Playing YUV video with SDL: Line wraps too late when resolution is not multiple of 4

Question

I've been battling with an issue when playing certain sources of uncompressed YUV 4:2:0 planar video data with SDL_Overlay (SDL 1.2.5).

I have no problems playing, say, 640x480 video. But I have just attempted playing a video with the resolution 854x480, and I get a strange effect. The line wraps 1-2 pixels too late (causing a shear-like transformation) and the chroma disappears, to be replaced with alternating R, G or B on each line. See this screenshot

The YUV data itself is correct, as I can save it to a file and play it in another player. It is not padded at this point - the pitch matches the line length.

My suspicion is that some issue occurs when the resolution is not a multiple of 4. Perhaps SDL_Surface expects an SDL_Overlay to have a chroma resolution as a multiple of 2?

Adding to my suspicion, I note that the RGB SDL_Surface that I create at a size of 854*480 has a pitch of 2564, not the 3*854 = 2562 I would expect.

If I add 1 or 2 pixels to the width of the SDL_Surface (but keep the overlay and rectangle the same), it works fine, albeit with a black border to the right. Of course this then breaks with videos which are a multiple of four.

Setup

screen = SDL_SetVideoMode(width, height, 24, SDL_SWSURFACE|SDL_ANYFORMAT|SDL_ASYNCBLIT);
if ( screen == NULL ) {
  return 0;
}
YUVOverlay = SDL_CreateYUVOverlay(width, height, SDL_IYUV_OVERLAY, screen);
Ydata = new unsigned char[luma_size];
Udata = new unsigned char[chroma_size];
Vdata = new unsigned char[chroma_size];

YUVOverlay->pixels[0] = Ydata;
YUVOverlay->pixels[1] = Udata;
YUVOverlay->pixels[2] = Vdata;

SDL_DisplayYUVOverlay(YUVOverlay, dest);

Rendering loop:

SDL_LockYUVOverlay(YUVOverlay);
memcpy(Ydata, buffer, luma_size);
memcpy(Udata, buffer+luma_size, chroma_size);
memcpy(Vdata, buffer+luma_size+chroma_size, chroma_size);
int i = SDL_DisplayYUVOverlay(YUVOverlay, dest);
SDL_UnlockYUVOverlay(YUVOverlay);

The easiest fix for me to do is increase the RGB SDL_Surface size so that it is a multiple of 4 in each dimension. But then this adds a black border.

Is there a correct way of fixing this issue? Should I try playing with padding on my YUV data?

Answer 1

Each plane of your input data must start on an address divisible by 8, and the stride of each row must be divisible by 8. To be clear: your chroma planes need to obey this too.

This requirement seems to be from the SDL library's use of MMX multimedia instructions on an x86 cpu. See the comments in src/video/SDL_yuv_mmx.c in the distribution.

update: I looked at the actual assembly code, and there are additional assumptions not mentioned in the source code comments. This is for SDL 1.2.14. In addition to the modulo 8 assumption described above, the code assumes that both the input luma and input chroma planes are packed perfectly (i.e. width == stride).