Minimal libpng interface to save SDL_Surfaces as PNG files.
You might want to take a look in "savepng.h" - it is much shorter and simpler than this README.
Add "savepng.c" and "savepng.h" to your project.
Link the libpng library, i.e. add the -lpng LDFLAG (even if you already have
-lSDL_image).
#include "savepng.h"
SDL_Surface *bmp = ... //your surface
if (SDL_SavePNG(bmp, "image.png")) { //boring way with error checking
printf("Unable to save png -- %s\n", SDL_GetError());
}
As you can see, SDL_SavePNG accepts an SDL_Surface and a filename for it's
input. Similar to SDL_SaveBMP, it is a wrapper around the actual RWops-based
SDL_SavePNG_RW function, so you could use that, if needed.
Lastly, there is SDL_PNGFormatAlpha, modeled after SDL_DisplayFormatAlpha,
that would convert any SDL_Surface to an SDL_Surface suitable for PNG
output. Each call to SDL_PNGFormatAlpha produces a new SDL_Surface that
must be freed using SDL_FreeSurface.
//safest way, usefull for 'screen' surface
SDL_Surface *tmp = SDL_PNGFormatAlpha(screen);
SDL_SavePNG(tmp, "screenshot.png");
SDL_FreeSurface(tmp)
Such conversion is actually only required for one surface format (see below), and would do nothing for all other formats, making it very fast. The format in question is:
There is a interesting caveat of combining naive libpng and cunning SDL in a 32-bpp video mode.
The screen surface (obtained by SDL_SetVideoMode or similarly) might (and
will!) ignore it's alpha-component even in the 32bpp mode. Meaning that an
0xAARRGGBB color would be blitted as 0xFFrrggbb irregardless, as if it was a
24bpp color.
Since screen itself is never blitted onto anything else, ignoring the alpha makes perfect sense. However, unlike 24bpp images, the alpha component does exist. Thus, when such surface is saved, it appears to be completely transparent, as the alpha values for each pixel are set to 0.
Depending on your video mode, you might or might not need to first convert your
surface using SDL_PNGFormatAlpha. If you have absolute control over the video
surface, you can force it to 24bpp (or less) mode, which would avoid the
problem.
If the surface passed to SDL_PNGFormatAlpha is already suitable, a no-op is
performed. It is very fast, so you should probably always convert your surfaces
before saving.
Unfortunately, a simplistic interface such as SDL_SavePNG provides no means to write PNG meta-data. If you need to add iTXT chunks to your PNGs, you would have to modify this code or write your own version.
If you have some kind of simple API, that would be thematically consistent with SDL, in mind -- please share.
See main.c and Makefile for an example program. It too is shorter than this
README.
The problem in question is very simple, and this little piece of functionality was implemented and re-implemented multiple times by multiple authors (notably, Angelo "Encelo" Theodorou and Darren Grant, among others). I decided to write my own version to ensure it's correctness, learn more about libpng, and to provide a copy-pastable, maintained, libpng15-aware, palette-supporting variation that I could link to. You can view it as a continuation of their efforts.
SDL_Image would've been perfect place for this, but that library has different purposes.
Next up: code to load SDL_Surfaces as OpenGL 1.1 textures. J/K ;)
SDL_SavePNG is available under the zlib/libpng license.