Rotate the image by 90 degree using pixel data

[nitinsharma0608]

Currently i m capturing the screen using the glreadpixels(). the image captured is generally mirrored image hence i flipped back the image to normal. Now i want to rotate the captured data (image) by 90'degree. any idea how to do that ?

The code i m using to capture the screen data is :

CGRect screenBounds = [[UIScreen mainScreen] bounds];

int backingWidth = screenBounds.size.width;
int backingHeight =screenBounds.size.height;

glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);


NSInteger myDataLength = backingWidth * backingHeight * 4;
GLuint *buffer;
if((buffer= (GLuint *) malloc(myDataLength)) == NULL )
    NSLog(@"error initializing the buffer");
glReadPixels(0, 0, backingWidth, backingHeight, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
// code for flipping back (mirroring the image data)    
for(int y = 0; y < backingHeight / 2; y++) {
    for(int xt = 0; xt < backingWidth; xt++) {
        GLuint top = buffer[y * backingWidth + xt];
        GLuint bottom = buffer[(backingHeight - 1 - y) * backingWidth + xt];
        buffer[(backingHeight - 1 - y) * backingWidth + xt] = top;
        buffer[y * backingWidth + xt] = bottom;
    }
}

Any idea how to rotate the data captured in buffer by 90'degree ? Thanks