Flashing screen with a high framerate

SDL Development
1

I have a simple program that blits some surfaces that use
transparency to the screen. I am getting around 700 fps
fullscreen, but the images that are blit’ed, are “flashing”.

If I switch back to softwaremode and not fullscreen, I can get
the fps down to around 15-30 and I do not see the “flashing”

Is the problem the screen is updating too fast? Should I be
using double buffering? Or is that not related to the problem?

//video mode I’m using
screen = SDL_SetVideoMode(SCREEN_W, SCREEN_H, screen_bpp,
SDL_HWSURFACE | SDL_RLEACCEL | SDL_FULLSCREEN);

Thanks,
jake__________________________________
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2

jake b wrote:

I have a simple program that blits some surfaces that use
transparency to the screen. I am getting around 700 fps
fullscreen, but the images that are blit’ed, are “flashing”.

If I switch back to softwaremode and not fullscreen, I can get
the fps down to around 15-30 and I do not see the “flashing”

Is the problem the screen is updating too fast? Should I be
using double buffering?

Yes. If you don’t use double buffering, things might be visible on
screen just after you called the blit, before you use the
SDL_UpdateRects function.
That usually cerates that “blinking” effect you see, since something
half-drawn is shown on screen.

Or is that not related to the problem?

//video mode I’m using
screen = SDL_SetVideoMode(SCREEN_W, SCREEN_H, screen_bpp,
SDL_HWSURFACE | SDL_RLEACCEL | SDL_FULLSCREEN);

Just add SDL_DOUBLEBUF to these flags.
Btw, SDL_RLEACCEL has no effect on the video surface, it’s only useful
on SDL surfaces.
(SDL_RLEACCEL should be set on the source surface of a blit and if the
blit is done in software, it should accelerate the blit)

Stephane

3

Just add SDL_DOUBLEBUF to these flags.
Btw, SDL_RLEACCEL has no effect on the video surface, it’s
only useful
on SDL surfaces.
(SDL_RLEACCEL should be set on the source surface of a blit
and if the
blit is done in software, it should accelerate the blit)

Stephane

  1. Does SDL_RLEACCEL have any negative impact if used on a
    surface, even if the screen is in hardware mode?

  2. Is this the correct way to use doublebuffering? It now
    averages 70 fps (instead of 600+) but the flashing is gone.

//screen
SDL_Surface *screen;

//init
screen = SDL_SetVideoMode(SCREEN_W, SCREEN_H, screen_bpp,
SDL_HWSURFACE | SDL_FULLSCREEN | SDL_DOUBLEBUF);

//render function
//blit some…
//flip
SDL_Flip(screen);

  1. Do I no longer need to use UpdateRect()? It says that if
    doublebuffering is not enabled, it will instead call
    UpdateRect() from SDL_Flip().

thanks,
jake__________________________________
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Friends. Fun. Try the all-new Yahoo! Messenger.

4

jake b wrote:

Just add SDL_DOUBLEBUF to these flags.
Btw, SDL_RLEACCEL has no effect on the video surface, it’s
only useful
on SDL surfaces.
(SDL_RLEACCEL should be set on the source surface of a blit
and if the
blit is done in software, it should accelerate the blit)

Stephane

  1. Does SDL_RLEACCEL have any negative impact if used on a
    surface, even if the screen is in hardware mode?

  • If the blit is done by the cpu, SDL_RLEACCEL will probably speed up
    the blits (that depends on the proportion of empty pixels and the layout
    of the pixels in the surface).

  • If the screen is a hardware surface and the blits is done in hardware,
    the SDL_RLEACCEL flag is ignored.

(notice there is a special case : most backends don’t hardware
accelerate alpha blits, so these are done by the cpu anyway, even if the
screen is a hardware surface. So the first case applies here)

So having a speed increase or not heavily depends on :

  • your hardware platform (cpu, video card…)
  • the backend you currently use (and whether it is able to accelerate
    colorkey or alpha blits)
  • whether the video surface is hardware or software
  1. Is this the correct way to use doublebuffering? It now
    averages 70 fps (instead of 600+) but the flashing is gone.

//screen
SDL_Surface *screen;

//init
screen = SDL_SetVideoMode(SCREEN_W, SCREEN_H, screen_bpp,
SDL_HWSURFACE | SDL_FULLSCREEN | SDL_DOUBLEBUF);

//render function
//blit some…
//flip
SDL_Flip(screen);

Yes.

  1. Do I no longer need to use UpdateRect()? It says that if
    doublebuffering is not enabled, it will instead call
    UpdateRect() from SDL_Flip().

You have to choose between SDL_UpdateRects() and SDL_Flip().
SDL_UpdateRects is meant to be used with single buffered surfaces, while
SDL_Flip is meant to be used with doublebuffered surfaces.
(and if you call the wrong one, SDL will call the correct one for you
anyway)

Stephane