Adding filters to video with AVFoundation (OSX) - how do I write the resulting image back to AVWriter?

Question

Setting the scene

I am working on a video processing app that runs from the command line to read in, process and then export video. I'm working with 4 tracks.

1. Lots of clips that I append into a single track to make one video. Let's call this the ugcVideoComposition.
2. Clips with Alpha which get positioned on a second track and using layer instructions, is set composited on export to play back over the top of the ugcVideoComposition.
3. A music audio track.
4. An audio track for the ugcVideoComposition containing the audio from the clips appended into the single track.

I have this all working, can composite it and export it correctly using AVExportSession.

The problem

What I now want to do is apply filters and gradients to the ugcVideoComposition.

My research so far suggests that this is done by using AVReader and AVWriter, extracting a CIImage, manipulating it with filters and then writing that out.

I haven't yet got all the functionality I had above working, but I have managed to get the ugcVideoComposition read in and written back out to disk using the AssetReader and AssetWriter.

    BOOL done = NO;
    while (!done)
    {
        while ([assetWriterVideoInput isReadyForMoreMediaData] && !done)
        {
            CMSampleBufferRef sampleBuffer = [videoCompositionOutput copyNextSampleBuffer];
            if (sampleBuffer)
            {
                // Let's try create an image....
                CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
                CIImage *inputImage = [CIImage imageWithCVImageBuffer:imageBuffer];

                // < Apply filters and transformations to the CIImage here

                // < HOW TO GET THE TRANSFORMED IMAGE BACK INTO SAMPLE BUFFER??? >

                // Write things back out.
                [assetWriterVideoInput appendSampleBuffer:sampleBuffer];

                CFRelease(sampleBuffer);
                sampleBuffer = NULL;
            }
            else
            {
                // Find out why we couldn't get another sample buffer....
                if (assetReader.status == AVAssetReaderStatusFailed)
                {
                    NSError *failureError = assetReader.error;
                    // Do something with this error.
                }
                else
                {
                    // Some kind of success....
                    done = YES;
                    [assetWriter finishWriting];

                }
            }
         }
      }

As you can see, I can even get the CIImage from the CMSampleBuffer, and I'm confident I can work out how to manipulate the image and apply any effects etc. I need. What I don't know how to do is put the resulting manipulated image BACK into the SampleBuffer so I can write it out again.

The question

Given a CIImage, how can I put that into a sampleBuffer to append it with the assetWriter?

Any help appreciated - the AVFoundation documentation is terrible and either misses crucial points (like how to put an image back after you've extracted it, or is focussed on rendering images to the iPhone screen which is not what I want to do.

Much appreciated and thanks!

Answer 1

I eventually found a solution by digging through a lot of half complete samples and poor AVFoundation documentation from Apple.

The biggest confusion is that while at a high level, AVFoundation is "reasonably" consistent between iOS and OSX, the lower level items behave differently, have different methods and different techniques. This solution is for OSX.

Setting up your AssetWriter

The first thing is to make sure that when you set up the asset writer, you add an adaptor to read in from a CVPixelBuffer. This buffer will contain the modified frames.

    // Create the asset writer input and add it to the asset writer.
    AVAssetWriterInput *assetWriterVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:[[videoTracks objectAtIndex:0] mediaType] outputSettings:videoSettings];
    // Now create an adaptor that writes pixels too!
    AVAssetWriterInputPixelBufferAdaptor *adaptor = [AVAssetWriterInputPixelBufferAdaptor
                                                   assetWriterInputPixelBufferAdaptorWithAssetWriterInput:assetWriterVideoInput
                                                 sourcePixelBufferAttributes:nil];
    assetWriterVideoInput.expectsMediaDataInRealTime = NO;
    [assetWriter addInput:assetWriterVideoInput];

Reading and Writing

The challenge here is that I couldn't find directly comparable methods between iOS and OSX - iOS has the ability to render a context directly to a PixelBuffer, where OSX does NOT support that option. The context is also configured differently between iOS and OSX.

Note that you should include the QuartzCore.Framework into your XCode Project as well.

Creating the context on OSX.

