iOS zoom for recording movies - ios

Question

I am using a AVAssetWriter to record movie frames.
I was wondering how I would go about zooming/scaling a frame (it can be set at the beginning of the movie, it does not have to be on a per frame basis..)
I assume I should be able to do that with the CMSampleBuffer that append to by assetWriterInput
Psydocode:
CMSampleBufferRef scaledSampleBuffer = [self scale:sampleBuffer];
[_videoWriterInput appendSampleBuffer:scaledSampleBuffer];
I have found some stackoverflow questions on this:
This one should create a vImage_Buffer, but I have no idea how I get that into my _videoWriterInput:
Stackoverflow
I tried to convert it back with something like this:
NSInteger cropX0 = 100,
cropY0 = 100,
cropHeight = 100,
cropWidth = 100,
outWidth = 480,
outHeight = 480;
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
CVPixelBufferLockBaseAddress(imageBuffer,0);
void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
vImage_Buffer inBuff;
inBuff.height = cropHeight;
inBuff.width = cropWidth;
inBuff.rowBytes = bytesPerRow;
NSInteger startpos = cropY0*bytesPerRow+4*cropX0;
inBuff.data = baseAddress+startpos;
unsigned char *outImg= (unsigned char*)malloc(4*outWidth*outHeight);
vImage_Buffer outBuff = {outImg, outHeight, outWidth, 4*outWidth};
vImage_Error err = vImageScale_ARGB8888(&inBuff, &outBuff, NULL, 0);
if (err != kvImageNoError) NSLog(#" error %ld", err);
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
CVPixelBufferRef pixelBuffer;
CVPixelBufferCreate(kCFAllocatorSystemDefault, 480, 480, kCVPixelFormatType_32BGRA, NULL, &pixelBuffer);
CVPixelBufferLockBaseAddress( pixelBuffer, 0 );
vImage_CGImageFormat format = {
.bitsPerComponent = 8,
.bitsPerPixel = 32,
.bitmapInfo = kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipFirst, //BGRX8888
.colorSpace = NULL, //sRGB
};
vImageBuffer_CopyToCVPixelBuffer(&outBuff,
&format,
pixelBuffer,
NULL,
NULL,
kvImageNoFlags);
CVPixelBufferUnlockBaseAddress( pixelBuffer, 0 );
CMSampleTimingInfo sampleTime = {
.duration = CMSampleBufferGetDuration(sampleBuffer),
.presentationTimeStamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer),
.decodeTimeStamp = CMSampleBufferGetDecodeTimeStamp(sampleBuffer)
};
CMVideoFormatDescriptionRef videoInfo = NULL;
CMVideoFormatDescriptionCreateForImageBuffer(kCFAllocatorDefault, pixelBuffer, &videoInfo);
CMSampleBufferRef oBuf;
CMSampleBufferCreateForImageBuffer(kCFAllocatorDefault, pixelBuffer, true, NULL, NULL, videoInfo, &sampleTime, &oBuf);
But that doesn't work either (corrupted frames)
Copied together form other Stackoverflow articles, I came up with this code, but it results in a broken video:
CGAffineTransform transform = CGAffineTransformMakeScale(2, 2); // zoom 2x
CIImage *ciImage = [CIImage imageWithCVPixelBuffer:CMSampleBufferGetImageBuffer(sampleBuffer)
options:[NSDictionary dictionaryWithObjectsAndKeys:[NSNull null], kCIImageColorSpace, nil]];
ciImage = [[ciImage imageByApplyingTransform:transform] imageByCroppingToRect:CGRectMake(0, 0, 480, 480)];
CVPixelBufferRef pixelBuffer;
CVPixelBufferCreate(kCFAllocatorSystemDefault, 480, 480, kCVPixelFormatType_32BGRA, NULL, &pixelBuffer);
CVPixelBufferLockBaseAddress( pixelBuffer, 0 );
CIContext * ciContext = [CIContext contextWithOptions: nil];
[ciContext render:ciImage toCVPixelBuffer:pixelBuffer];
CVPixelBufferUnlockBaseAddress( pixelBuffer, 0 );
CMSampleTimingInfo sampleTime = {
.duration = CMSampleBufferGetDuration(sampleBuffer),
.presentationTimeStamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer),
.decodeTimeStamp = CMSampleBufferGetDecodeTimeStamp(sampleBuffer)
};
CMVideoFormatDescriptionRef videoInfo = NULL;
CMVideoFormatDescriptionCreateForImageBuffer(kCFAllocatorDefault, pixelBuffer, &videoInfo);
CMSampleBufferRef oBuf;
CMSampleBufferCreateForImageBuffer(kCFAllocatorDefault, pixelBuffer, true, NULL, NULL, videoInfo, &sampleTime, &oBuf);
Any ideas?