Tutorial 08: Software Scaling

libswscale

ffmpeg has recently added a new interface, libswscale to handle image scaling. Whereas before in our player we would use img_convert to go from RGB to YUV12, we now use the new interface. This new interface is more modular, faster, and I believe has MMX optimization stuff. In other words, it's the preferred way to do scaling.

The basic function we're going to use to scale is sws_scale. But first, we're going to have to set up what's called an SwsContext. This allows us to compile the conversion we want, and then pass that in later to sws_scale. It's kind of like a prepared statement in SQL or a compiled regexp in Python. To prepare this context, we use the sws_getContext function, which is going to want our source width and height, our desired width and height, the source format and desired format, along with some other options and flags. Then we use sws_scale the same way as img_convert except we pass it our SwsContext:

#include <ffmpeg/swscale.h> // include the header!

int queue_picture(VideoState *is, AVFrame *pFrame, double pts) {

  static struct SwsContext *img_convert_ctx;
  ...

  if(vp->bmp) {

    SDL_LockYUVOverlay(vp->bmp);
    
    dst_pix_fmt = PIX_FMT_YUV420P;
    /* point pict at the queue */

    pict.data[0] = vp->bmp->pixels[0];
    pict.data[1] = vp->bmp->pixels[2];
    pict.data[2] = vp->bmp->pixels[1];
    
    pict.linesize[0] = vp->bmp->pitches[0];
    pict.linesize[1] = vp->bmp->pitches[2];
    pict.linesize[2] = vp->bmp->pitches[1];
    
    // Convert the image into YUV format that SDL uses
    if(img_convert_ctx == NULL) {
      int w = is->video_st->codec->width;
      int h = is->video_st->codec->height;
      img_convert_ctx = sws_getContext(w, h, 
                        is->video_st->codec->pix_fmt, 
                        w, h, dst_pix_fmt, SWS_BICUBIC, 
                        NULL, NULL, NULL);
      if(img_convert_ctx == NULL) {
	fprintf(stderr, "Cannot initialize the conversion context!\n");
	exit(1);
      }
    }
    sws_scale(img_convert_ctx, pFrame->data, 
              pFrame->linesize, 0, 
              is->video_st->codec->height, 
              pict.data, pict.linesize);

That's it! We're done! Go ahead and compile your player:

gcc -o tutorial08 tutorial08.c -lavutil -lavformat -lavcodec -lz -lm `sdl-config --cflags --libs`

Of course, there's a lot of things we glanced over that we could add.

>> What's left?