFmpeg:
Build Version: git-cf16b45 (2015-03-29)
libavutil      54. 20.101 / 54. 20.101
libavcodec     56. 31.100 / 56. 31.100
libavformat    56. 26.101 / 56. 26.101
libavdevice    56.  4.100 / 56.  4.100
libavfilter     5. 13.101 /  5. 13.101
libswscale      3.  1.101 /  3.  1.101
libswresample   1.  1.100 /  1.  1.100
libpostproc    53.  3.100 / 53.  3.100

I am just testing the following code with a .png file:
http://ffmpeg.org/doxygen/trunk/lavfutils_8c_source.html#l00024

Problem:
At line 77
ret = avcodec_decode_video2(codec_ctx, frame, &frame_decoded, &pkt);

ret is >0 and is equal to PNG file size, but frame_decoded=0!!
AVFrame *frame;

According to documentation:
​http://ffmpeg.org/doxygen/trunk/group__lavc__decoding.html#ga99ee61b6dcffb7817a275d39da58cc74
"

• got_picture_ptr: Zero if no frame could be decompressed, otherwise, it is nonzero.
• Returns: On error a negative value is returned, otherwise the number of bytes used or zero if no frame could be decompressed."

In our case, we have at the same time :

• got_picture_ptr=0
• Returns=number of bytes used

It seems incompatible.

As said, the code is working fine with jpeg or bmp files.
Problem occurs on 32 or 64 bits dll.

[sgan]

Example of png file.

[sgan]

So I am still working on it and I think I finally found the solution.

As I am not an libavcodec expert, please fill free to comment.

According to avcodec_open2 documentation,
​http://ffmpeg.org/doxygen/trunk/group__lavc__core.html#ga11f785a188d7d9df71621001465b0f1d
Prior to using this function the context has to be allocated with avcodec_alloc_context3().

So new code should be something like that (modifications have been commented as [sgan]:

int ff_load_image(uint8_t *data[4], int linesize[4],
                  int *w, int *h, enum AVPixelFormat *pix_fmt,
                  const char *filename, void *log_ctx)
{
    AVInputFormat *iformat = NULL;
    AVFormatContext *format_ctx = NULL;
    AVCodec *codec;
    AVCodecContext *codec_ctx;
    AVFrame *frame;
    int frame_decoded, ret = 0;
    AVPacket pkt;

    av_init_packet(&pkt);

    av_register_all();

    iformat = av_find_input_format("image2");
    if ((ret = avformat_open_input(&format_ctx, filename, iformat, NULL)) < 0) {
        av_log(log_ctx, AV_LOG_ERROR,
               "Failed to open input file '%s'\n", filename);
        return ret;
    }

    if ((ret = avformat_find_stream_info(format_ctx, NULL)) < 0) {
        av_log(log_ctx, AV_LOG_ERROR, "Find stream info failed\n");
        return ret;
    }

    //codec_ctx = format_ctx->streams[0]->codec;            //[sgan]To remove
    codec = avcodec_find_decoder(codec_ctx->codec_id);   //[sgan]To modify 
    if (!codec) {
        av_log(log_ctx, AV_LOG_ERROR, "Failed to find codec\n");
        ret = AVERROR(EINVAL);
        goto end;
    }
    
    codec_ctx = avcodec_alloc_context3(codec);   //[sgan]To add
      if (!codec_ctx) return -1;                           //[sgan]To add

   
   codec_ctx->width=format_ctx->streams[0]->codec->width;         //[sgan]To add
   codec_ctx->height=format_ctx->streams[0]->codec->height;      //[sgan]To add
   codec_ctx->pix_fmt=format_ctx->streams[0]->codec->pix_fmt;   //[sgan]To add

    if ((ret = avcodec_open2(codec_ctx, codec, NULL)) < 0) {
        av_log(log_ctx, AV_LOG_ERROR, "Failed to open codec\n");
        goto end;
    }

    if (!(frame = av_frame_alloc()) ) {
        av_log(log_ctx, AV_LOG_ERROR, "Failed to alloc frame\n");
        ret = AVERROR(ENOMEM);
        goto end;
    }

    ret = av_read_frame(format_ctx, &pkt);
    if (ret < 0) {
        av_log(log_ctx, AV_LOG_ERROR, "Failed to read frame from file\n");
        goto end;
    }

    ret = avcodec_decode_video2(codec_ctx, frame, &frame_decoded, &pkt);
    if (ret < 0 || !frame_decoded) {
        av_log(log_ctx, AV_LOG_ERROR, "Failed to decode image from file\n");
        if (ret >= 0)
            ret = -1;
        goto end;
    }

    *w       = frame->width;
    *h       = frame->height;
    *pix_fmt = frame->format;

    if ((ret = av_image_alloc(data, linesize, *w, *h, *pix_fmt, 16)) < 0)
        goto end;
    ret = 0;

    av_image_copy(data, linesize, (const uint8_t **)frame->data, frame->linesize, *pix_fmt, *w, *h);

end:
    av_free_packet(&pkt);
    //avcodec_close(codec_ctx);               //[sgan]To remove
    avcodec_free_context(&codec_ctx);      //[sgan]To add
    avformat_close_input(&format_ctx);
    av_frame_free(&frame);

    if (ret < 0)
        av_log(log_ctx, AV_LOG_ERROR, "Error loading image file '%s'\n", filename);
    return ret;
}

It seems that after:

 codec_ctx = avcodec_alloc_context3(codec);

, some attibutes have to be set (width,height, ...).

Now it is working with PNG and TIFF files also.

[sgan]

I am following investigations!!

In fact, the original code ​http://ffmpeg.org/doxygen/trunk/lavfutils_8c_source.html#l00024 is working for PNG if we set the AVCodecContext::thread_count attribute to 1.

ex at line 59: format_ctx->streams[0]->codec->thread_count=1;

This is a workaround and I do not know why it is working. Problem should be analyse now deeply by developers.

The problem is connected to the following point:
​http://stackoverflow.com/questions/22930109/call-to-avformat-find-stream-info-prevents-decoding-of-simple-png-image/22994263#22994263

Regards

Sebastien.

[Michael Niedermayer]

Yes, thread_count has to be set to 1 or the user application must call the decoder in a loop until the frame has been decoded.