How to grab YUV formatted video from the camera, display it and process it

Question

I am writing an iphone (IOS 4) program that capture live video from the camera and process it in real time.

I prefer to capture in kCVPixelFormatType_420YpCbCr8BiPlanarFullRange format for easier processing (I need to process the Y Channel). how do I display data in this format? I suppose I need to somehow convert it to a UIImage and then put it in some ImageView?

Currently I have code that displays kCVPixelFormatType_32BGRA data, but naturally it does not work with kCVPixelFormatType_420YpCbCr8BiPlanarFullRange.

This is the code I use now for the transformation, any help/sample on how to do the same for kCVPixelFormatType_420YpCbCr8BiPlanarFullRange will be appreciated. (Also criticism of my current method).

// Create a UIImage from sample buffer data
- (UIImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer 
{
    // Get a CMSampleBuffer's Core Video image buffer for the media data
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer); 
    // Lock the base address of the pixel buffer
    CVPixelBufferLockBaseAddress(imageBuffer, 0); 

    // Get the number of bytes per row for the pixel buffer
    void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer); 

    // Get the number of bytes per row for the pixel buffer
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer); 
    // Get the pixel buffer width and height
    size_t width = CVPixelBufferGetWidth(imageBuffer); 
    size_t height = CVPixelBufferGetHeight(imageBuffer); 

    // Create a device-dependent RGB color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 

    // Create a bitmap graphics context with the sample buffer data
    CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8, 
                                                 bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst); 
    // Create a Quartz image from the pixel data in the bitmap graphics context
    CGImageRef quartzImage = CGBitmapContextCreateImage(context);

    // Unlock the pixel buffer
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);

    // Free up the context and color space
    CGContextRelease(context); 
    CGColorSpaceRelease(colorSpace);

    // Create an image object from the Quartz image
    UIImage *image = [UIImage imageWithCGImage:quartzImage];

    // Release the Quartz image
    CGImageRelease(quartzImage);

    return (image);
}

Answer 1

Answering my own question. this solved the problem I had (which was to grab yuv output, display it and process it), although its not exactly the answer to the question:

To grab YUV output from the camera:

AVCaptureVideoDataOutput *videoOut = [[AVCaptureVideoDataOutput alloc] init];
[videoOut setAlwaysDiscardsLateVideoFrames:YES];
[videoOut setVideoSettings:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange] forKey:(id)kCVPixelBufferPixelFormatTypeKey]];

To display it as is, use AVCaptureVideoPreviewLayer, it does not require any much code. (You can see the FindMyiCon sample in the WWDC samples pack for example).

To process the YUV y channel (bi-planer in this case so it's all in a single chunk, you can also use memcpy instead of looping) :

- (void)processPixelBuffer: (CVImageBufferRef)pixelBuffer {

    CVPixelBufferLockBaseAddress( pixelBuffer, 0 );

    int bufferHeight = CVPixelBufferGetHeight(pixelBuffer);
    int bufferWidth = CVPixelBufferGetWidth(pixelBuffer);

    // allocate space for ychannel, reallocating as needed.
    if (bufferWidth != y_channel.width || bufferHeight != y_channel.height)
    {
        if (y_channel.data) free(y_channel.data);
        y_channel.width = bufferWidth;
        y_channel.height = bufferHeight;
        y_channel.data = malloc(y_channel.width * y_channel.height);        
    }

    uint8_t *yc = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0);
    int total = bufferWidth * bufferHeight;
    for(int k=0;k<total;k++)
    {
        y_channel.data[k] = yc[k++]; // copy y channel
    }

    CVPixelBufferUnlockBaseAddress( pixelBuffer, 0 );
}