[SDL] truble to get the color of a surface pixel

[f]

i need to get the color of a pixel of specific position of a surface,
i'm trying something like this:


int frogoutofbounds(SDL_Surface *maskbackground)
{
        SDL_Color col;
        Uint32 pixupleft;
        SDL_PixelFormat *fmt;
        Uint8 r,g,b;
        pixupleft=frog.pos.x+(frog.pos.y*800);
        fmt=maskbackground->format;
        SDL_GetRGB(pixupleft,fmt,&r,&g,&b);
        col.g=g;
        printf("green color:%d\n",col.g);
.....


but this don't return me the rgb code, it returns me an non-sense
number that isn't correct.

frog is a global struct that have the position of a sprite (frog.pos)
, i want to know if the upper left corner of the sprite  (800x600
screen size) is over the "black zone" of the mask of the background.

thanks in advance


-- 
f || www.ct.upc.edu/%7eferran