[SDL] Reading a bunch of pixels's alpha from a PNG file using SDL_Image? (OpenGL+SDL & Linux)

[DARKGuy .]

I hate to bump but... nobody knows? :(

On 5/2/07, DARKGuy . <dark.guy.2008 at gmail.com> wrote:
>
> Hey!
>
> Could somebody assist me with making a code that can read a 10x10 (or any
> other dimension) region and determine the alpha of each pixel? I'm trying to
> use this code but I always seem to get wrong data >.<! according to the main
> code, I should get something like this:
>
> (Swap the 1's by 255. I just did it here with 1's to make it clearer)
>
> 0 0 0 0 1 0 0 0 0
> 0 0 0 1 1 1 0 0 0
> 0 0 1 1 1 1 1 0 0
> 0 1 1 1 1 1 1 1 0
> 1 1 1 1 1 1 1 1 1
>
> But instead I get weird stuff like this:
>
> 128 170 255 128 170 255 128 170 255
> 128 170 255 128 170 255 128 170 255
> 128 170 255 128 170 255 128 170 255
> 128 170 255 128 170 255 128 170 255
> 128 170 255 128 170 255 128 170 255
> 128 170 255 128 170 255 128 170 255
>
> Which, even if they're alpha values, they aren't the right ones in the
> positions I'm requesting. Any help would be GREATLY appreciated!
>
> - DARKGuy
>
> P.D.: Code goes below.
>
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>
> int main()
> {
>     ...
>
>     Terrain.Load("terrain.png");
>
>     Uint32 pixel;
>     int px,py;
>     Uint8 red,green,blue,alpha2;
>
>     SDL_LockSurface(Terrain.pix[0]);
>
>     for(py = 200; py <= 210; py++){
>         for(px = 200; px <= 230; px++){
>             pixel = getpixel(Terrain.pix[0], px, py);
>             SDL_GetRGBA(pixel, Terrain.pix[0]->format,
> &red,&green,&blue,&alpha2);
>             printf("%u ",alpha2);
>         }
>         printf("\n");
>     }
>     SDL_UnlockSurface(Terrain.pix[0]);
>
>     exit(0);
> }
>
>
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
> FUNCTIONS
>
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Uint32 getpixel(SDL_Surface *surface, int x, int y)
> {
>     int bpp = surface->format->BytesPerPixel;
>     /* Here p is the address to the pixel we want to retrieve */
>     Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp /
> 8;
>     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;       /* shouldn't happen, but avoids warnings */
>     }
> }
>
> Terrain.pix[0] is generated from this:
>
>     void Texture::Load(const char filename[]){
>         GLuint texture;
>         SDL_Surface * image2 = IMG_Load(filename);
>         SDL_Surface * imgFile = SDL_DisplayFormatAlpha(image2);
>
>         width = (GLfloat)image2->w;
>         height = (GLfloat)image2->h;
>
>         imgF = image2;   // here. Look below for further info
>
>         glGenTextures(1, &texture);
>         glBindTexture(GL_TEXTURE_2D, texture);
>
>         if(imgFile->format->BitsPerPixel == 32 ||
> imgFile->format->BitsPerPixel == 24){
>             glTexImage2D(GL_TEXTURE_2D, 0, 4, imgFile->w, imgFile->h, 0,
> GL_BGRA, GL_UNSIGNED_BYTE, imgFile->pixels);
>         } else { glTexImage2D(GL_TEXTURE_2D, 0, 4, imgFile->w, imgFile->h,
> 0, GL_RGBA, GL_UNSIGNED_BYTE, imgFile->pixels); }
>
>         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
>         glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
>     }
>
> imgF is this:
>
>     class Texture {
>        public:
>             GLuint id;
>             GLfloat width, height;
>             SDL_Surface * imgF;
>           void Load(const char filename[]);
>     };
>
> and pix[0] is this:
>
>     class Sprite {
>         public:
>                 ...
>             vector <SDL_Surface*> pix;
>     };
>
> When I create a sprite, I do this:
>
>     int Sprite::Load(const char texfilename[]) {
>         Texture tex;
>         tex.Load(texfilename);
>         img.push_back(tex.id);
>         width.push_back(tex.width);
>         height.push_back(tex.height);
>         alpha.push_back(1.0f);
>         pix.push_back (tex.imgF);
>         animImg=0;
>         animCounter=0;
>         angle=0;
>         Mirror=false;
>         Flip=false;
>         return img.size()-1;
>     }
>
> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>
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