There's a strange behaviour I've found when trying to merge videos with AVFoundation. I'm pretty sure that I've made a mistake somewhere but I'm too blind to see it. My goal is just to merge 4 videos (later there will be crossfade transition between them). Everytime I'm trying to export video I get this error:
Error Domain=AVFoundationErrorDomain Code=-11821 "Cannot Decode" UserInfo=0x7fd94073cc30 {NSLocalizedDescription=Cannot Decode, NSLocalizedFailureReason=The media data could not be decoded. It may be damaged.}
The funniest thing is that if I don't provide AVAssetExportSession with AVMutableVideoComposition, then everything works fine! I can't understand what I'm doing wrong. The source videos are downloaded from youtube and have .mp4 extension. I can play them with MPMoviePlayerController. While checking the source code, please, look carefully at AVMutableVideoComposition. I was testing this code in Xcode 6.0.1 on iOS simulator.
#import "VideoStitcher.h"
#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>
#import <AssetsLibrary/AssetsLibrary.h>
@implementation VideoStitcher
{
VideoStitcherCompletionBlock _completionBlock;
AVMutableComposition *_composition;
AVMutableVideoComposition *_videoComposition;
}
- (instancetype)init
{
self = [super init];
if (self)
{
_composition = [AVMutableComposition composition];
_videoComposition = [AVMutableVideoComposition videoComposition];
}
return self;
}
- (void)compileVideoWithAssets:(NSArray *)assets completion:(VideoStitcherCompletionBlock)completion
{
_completionBlock = [completion copy];
if (assets == nil || assets.count < 2)
{
// We need at least two video to make a stitch, right?
NSAssert(NO, @"VideoStitcher: assets parameter is nil or has not enough items in it");
}
else
{
[self composeAssets:assets];
if (_composition != nil) // if stitching went good and no errors were found
[self exportComposition];
}
}
- (void)composeAssets:(NSArray *)assets
{
AVMutableCompositionTrack *compositionVideoTrack = [_composition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
NSError *compositionError = nil;
CMTime currentTime = kCMTimeZero;
AVAsset *asset = nil;
for (int i = (int)assets.count - 1; i >= 0; i--) //For some reason videos are compiled in reverse order. Find the bug later. 06.10.14
{
asset = assets[i];
AVAssetTrack *assetVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] firstObject];
BOOL success = [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, assetVideoTrack.timeRange.duration)
ofTrack:assetVideoTrack
atTime:currentTime
error:&compositionError];
if (success)
{
CMTimeAdd(currentTime, asset.duration);
}
else
{
NSLog(@"VideoStitcher: something went wrong during inserting time range in composition");
if (compositionError != nil)
{
NSLog(@"%@", compositionError);
_completionBlock(nil, compositionError);
_composition = nil;
return;
}
}
}
AVMutableVideoCompositionInstruction *videoCompositionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
videoCompositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, _composition.duration);
videoCompositionInstruction.backgroundColor = [[UIColor redColor] CGColor];
_videoComposition.instructions = @[videoCompositionInstruction];
_videoComposition.renderSize = [self calculateOptimalRenderSizeFromAssets:assets];
_videoComposition.frameDuration = CMTimeMake(1, 600);
}
- (void)exportComposition
{
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *myPathDocs = [documentsDirectory stringByAppendingPathComponent:@"testVideo.mov"];
NSURL *url = [NSURL fileURLWithPath:myPathDocs];
NSString *filePath = [url path];
NSFileManager *fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:filePath]) {
NSError *error;
if ([fileManager removeItemAtPath:filePath error:&error] == NO) {
NSLog(@"removeItemAtPath %@ error:%@", filePath, error);
}
}
AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:_composition
presetName:AVAssetExportPreset1280x720];
exporter.outputURL = url;
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.shouldOptimizeForNetworkUse = YES;
exporter.videoComposition = _videoComposition;
[exporter exportAsynchronouslyWithCompletionHandler:^{
[self exportDidFinish:exporter];
}];
}
- (void)exportDidFinish:(AVAssetExportSession*)session
{
NSLog(@"%li", session.status);
if (session.status == AVAssetExportSessionStatusCompleted)
{
NSURL *outputURL = session.outputURL;
// time to call delegate methods, but for testing purposes we save the video in 'photos' app
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
if ([library videoAtPathIsCompatibleWithSavedPhotosAlbum:outputURL])
{
[library writeVideoAtPathToSavedPhotosAlbum:outputURL completionBlock:^(NSURL *assetURL, NSError *error){
if (error == nil)
{
NSLog(@"successfully saved video");
}
else
{
NSLog(@"saving video failed.\n%@", error);
}
}];
}
}
else if (session.status == AVAssetExportSessionStatusFailed)
{
NSLog(@"VideoStitcher: exporting failed.\n%@", session.error);
}
}
- (CGSize)calculateOptimalRenderSizeFromAssets:(NSArray *)assets
{
AVAsset *firstAsset = assets[0];
AVAssetTrack *firstAssetVideoTrack = [[firstAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];
CGFloat maxWidth = firstAssetVideoTrack.naturalSize.height;
CGFloat maxHeight = firstAssetVideoTrack.naturalSize.width;
for (AVAsset *asset in assets)
{
AVAssetTrack *assetVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] firstObject];
if (assetVideoTrack.naturalSize.width > maxWidth)
maxWidth = assetVideoTrack.naturalSize.width;
if (assetVideoTrack.naturalSize.height > maxHeight)
maxHeight = assetVideoTrack.naturalSize.height;
}
return CGSizeMake(maxWidth, maxHeight);
}
@end
Thank you for your attention. I am really tired, I've been trying to find the bug for four hours straight. I'll go to sleep now.
I've finally found the solution. The description of error lead me in the wrong direction: "Cannot Decode. The media data could not be decoded. It may be damaged.". From this description you may think that there is something wrong with your video files. I've spent 5 hours experimenting with formats, debugging and etc.
Well, THE ANSWER IS COMPLETELY DIFFERENT!
My mistake was that I forgot that CMTimeADD() returns value. I thought that it changes the value of its first argument, and in the code you can see this:
CMTime currentTime = kCMTimeZero;
for (int i = (int)assets.count - 1; i >= 0; i--)
{
CMTimeAdd(currentTime, asset.duration); //HERE!! I don't actually increment the value! currentTime is always kCMTimeZero
}
videoCompositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, _composition.duration); // And that's where everything breaks!
The lesson that I've learned: When working with AVFoundation always check your time values! It's very important, otherwise you'll get a lot of bugs.