Drawing on a surface after displaying a YUY2 overlay

I grab frames from a webcam and show them on a window through a
overlay (SDL_YUY2_OVERLAY); then I draw on them while I see the video.
The problem is that the video is always flashing (e.g. I see flashing
also the area which is supposed to be covered by the drawings). More
specificly, what I draw is a color histogram.

I’ve seen some discussions on this list about similar problems, but no
one had a definitive solution. What I do in my app now is more or less

// Init
pscreen = SDL_SetVideoMode(w, h, 24, SDL_HWSURFACE|SDL_DOUBLEBUF);
// …
overlay = SDL_CreateYUVOverlay(w, h, SDL_YUY2_OVERLAY, pscreen);
pbuff = (unsigned char *) overlay->pixels[0];

while (…) {
// …
// Grab a frame
// …
memcpy(pbuff, frame, wh2);
SDL_DisplayYUVOverlay(overlay, &drect);


// Then draw lines with putpixel on pscreen surface
// …


SDL_UpdateRect(pscreen, x,y,etc.);

Other info:

  • I tried with and without locking pscreenm and/or using SDL_flip()
    instead of updaterect
  • It happens with both HWSURFACE (with double buffering or not) and SWSURFACE
  • I’m on Linux, SDL 1.2.12
  • I can’t draw on the frame before painting it, because it’s in YUYV
    and it means that U and V are shared between close pixels (so I can’t
    draw on single pixels)

I partially solved by reducing the overlay size, but what if the area
I want to draw on is not rectangular?

Any suggestion?