iphone – 为什么这个GLView截图代码返回空白/黑色UIImage？

- (UIImage*) getGLScreenshot {

NSLog(@"1");

float scale = 0.0;
if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)])
{
    // scale value should be 1.0 on 3G and 3GS,and 2.0 on iPhone 4.
    scale = [[UIScreen mainScreen] scale]; 
}

// these are swapped since the screen is rotatey
float h = 768 * scale;
float w = 924 * scale;

NSInteger myDataLength = w * h * 4;

// allocate array and read pixels into it.
GLubyte *buffer = (GLubyte *) malloc(myDataLength);
glReadPixels(0,w,h,GL_RGBA,GL_UNSIGNED_BYTE,buffer);

// gl renders "upside down" so swap top to bottom into new array.
// there's gotta be a better way,but this works.
GLubyte *buffer2 = (GLubyte *) malloc(myDataLength);
for(int y = 0; y <h; y++)
{
    for(int x = 0; x <w * 4; x++)
    {
        buffer2[(((int)h-1) - y) * (int)w * 4 + x] = buffer[y * 4 * (int)w + x];
    }
}

// make data provider with data.
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL,buffer2,myDataLength,NULL);

// prep the ingredients
int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerRow = 4 * w;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;

// make the cgimage
CGImageRef imageRef = CGImageCreate(w,bitsPerComponent,bitsPerPixel,bytesPerRow,colorSpaceRef,bitmapInfo,provider,NULL,NO,renderingIntent);

// then make the uiimage from that
UIImage *myImage = [UIImage imageWithCGImage:imageRef];
return myImage;

}

- (void)saveGLScreenshotToPhotosAlbum {
UIImageWriteToSavedPhotosAlbum([self getGLScreenshot],nil,nil);
}