objective c - AvAssetReader and Writer to overlay video - objective-c

Question

I am trying to overlay a recorded video with AvAssetReader and AvAssetWriter with some images. Following this tutorial, I am able to copy a video (and audio) into a new file. Now my objective is to overlay some of the initial video frames with some images with this code:
while ([assetWriterVideoInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next video sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [assetReaderVideoOutput copyNextSampleBuffer];
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
CVPixelBufferLockBaseAddress(pixelBuffer, 0);
EAGLContext *eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
CIContext *ciContext = [CIContext contextWithEAGLContext:eaglContext options:#{kCIContextWorkingColorSpace : [NSNull null]}];
UIFont *font = [UIFont fontWithName:#"Helvetica" size:40];
NSDictionary *attributes = #{NSFontAttributeName:font, NSForegroundColorAttributeName:[UIColor lightTextColor]};
UIImage *img = [self imageFromText:#"test" :attributes];
CIImage *filteredImage = [[CIImage alloc] initWithCGImage:img.CGImage];
[ciContext render:filteredImage toCVPixelBuffer:pixelBuffer bounds:[filteredImage extent] colorSpace:CGColorSpaceCreateDeviceRGB()];
CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
if (sampleBuffer != NULL)
{
BOOL success = [assetWriterVideoInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
And to create image from text:
-(UIImage *)imageFromText:(NSString *)text :(NSDictionary *)attributes{
CGSize size = [text sizeWithAttributes:attributes];
UIGraphicsBeginImageContextWithOptions(size, NO, 0.0);
[text drawAtPoint:CGPointMake(0.0, 0.0) withAttributes:attributes];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return image;
}
The video and audio are copied, but I haven't any text on my video.
Question 1: Why this code is not working?
Moreover, I want to be able to check the timecode of the current read frame. For example I would like to insert a text with the current timecode in the video.
I try this code following this tutorial:
AVAsset *localAsset = [AVAsset assetWithURL:mURL];
NSError *localError;
AVAssetReader *assetReader = [[AVAssetReader alloc] initWithAsset:localAsset error:&localError];
BOOL success = (assetReader != nil);
// Create asset reader output for the first timecode track of the asset
if (success) {
AVAssetTrack *timecodeTrack = nil;
// Grab first timecode track, if the asset has them
NSArray *timecodeTracks = [localAsset tracksWithMediaType:AVMediaTypeTimecode];
if ([timecodeTracks count] > 0)
timecodeTrack = [timecodeTracks objectAtIndex:0];
if (timecodeTrack) {
AVAssetReaderTrackOutput *timecodeOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:timecodeTrack outputSettings:nil];
[assetReader addOutput:timecodeOutput];
} else {
NSLog(#"%# has no timecode tracks", localAsset);
}
}
But I get the log:
[...] has no timecode tracks
Question 2: Why my video hasn't any AVMediaTypeTimecode? Ad so how can I get the current frame timecode?
Thanks for your help

Answer 1

I found the solutions:
To overlay video frames, you need to fix the decompression settings:
NSString* key = (NSString*)kCVPixelBufferPixelFormatTypeKey;
NSNumber* value = [NSNumber numberWithUnsignedInt:kCVPixelFormatType_32BGRA];
NSDictionary* decompressionVideoSettings = [NSDictionary dictionaryWithObject:value forKey:key];
// If there is a video track to read, set the decompression settings for YUV and create the asset reader output.
assetReaderVideoOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetVideoTrack outputSettings:decompressionVideoSettings];
To get the frame timestamp, you have to read the video informations and then use a counter to increment the current timestamp:
durationSeconds = CMTimeGetSeconds(asset.duration);
timePerFrame = 1.0 / (Float64)assetVideoTrack.nominalFrameRate;
totalFrames = durationSeconds * assetVideoTrack.nominalFrameRate;
Then in this loop
while ([assetWriterVideoInput isReadyForMoreMediaData] && !completedOrFailed)
You can found the timestamp:
CMSampleBufferRef sampleBuffer = [assetReaderVideoOutput copyNextSampleBuffer];
if (sampleBuffer != NULL){
CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
if (pixelBuffer) {
Float64 secondsIn = ((float)counter/totalFrames)*durationSeconds;
CMTime imageTimeEstimate = CMTimeMakeWithSeconds(secondsIn, 600);
mergeTime = CMTimeGetSeconds(imageTimeEstimate);
counter++;
}
}
I hope it could help!