sws_scale generates malformed video

Question

I have to encode a series of frames from CAIRO_FORMAT_ARGB32 to AV_PIX_FMT_YUV420P with sws_scale. From the ffmpeg docs I came to know the AV equivalent of the source format is AV_PIX_FMT_ARGB so here is my code:

 // Set up conversion context
    img->sws_ctx = sws_getCachedContext(
        img->sws_ctx,
        img->video_size[0],
        img->video_size[1],
        AV_PIX_FMT_ARGB,
        img->video_size[0],
        img->video_size[1],
        AV_PIX_FMT_YUV420P,
        SWS_BILINEAR,
        NULL,
        NULL,
        NULL);

    width  = cairo_image_surface_get_width( surface );
    height = cairo_image_surface_get_height( surface );
    stride = cairo_image_surface_get_stride( surface );
    pix    = cairo_image_surface_get_data( surface );
    const int in_linesize[1] = { stride };
    
    sws_scale(  img->sws_ctx, (const uint8_t * const *) &pix, in_linesize, 0,
                img->video_size[1], img->video_frame->data, img->video_frame->linesize);
    img->video_frame->pts++;

Sadly the video doesn't play and VLC shows a bunch of these useless messages:

[h264 @ 0x7f6ce0cbc1c0] mmco: unref short failure
[h264 @ 0x7f6ce0c39a80] co located POCs unavailable
[h264 @ 0x7f6ce0c82800] co located POCs unavailable
[h264 @ 0x7f6ce0c9f400] mmco: unref short failure

The encoding process runs just fine. I also tried with const int in_linesize[1] = { 3 * width }; Where am I wrong?

Answer 1

The following answer shows how to use sws_scale for converting ARGB to YUV420p.
You have to make some adaptations for integration the conversion in your code.

The code sample is "stand alone" sws_scale example, that doesn't use CAIRO.

---

Create BGRA input sample using FFmpeg (command line tool):

ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1 -vcodec rawvideo -pix_fmt argb -frames 1 -f rawvideo argb_image.bin

---

The following code sample applies the following stages:

• Read ARGB (sample) input from binary file.
• Allocate memory buffers from storing the YUV420p output.
• Get SWS context.
• Apply color conversion.
• Write YUV420p output image to binary file (for testing).
• Free allocated memory.

C++ code sample:

#include <stdio.h>
#include <string.h>
#include <stdint.h>

extern "C"
{
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>
}


int main()
{
    //Use FFmpeg for building raw ARGB image (used as input).
    //ffmpeg -y -f lavfi -i testsrc=size=192x108:rate=1 -vcodec rawvideo -pix_fmt argb -frames 1 -f rawvideo argb_image.bin
    
    const int width = 192;
    const int height = 108;
    unsigned char* argb_in = new uint8_t[width * height * 4];   //Allocate 4 bytes per pixel (applies ARGB)

    const enum AVPixelFormat out_pix_fmt = AV_PIX_FMT_YUV420P;

    //Read input image for binary file (for testing)
    ////////////////////////////////////////////////////////////////////////////
    FILE* f = fopen("argb_image.bin", "rb"); //For using fopen in Visual Studio, define: _CRT_SECURE_NO_WARNINGS (or use fopen_s).
    fread(argb_in, 1, width * height * 4, f);
    fclose(f);
    ////////////////////////////////////////////////////////////////////////////    

    //Allocate output buffers:
    ////////////////////////////////////////////////////////////////////////////
    // YUV420p data is separated in three planes
    // 1. Y - intensity plane, resolution: width x height
    // 2. U - Color plane, resolution: width/2 x height/2
    // 3. V - Color plane, resolution: width/2 x height/2

    int out_linesize[4] = {0, 0, 0, 0};
    uint8_t* out_planes[4] = { nullptr, nullptr, nullptr, nullptr };   

    int sts = av_image_alloc(out_planes,    //uint8_t * pointers[4], 
                             out_linesize,  //int linesizes[4], 
                             width,         //int w, 
                             height,        //int h, 
                             out_pix_fmt,   //enum AVPixelFormat pix_fmt, 
                             32);           //int align);   //Align to 32 bytes address may result faster execution time compared to 1 byte aligenment.

    if (sts < 0)
    {
        printf("Error: av_image_alloc response = %d\n", sts);
        return -1;
    }
    ////////////////////////////////////////////////////////////////////////////

    
    
    //Get SWS context
    ////////////////////////////////////////////////////////////////////////////
    struct SwsContext* sws_context = nullptr;

    sws_context = sws_getCachedContext(sws_context,         //struct SwsContext *context,
                                       width,               //int srcW,
                                       height,              //int srcH,
                                       AV_PIX_FMT_ARGB,     //enum AVPixelFormat srcFormat,
                                       width,               //int dstW,
                                       height,              //int dstH,
                                       out_pix_fmt,         //enum AVPixelFormat dstFormat,
                                       SWS_BILINEAR,        //int flags,
                                       nullptr,             //SwsFilter *srcFilter,
                                       nullptr,             //SwsFilter *dstFilter,
                                       nullptr);            //const double *param);

    if (sws_context == nullptr)
    {
        printf("Error: sws_getCachedContext returned nullptr\n");
        return -1;
    }
    ////////////////////////////////////////////////////////////////////////////


    //Apply color conversion
    ////////////////////////////////////////////////////////////////////////////
    const int in_linesize[1] = { 4 * width }; // ARGB stride (4 bytes per pixel - assume data is continuous).
    const uint8_t* in_planes[1] = { argb_in };

    int response = sws_scale(sws_context,   //struct SwsContext *c, 
                             in_planes,     //const uint8_t *const srcSlice[],
                             in_linesize,   //const int srcStride[], 
                             0,             //int srcSliceY, 
                             height,        //int srcSliceH,
                             out_planes,    //uint8_t *const dst[], 
                             out_linesize); //const int dstStride[]);


    if (response < 0)
    {
        printf("Error: sws_scale response = %d\n", response);
        return -1;
    }
    ////////////////////////////////////////////////////////////////////////////


    //Write YUV420p output image to binary file (for testing)
    //You may execute FFmpeg after conversion for testing the output:
    //ffmpeg -y -f rawvideo -s 192x108 -pixel_format yuv420p -i yuv420p_image.bin rgb.png
    ////////////////////////////////////////////////////////////////////////////
    f = fopen("yuv420p_image.bin", "wb");
    fwrite(out_planes[0], 1, width * height, f);
    fwrite(out_planes[1], 1, width * height / 4, f);
    fwrite(out_planes[2], 1, width * height / 4, f);
    fclose(f);
    ////////////////////////////////////////////////////////////////////////////


    //Free allocated memory
    ////////////////////////////////////////////////////////////////////////////
    av_freep(out_planes);
    sws_freeContext(sws_context);
    delete[] argb_in;
    ////////////////////////////////////////////////////////////////////////////

    return 0;
}

---

For testing the output, convert yuv420p_image.bin to PNG image using FFmpeg:

ffmpeg -y -f rawvideo -s 192x108 -pixel_format yuv420p -i yuv420p_image.bin rgb.png

---

rgb.png (result of FFmpeg conversion):