iphone - Convert UIImage to CMSampleBufferRef

Question

I am doing video recording using AVFoundation. I have to crop the video to 320x280. I am getting the CMSampleBufferRef and converting it to UIImage using the below code.

CGImageRef _cgImage = [self imageFromSampleBuffer:sampleBuffer];
UIImage *_uiImage = [UIImage imageWithCGImage:_cgImage];
CGImageRelease(_cgImage);
_uiImage = [_uiImage resizedImageWithSize:CGSizeMake(320, 280)];

CMSampleBufferRef croppedBuffer = /* NEED HELP WITH THIS */

[_videoInput appendSampleBuffer:sampleBuffer]; 
// _videoInput is a AVAssetWriterInput

The imageFromSampleBuffer: method looks like this:

- (CGImageRef) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer // Create a CGImageRef from sample buffer data
{
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    CVPixelBufferLockBaseAddress(imageBuffer,0);        // Lock the image buffer

    uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddressOfPlane(imageBuffer, 0);   // Get information of the image
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef newContext = CGBitmapContextCreate(baseAddress, width, height, 8, bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
    CGImageRef newImage = CGBitmapContextCreateImage(newContext);
    CGContextRelease(newContext);

    CGColorSpaceRelease(colorSpace);
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);
    /* CVBufferRelease(imageBuffer); */  // do not call this!

    return newImage;
}

Now I have to convert the resized image back to CMSampleBufferRef to write in AVAssetWriterInput.

How do I convert UIImage to CMSampleBufferRef?

Thanks everyone!

Answer 1

While you could create your own Core Media sample buffers from scratch, it's probably easier to use a AVPixelBufferAdaptor.

You describe the source pixel buffer format in the inputSettings dictionary and pass that to the adaptor initializer:

NSMutableDictionary* inputSettingsDict = [NSMutableDictionary dictionary];
[inputSettingsDict setObject:[NSNumber numberWithInt:pixelFormat] forKey:(NSString*)kCVPixelBufferPixelFormatTypeKey];
[inputSettingsDict setObject:[NSNumber numberWithUnsignedInteger:(NSUInteger)(image.uncompressedSize/image.rect.size.height)] forKey:(NSString*)kCVPixelBufferBytesPerRowAlignmentKey];
[inputSettingsDict setObject:[NSNumber numberWithDouble:image.rect.size.width] forKey:(NSString*)kCVPixelBufferWidthKey];
[inputSettingsDict setObject:[NSNumber numberWithDouble:image.rect.size.height] forKey:(NSString*)kCVPixelBufferHeightKey];
[inputSettingsDict setObject:[NSNumber numberWithBool:YES] forKey:(NSString*)kCVPixelBufferCGImageCompatibilityKey];
[inputSettingsDict setObject:[NSNumber numberWithBool:YES] forKey:(NSString*)kCVPixelBufferCGBitmapContextCompatibilityKey];
AVAssetWriterInputPixelBufferAdaptor* pixelBufferAdapter = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput:assetWriterInput sourcePixelBufferAttributes:inputSettingsDict];

You can then append CVPixelBuffers to your adaptor:

[pixelBufferAdapter appendPixelBuffer:completePixelBuffer withPresentationTime:pixelBufferTime]

The pixelbufferAdaptor accepts CVPixelBuffers, so you have to convert your UIImages to pixelBuffers, which is described here: https://stackoverflow.com/a/3742212/100848
Pass the CGImage property of your UIImage to newPixelBufferFromCGImage.