Question

I would like to create a fake “explosion” effect in SDL. For this, I would like the screen to go from what it is currently, and fade to white.

Originally, I thought about using SDL_FillRect like so (where explosionTick is the current alpha value):

SDL_FillRect(screen , NULL , SDL_MapRGBA(screen->format , 255, 255 , 255, explosionTick ));

But instead of a reverse fading rectangle, it shows up completely white with no alpha. The other method I tried involved using a fullscreen bitmap filled with a transparent white (with an alpha value of 1), and blit it once for each explosionTick like so:

        for(int a=0; a<explosionTick; a++){
            SDL_BlitSurface(boom, NULL, screen, NULL);
        }

But, this ended up being to slow to run in real time.
Is there any easy way to achieve this effect without losing performance? Thank you for your time.

Answer 1

Well, you need blending and AFAIK the only way SDL does it is with SDL_Blitsurface. So you just need to optimize that blit. I suggest benchmarking those:

• try to use SDL_SetAlpha to use per-surface alpha instead of per-pixel alpha. In theory, it’s less work for SDL, so you may hope some speed gain. But I never compared it and had some problem with this in the past.
• you don’t really need a fullscreen bitmap, just repeat a thick row. It should be less memory intensive and maybe there is a cache gain. Also you can probably fake some smoothness by doing half the lines at each pass (less pixels to blit and should still look like a global screen effect).
• for optimal performance, verify that your bitmap is at the display format. Check SDL_DisplayFormatAlpha or possibly SDL_DisplayFormat if you use per-surface alpha