glReadPixel() return black image - ios

Question

I'm using AVCaptureSession for live cameraview feed then i render some images on the camera overlay view. I didn't use any EAGLView, just overlaying some images using AVCaptureSession with previewlayer. I want to take screenshot live camera feed image and overlay image. So, i searched some links finally got with glReadPixel(), but when i implement this code it returns black image. Just added OpenGLES.framework library and imported.
- (void)viewDidLoad
{
[super viewDidLoad];
[self setCaptureSession:[[AVCaptureSession alloc] init]];
[self addVideoInputFrontCamera:NO]; // set to YES for Front Camera, No for Back camera
[self addStillImageOutput];
[self setPreviewLayer:[[AVCaptureVideoPreviewLayer alloc] initWithSession:[self captureSession]] ];
[[self previewLayer] setVideoGravity:AVLayerVideoGravityResizeAspectFill];
CGRect layerRect = [[[self view] layer] bounds];
[[self previewLayer]setBounds:layerRect];
[[self previewLayer] setPosition:CGPointMake(CGRectGetMidX(layerRect),CGRectGetMidY(layerRect))];
[[[self view] layer] addSublayer:[self previewLayer]];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(saveImageToPhotoAlbum) name:kImageCapturedSuccessfully object:nil];
[[self captureSession] startRunning];
UIImageView *dot =[[UIImageView alloc] initWithFrame:CGRectMake(50,50,200,200)];
dot.image=[UIImage imageNamed:#"draw.png"];
[self.view addSubview:dot];
}
capturing the live camera feed with overlay content using glReadPixel():
- (UIImage*) glToUIImage
{
CGFloat scale = [[UIScreen mainScreen] scale];
// CGRect s = CGRectMake(0, 0, 320.0f * scale, 480.0f * scale);
CGRect s = CGRectMake(0, 0, 768.0f * scale, 1024.0f * scale);
uint8_t *buffer = (uint8_t *) malloc(s.size.width * s.size.height * 4);
glReadPixels(0, 0, s.size.width, s.size.height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, buffer, s.size.width * s.size.height * 4, NULL);
CGImageRef iref = CGImageCreate(s.size.width, s.size.height, 8, 32, s.size.width * 4, CGColorSpaceCreateDeviceRGB(), kCGBitmapByteOrderDefault, ref, NULL, true, kCGRenderingIntentDefault);
size_t width = CGImageGetWidth(iref);
size_t height = CGImageGetHeight(iref);
size_t length = width * height * 4;
uint32_t *pixels = (uint32_t *)malloc(length);
CGContextRef context1 = CGBitmapContextCreate(pixels, width, height, 8, width * 4,
CGImageGetColorSpace(iref), kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Big);
CGAffineTransform transform = CGAffineTransformIdentity;
transform = CGAffineTransformMakeTranslation(0.0f, height);
transform = CGAffineTransformScale(transform, 1.0, -1.0);
CGContextConcatCTM(context1, transform);
CGContextDrawImage(context1, CGRectMake(0.0f, 0.0f, width, height), iref);
CGImageRef outputRef = CGBitmapContextCreateImage(context1);
outputImage = [UIImage imageWithCGImage: outputRef];
CGDataProviderRelease(ref);
CGImageRelease(iref);
CGContextRelease(context1);
CGImageRelease(outputRef);
free(pixels);
free(buffer);
UIImageWriteToSavedPhotosAlbum(outputImage, nil, nil, nil);
NSLog(#"Screenshot size: %d, %d", (int)[outputImage size].width, (int)[outputImage size].height);
return outputImage;
}
-(void)screenshot:(id)sender{
[self glToUIImage];
}
But it returns black image.

Answer 1

glReadPixels() won't work with an AV Foundation preview layer. There's no OpenGL ES context to capture pixels from, and even if there was, you'd need to capture from it before the scene was presented to the display.
If what you're trying to do is to capture an image overlaid on live video from the camera, my GPUImage framework could handle that for you. All you'd need to do would be to set up a GPUImageVideoCamera, a GPUImagePicture instance for what you needed to overlay, and a blend filter of some sort. You would then feed the output to a GPUImageView for display, and be able to capture still images from the blend filter at any point. The framework handles the heavy lifting for you with all of this.