iOS 4: video quality with AVAsset based application - ios

Question

I'm trying to create a simple video application(loading existing video file from ios 4 device, edit it using direct pixel access and save under a different name).
I managed to load, edit and save my movie file on real device(ipod 4g). The only problem that I have is related to movie quality(original vs edited one). I don't know what am I doing wrong, but quality of my output file is very bad comparing to the input one.
Below you can find how am I loading my movie:
// // *** tmp file ***
NSURL *movieUrl = [info objectForKey:#"UIImagePickerControllerMediaURL"];
NSLog(#"picker controller movie url: %#", [movieUrl absoluteString]);
## picker controller movie url: /private/var/mobile/Applications/6253D1-8C0-41-B780-638250817/tmp//trim.2q03uz.MOV
AVURLAsset *movieAsset = [[AVURLAsset alloc] initWithURL:movieUrl options:nil];
CGSize size = [movieAsset naturalSize];
NSLog(#"movie asset natual size: size.width = %f size.height = %f", size.width, size.height);
## movie asset natual size: size.width = 224.000000 size.height = 128.000000
// *** ASSET READER ***
AVAssetReader *assetReader = [[AVAssetReader alloc] initWithAsset:movieAsset error:&error];
NSArray* videoTracks = [movieAsset tracksWithMediaType:AVMediaTypeVideo];
// asset track
AVAssetTrack* videoTrack = [videoTracks objectAtIndex:0];
NSDictionary* dictionary = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange]
forKey:(id)kCVPixelBufferPixelFormatTypeKey];
NSLog(#"video track %f %f %d", [videoTrack naturalSize].width, [videoTrack naturalSize].height, [videoTrack nominalFrameRate]);
## video track 224.000000 128.000000 0
/*
also tried
kCVPixelFormatType_32BGRA
kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange
*/
// asset reader track output
AVAssetReaderTrackOutput* assetReaderOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:videoTrack
outputSettings:dictionary];
if(![assetReader canAddOutput:assetReaderOutput])
NSLog(#"unable to add reader output");
else
[assetReader addOutput:assetReaderOutput];
// *** ASSET WRITER ***
AVAssetWriter *videoWriter = [[AVAssetWriter alloc] initWithURL:exportURL
fileType:AVFileTypeQuickTimeMovie error:&error];
NSParameterAssert(videoWriter);
NSLog(#"asset writer %d %d", [videoWriter status], [error code]);
NSDictionary *videoCompressionProps = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithDouble:128.0*1024.0], AVVideoAverageBitRateKey,
nil];
NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
AVVideoCodecH264, AVVideoCodecKey,
[NSNumber numberWithInt:size.width], AVVideoWidthKey,
[NSNumber numberWithInt:size.height], AVVideoHeightKey,
//videoCompressionProps, AVVideoCompressionPropertiesKey,<- no difference at all
nil];
AVAssetWriterInput* writerInput = [[AVAssetWriterInput
assetWriterInputWithMediaType:AVMediaTypeVideo
outputSettings:videoSettings] retain];
// set preffered transform for output video
writerInput.transform = [videoTrack preferredTransform];
NSParameterAssert(writerInput);
NSParameterAssert([videoWriter canAddInput:writerInput]);
[videoWriter addInput:writerInput];
writerInput.expectsMediaDataInRealTime = NO;
Then I simply read next sample buffer, modify pixel data and save it with a different name. That already works, but output quality is so bad...
Please guys give me some advice:-).

Answer 1

Try setting:
imagePickerController.videoQuality = UIImagePickerControllerQualityTypeHigh;
before picking the movie.

Answer 2

It looks like you are using the UIImagePickerController. That will compress your video I believe, resulting in the bad quality you're getting.
Also, you may consider setting expectsMediaDataInRealTime to YES, to optimize the dropping of frames.