GPF on SDL_GetRGB

SDL Development
1

Hi All,

I’m faced with what must surely be a common problem - reading the color of a
pixel on an SDL surface. I’ve struggled with three methods (shown below)
gleamed from the SDL documentation and other sources on the web, but in all
cases the program GPF’s on the SDL_GetRGBA call.

The display surface i’m attempting to read doesn’t need locking (according to
SDL_MUSTLOCK) and has 4 bytes per pixel. I’m sure its just something in the way
i’m passing the first parameter to SDL_GetRGBA but its got me stumped.

Any advice on how to successfully read a pixel would be greatly appreciated.
Thanks in advance.-----------------------------------------------------------------------------
METHOD 1

Uint32 getpixel(SDL_Surface *surface, int x, int y) {
int bpp = surface->format->BytesPerPixel;
Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;
switch(bpp) {
case 1: return *p;
case 2: return *(Uint16 *)p;
case 3: if(SDL_BYTEORDER == SDL_BIG_ENDIAN)
return p[0] << 16 | p[1] << 8 | p[2];
else
return p[0] | p[1] << 8 | p[2] << 16;
case 4: return *(Uint32 *)p;
default: return 0;
}
}

Uint32 c = getpixel(screen,x,y);
Uint8 R, G, B, A;
SDL_GetRGBA(c,screen->format,&R,&G,&B,&A);

METHOD 2

char* p = (char*)screen->pixels;
p += (screen->pitch * y);
p += (screen->format->BytesPerPixel * x);
Uint8 R, G, B, A;
SDL_GetRGBA((Uint32)(*p),screen->format,&R,&G,&B,&A);

METHOD 3

Uint16 getpixel(SDL_Surface *surface, int x, int y) {
Uint8 *p = (Uint8 )surface->pixels + y * surface->pitch
+ x * surface->format->BytesPerPixel;
return ((Uint16 *)p);
}

Uint16 c = getpixel(screen,x,y);
Uint8 R, G, B, A;
SDL_GetRGBA((Uint32)c,screen->format,&R,&G,&B,&A);

2

Damian <damian_rajkowski technologyonecorp.com> writes:

I’m faced with what must surely be a common problem - reading the color of a
pixel on an SDL surface. I’ve struggled with three methods (shown below)
gleamed from the SDL documentation and other sources on the web, but in all
cases the program GPF’s on the SDL_GetRGBA call.

Oops. Its not SDL_GetRGBA thats failing, it seems to be the reference to the
location in the SDL surface (*p) that kills it.

METHOD 1
Uint32 getpixel(SDL_Surface *surface, int x, int y) {
case 4: return *(Uint32 *)p;

In Method 1, it fails on this line, where it refers to *p.

METHOD 2
SDL_GetRGBA((Uint32)(*p),screen->format,&R,&G,&B,&A);

In Method 2, it fails on this line (again reference to *p).

I though it might be caused by using a hardware surface, but a software surface
exhibits the same problem.

Is anyone willing to post an example of a working getpixel() func?

3

I’m faced with what must surely be a common problem - reading the color of a
pixel on an SDL surface. I’ve struggled with three methods (shown below)
gleamed from the SDL documentation and other sources on the web, but in all
cases the program GPF’s on the SDL_GetRGBA call.

Make sure surface->pixels isn’t NULL, if it is, you need to lock the surface.
Make sure that x and y are not out of bounds. They range from 0 to (w or h)-1

See ya,
-Sam Lantinga, Software Engineer, Blizzard Entertainment