    CIContext *context = [CIContext contextWithCGContext:
                      [[NSGraphicsContext currentContext] graphicsPort]
                                             options: nil]; // We don't want to always create a context so we put it outside the loop

Now you want want to loop through, reading off the AssetReader and writing to the AssetWriter... but note that you are writing via the adaptor created previously, not with the SampleBuffer.

    while ([adaptor.assetWriterInput isReadyForMoreMediaData] && !done)
    {
        CMSampleBufferRef sampleBuffer = [videoCompositionOutput copyNextSampleBuffer];
        if (sampleBuffer)
        {
            CMTime currentTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);

            // GRAB AN IMAGE FROM THE SAMPLE BUFFER
            CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
            NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:kCVPixelFormatType_32BGRA], kCVPixelBufferPixelFormatTypeKey,
                                     [NSNumber numberWithInt:640.0], kCVPixelBufferWidthKey,
                                     [NSNumber numberWithInt:360.0], kCVPixelBufferHeightKey,
                                     nil];

            CIImage *inputImage = [CIImage imageWithCVImageBuffer:imageBuffer options:options];

            //-----------------
            // FILTER IMAGE - APPLY ANY FILTERS IN HERE

            CIFilter *filter = [CIFilter filterWithName:@"CISepiaTone"];
            [filter setDefaults];
            [filter setValue: inputImage forKey: kCIInputImageKey];
            [filter setValue: @1.0f forKey: kCIInputIntensityKey];

            CIImage *outputImage = [filter valueForKey: kCIOutputImageKey];


            //-----------------
            // RENDER OUTPUT IMAGE BACK TO PIXEL BUFFER
            // 1. Firstly render the image
            CGImageRef finalImage = [context createCGImage:outputImage fromRect:[outputImage extent]];

            // 2. Grab the size
            CGSize size = CGSizeMake(CGImageGetWidth(finalImage), CGImageGetHeight(finalImage));

            // 3. Convert the CGImage to a PixelBuffer
            CVPixelBufferRef pxBuffer = NULL;
            // pixelBufferFromCGImage is documented below.
            pxBuffer = [self pixelBufferFromCGImage: finalImage andSize: size];

            // 4. Write things back out.
            // Calculate the frame time
            CMTime frameTime = CMTimeMake(1, 30); // Represents 1 frame at 30 FPS
            CMTime presentTime=CMTimeAdd(currentTime, frameTime); // Note that if you actually had a sequence of images (an animation or transition perhaps), your frameTime would represent the number of images / frames, not just 1 as I've done here.

            // Finally write out using the adaptor.
            [adaptor appendPixelBuffer:pxBuffer withPresentationTime:presentTime];

            CFRelease(sampleBuffer);
            sampleBuffer = NULL;
        }
        else
        {
            // Find out why we couldn't get another sample buffer....
            if (assetReader.status == AVAssetReaderStatusFailed)
            {
                NSError *failureError = assetReader.error;
                // Do something with this error.
            }
            else
            {
                // Some kind of success....
                done = YES;
                [assetWriter finishWriting];
            }
        }
    }
}

Creating the PixelBuffer

There MUST be an easier way, however for now, this works and is the only way I found to get directly from a CIImage to a PixelBuffer (via a CGImage) on OSX. The following code is cut and paste from AVFoundation + AssetWriter: Generate Movie With Images and Audio

    - (CVPixelBufferRef) pixelBufferFromCGImage: (CGImageRef) image andSize:(CGSize) size
    {
        NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                         [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
                         [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
                         nil];
        CVPixelBufferRef pxbuffer = NULL;

        CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, size.width,
                                      size.height, kCVPixelFormatType_32ARGB, (__bridge CFDictionaryRef) options,
                                      &pxbuffer);
        NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL);

        CVPixelBufferLockBaseAddress(pxbuffer, 0);
        void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
        NSParameterAssert(pxdata != NULL);

        CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
        CGContextRef context = CGBitmapContextCreate(pxdata, size.width,
                                             size.height, 8, 4*size.width, rgbColorSpace,
                                             kCGImageAlphaNoneSkipFirst);
        NSParameterAssert(context);
        CGContextConcatCTM(context, CGAffineTransformMakeRotation(0));
        CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image),
                                       CGImageGetHeight(image)), image);
        CGColorSpaceRelease(rgbColorSpace);
        CGContextRelease(context);

        CVPixelBufferUnlockBaseAddress(pxbuffer, 0);

        return pxbuffer;
    }

Answer 2

Try using: SDAVAssetExportSession

SDAVAssetExportSession on GITHub

and then implementing a delegate to process the pixels

- (void)exportSession:(SDAVAssetExportSession *)exportSession renderFrame:(CVPixelBufferRef)pixelBuffer withPresentationTime:(CMTime)presentationTime toBuffer:(CVPixelBufferRef)renderBuffer

{ Do CIImage and CIFilter inside here }