How to ensure that ffmpeg libraries uses/ not uses GPU - linux

Question

My library ( Linux, Debian) uses FFMpeg libraries ( avformat, avcodec, swscale etc) for reading video stream from network cameras. Actually, I need to capture each video frame from network camera, decode it, scale and store in memory- and other thread pass this data to calling program for display.
Problem is, that all works in CPU and take a huge amount of CPU resource. How can I enforce usage of GPU accelerator for processing?
I have video card: VGA compatible controller: Intel Corporation HD Graphics 620 (rev 02)
My decode thread look like this ( I omit declarations, error handling etc, so pls don't look for grammar mistakes:)))
fmt = avformat_alloc_context();
//initialising, setting option by av_dict_set
// finding video stream index
***
// finding decoder and allocate its contexts
frame = av_frame_alloc();
while ( av_read_frame(ctx->fmt, &pkt) >= 0)
{
AVPacket orig_pkt = pkt;
avcodec_send_packet(ctx->dec_ctx, pkt);
avcodec_receive_frame(ctx->dec_ctx, frame);
***
// get buffer allocated for store of frame data
buff = get_free_buffer(ctx);
sws_scale(ctx->sws, (const uint8_t * const*)frame->data,
frame->linesize, 0, ctx->dec_ctx->height, buff->data,
buff->linesize);
ret = decode_packet(ctx, frame, &pkt, &got_frame);
if (ret < 0)
break;
pkt.data += ret;
pkt.size -= ret;
}
while (pkt.size > 0);
av_packet_unref(&orig_pkt);
}
*****

Answer 1

You can find HW accelerated ffmpeg recoding commands on the internet, i am using
ffmpeg -vaapi_device /dev/dri/renderD128 -i "inputfile" -vf format=nv12,hwupload -c:v h264_vaapi -f mp4 -qp 18 -map 0 "outputfile.mp4"
You can list HW accelerators by command ffmpeg -hwaccels and DRI framework path using command ls /dev/dri/, then video codec/encoder (h264_vaapi in above example) you can find using command ffmpeg -encoders. -f mp4 parameter may not be necessary to define file format, -qp sets quality (in this case similar to original), -map 0 will try to use all streams of the input file, not just the stream with highest quality, first/default subtitle...
On another hand, when i do not define the HW accelerator device and use default encoder libx264, i can see CPU is maxed out and so no HW acceleration is likely used.