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• OpenCL generating YUV420P black pixel

  2 mai 2021, par Albert Tinon

  I'm working on an FFmpeg OpenCL filter for converting GoPro Max .360 files in Google EAC projected files.
For doing that I need to "stack" the 2 input streams to a twice height output.
It is working very well at a realtime speed. (The working code is in comment)

  


  For going further I need to replace some pixels with some specific colors.
I wrote some macros for making RGB->YUV conversion. But I get only some green or pink pixel (only grey is OK).
this is my test code (with stacking in comment)

  


  #define Y(R,G,B) 0.299 * R + 0.587 * G + 0.114 * B
#define U(R,G,B) -0.147 * R - 0.289 * G + 0.436 * B
#define V(R,G,B) 0.615 * R - 0.515 * G - 0.100 * B
#define YUV(R,G,B) (float4)(Y(R,G,B),U(R,G,B),V(R,G,B),0)

__kernel void gopromax_stack(__write_only image2d_t dst,
                                __read_only  image2d_t gopromax_front,
                                __read_only  image2d_t gopromax_rear)
{
    const sampler_t sampler = (CLK_NORMALIZED_COORDS_FALSE |
                               CLK_FILTER_NEAREST);
    
    float4 val;
    int2 dst_size = get_image_dim(dst);
    int2 loc = (int2)(get_global_id(0), get_global_id(1));
    int split_loc = dst_size.y/2;

        if (loc.y < split_loc)
        {
          // working code for stacking
          //  val = read_imagef(gopromax_front, sampler, (int2)(loc.x, loc.y));

          // testing to put grey (working)
            val = YUV(0.5f,0.5f,0.5f);
        }
        else
        {
          // working code for stacking
          //  val = read_imagef(gopromax_rear, sampler, (int2)(loc.x, loc.y-split_loc));

          // testing to put black (gives green !)
            val = YUV(0,0,0);
        }

    if ((loc.xcode>

  


  I tried many think I cannot succeed to generate black or anything except grey.
What did I make wrong ?
I supposed that my pixels are YUV because I specified yuv420p as the format in my filter :

  


  -filter_complex '[0:0]format=yuv420p,hwupload[a] , [0:4]format=yuv420p,hwupload[b], [a][b]gopromax_opencl, hwdownload,format=yuv420p'


  


  The source streams are in hevc / nv12.

  


  Thanks all for your help.

  


• Use ffmpeg to generate a video of black/blank frames [duplicate]

  2 novembre 2016, par Bird

  This question already has an answer here :

  • How can I generate a video file directly from an FFmpeg filter with no actual input file ?
    
    2 answers
    

  I want to generate a 3 second video of just black frames using ffmpeg on Windows 7. Based on the answer to this question, it seems possible.

  Running the command :

   ffmpeg -t 3 -s 640x480 -f rawvideo -pix_fmt rgb24 -r 25 -i /dev/zero empty.mpeg

  returns the error :

  /dev/zero: No such file or directory  

  and no file is generated.

  I also tried changing /dev/zero to NUL as suggested here, but the result is a size 0 KB file that does not play.

  Anyone know how to generate a black video using ffmpeg on Windows ?

• Is there a way to prevent black frames from appearing at the start of a video when using -c:v copy ?

  11 juillet 2023, par MethanE

  My task seems simple. Using ffmpeg in terminal, I'm trying to pass through the exact video settings minus the audio, and i've been successful, except for the fact that the videos start with a frame of black before the actual video plays.

  


  currently using this :

  


  for i in *.*;do ffmpeg -i "$i" -an -c:v copy "${i%.*}_noaudio.mp4";done


  


  My current thought is it may be related to the error I'm getting (the vid still exports), but I'm not certain :
Missing key frame while searching for timestamp : 1001

  


  Input video metadata :

  


  Metadata :
major_brand : XAVC
minor_version : 16785407
compatible_brands : XAVCmp42iso2
creation_time : 2021-09-12T20:13:46.000000Z
Duration : 00:00:17.52, start : 0.000000, bitrate : 107315 kb/s
Stream #0:0(und) : Video : h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709), 3840x2160 [SAR 1:1 DAR 16:9], 96798 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)

  


  Googling led me to try not using -c:v copy as there might be a glitch related to h264 keyframes and instead using -c:v libx264 but i really don't want to have to re-encode as I don't want to lose any quality. The file I tried this on went from 135 mb down to 6.

  


  Thanks for your help. Kind of a newb at this.