When I tried to convert SDL Surface to a GL texture, I got texture where red colour was replaced by blue. The problem is not in the GL texture, because the colors are correctly if I use Native::loadTexture() in the following example. And problem isn’t in SDL_image, because If I load a SDL Surface with a SDL_image and use a SDL for render it, the colors are correct.
Where is the problem? It’s a bug in SDL Surface? Or maybe it’s a feature? 
Code:
namespace SDL {
void loadTexture() {
SDL_Surface *texSurf = IMG_Load(“texture.png”);
if(!texSurf)
error(“Unable to load texture”);
GLint textureColors = texSurf->format->BytesPerPixel;
GLenum textureFormat = textureColors == 4 ? GL_RGBA : GL_RGB;
glTexImage2D(GL_TEXTURE_2D, 0, textureFormat, texSurf->w, texSurf->h, 0, textureFormat, GL_UNSIGNED_BYTE, texSurf->pixels);
SDL_FreeSurface(texSurf);
}
};
namespace Native {
void loadTexture() {
NSString *path = [[NSBundle mainBundle] pathForResource:@“texture” ofType:@“png”];
NSData *texData = [[NSData alloc] initWithContentsOfFile:path];
UIImage *image = [[UIImage alloc] initWithData:texData];
if (image == nil)
error(“Unable to load texture”);
GLuint width = CGImageGetWidth(image.CGImage);
GLuint height = CGImageGetHeight(image.CGImage);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
void *imageData = malloc(height * width * 4);
CGContextRef context = CGBitmapContextCreate(
imageData,
width,
height,
8, 4 * width,
colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big
);
CGColorSpaceRelease(colorSpace);
CGContextClearRect(context, CGRectMake( 0, 0, width, height));
CGContextTranslateCTM(context, 0, height - height);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), image.CGImage);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);
CGContextRelease(context);
free(imageData);
[image release];
[texData release];
}
};
int main(int argc, char *argv[])
{
//// Init SDL
if(SDL_Init(SDL_INIT_VIDEO) != 0)
error(“Unable to initialize SDL”);
atexit(SDL_Quit);
//// Enable OpenGL double buffering
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
//// Set the color depth (16-bit 565)
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 5);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 6);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 5);
//// Set video mode
if(!SDL_SetVideoMode(Width, Height, 16, SDL_OPENGL))
error("Unable to set video mode");
//// Generate a texture object
GLuint texId;
glGenTextures(1, &texId);
//// Bind the texture object
glBindTexture(GL_TEXTURE_2D, texId);
//// Load Texture
SDL::loadTexture();
// Native::loadTexture();
//// Set the texture's stretching properties
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
using namespace OpenGL::ES1::D2;
Vertex vertices[] = {
Vertex(-1.0f, 1.0f),
Vertex( 1.0f, 1.0f),
Vertex( 1.0f, -1.0f),
Vertex(-1.0f, -1.0f)
};
Texture::Coords textureCoords(
Texture::Coord(0.0f, 0.0f),
Texture::Coord(1.0f, 0.0f),
Texture::Coord(1.0f, 1.0f),
Texture::Coord(0.0f, 1.0f)
);
//// Setting up an orthogonal viewport
glViewport(0, 0, Width, Height);
glLoadIdentity();
glOrthof(-2.0f, 2.0f, -2.0/(Width/Height), 2.0f/(Width/Height), -2.0f, 2.0f);
//// Display
glEnable(GL_TEXTURE_2D);
glEnableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBindTexture(GL_TEXTURE_2D, texId);
glVertexPointer(2, GL_FLOAT, 0, vertices);
glTexCoordPointer(2, GL_FLOAT, 0, &textureCoords);
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
SDL_GL_SwapBuffers();
SDL_Delay(5000);
//// Free OpenGL texture
glDeleteTextures(1, &texId);
return 0;
}
Code:
int main(int argc, char **argv) {
atexit(SDL_Quit);
if(SDL_Init(SDL_INIT_VIDEO) != 0)
error(“Unable to initialize SDL”);
SDL_Surface *screen = SDL_SetVideoMode(Width, Height, 16, 0 /* SDL_FULLSCREEN */);
if(!screen)
error("Unable to set video mode");
SDL_Surface *image = IMG_Load("bg.png");
if(!image)
error("Unable to load image");
SDL_Rect src, dest;
dest.x = dest.y = src.x = src.y = 0;
dest.w = src.w = image->w;
dest.h = src.h = image->h;
SDL_BlitSurface(image, &src, screen, &dest);
SDL_UpdateRect(screen, 0, 0, 0, 0);
SDL_Delay(3000);
SDL_FreeSurface(image);
return 0;
}