Too much time to get and merge video frames and write movies - iphone

Question

I am building an app in which I have to get a thumbnail from a recorded video. I have an array of alpha video frames. I have to merge each set of frames and then make a movie with these final frames. Here is the piece of code I am dealing with:
-(void)createFrameForVideo
{
NSString *filePath = [NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:#"/Documents/movie.mp4"]];
NSURL *outputURL = [NSURL fileURLWithPath:filePath];
player = [[MPMoviePlayerController alloc]initWithContentURL:outputURL];
float frame = 0.00;
int count = 14;
NSFileManager *fileManager = [NSFileManager defaultManager];
NSString *docPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
docPath = [docPath stringByAppendingPathComponent:#"OutPut"];
BOOL success = [fileManager fileExistsAtPath:docPath];
if (success) {
[fileManager removeItemAtPath:docPath error:nil];
}
[fileManager createDirectoryAtPath:docPath withIntermediateDirectories:YES attributes:nil error:nil];
for (frame = (frameStartTime); frame < (frameStartTime+7); frame+=0.033)
{
#autoreleasepool
{
UIImage * singleFrameImage = [player thumbnailImageAtTime:frame timeOption:MPMovieTimeOptionExact];
[player pause];
NSString *imageName = [NSString stringWithFormat:#"export2%d.png",count];
NSString * file = [[NSBundle mainBundle]pathForResource:imageName ofType:nil];
UIImage *overlayImage = [UIImage imageWithData:[NSData dataWithContentsOfFile:file]];
count = count + 1;
NSString *imagePath = [NSString stringWithFormat:#"%#/%#", docPath, imageName];
if (overlayImage)
{
UIImage * outImage = [self mergeImage:singleFrameImage withImage:overlayImage];
NSData *imgData = [[NSData alloc] initWithData:UIImagePNGRepresentation(outImage)];
[fileManager createFileAtPath:imagePath contents:imgData attributes:nil];
[imgData release];
}
else
{
NSData *imgData = UIImagePNGRepresentation(singleFrameImage);
[fileManager createFileAtPath:imagePath contents:imgData attributes:nil];
}
[outputFramesArray addObject:imagePath];
}
}
[player release];
if([fileManager fileExistsAtPath:filePath])
{
[fileManager removeItemAtPath:filePath error:nil];
}
NSString *path = [NSHomeDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:#"/Documents/movie1.mp4"]];
NSLog(#"filePath %#", path);
if ([[NSFileManager defaultManager] fileExistsAtPath:path]) {
[[NSFileManager defaultManager] removeItemAtPath:path error:nil];
}
[self writeImageAsMovie:outputFramesArray toPath:path size:CGSizeMake(480, 320) duration:10];
NSLog(#"hello Your layering is completed");
[outputFramesArray removeAllObjects];
[outputFramesArray release];
success = [fileManager fileExistsAtPath:docPath];
if (success) {
[fileManager removeItemAtPath:docPath error:nil];
}
//[self performSelectorOnMainThread:#selector(CompileFilesToMakeMovieWithAudio) withObject:Nil waitUntilDone:YES];
[self CompileFilesToMakeMovieWithAudio];
// [self compileFinalOutputMovie];
}
The problem is its taking much time to deal with frames in the whole loop. Could anyone please help speed up the process. I already have tried ffmpeg, but I think the problem is in merging. If anyone has suggestions, please share them.

Answer 1

Try using instruments to see where your memory is getting tied up and if there are any leaks.
I had a similar problem recently with leaks in a piece of code that ran constantly in a loop like that. Changing the local variables into instance variables and then just reusing the variable kept it from allocating too much memory for the application.

Answer 2

Your issue is one of basic approach. If you want to get better execution time then you need to change the basic way you app works. There is not a specific "change this code" step that will make your existing code execute faster.
What you should try is to encode the video frame and then write directly into the compiled h.264, instead of what you do now which is to create each frame and then combine them all together at the end in another loop. Like so:
1: Read Input PNG
2: Read Video Frame
3: Combine Video frame and input PNG
4: Write combined frame to movie via AVAsset apis.
That avoids having to read each frame 2 times and it avoids writing all the PNG images to disk again. IO takes a lot of time and compressing the images to PNG takes a lot of time. The suggested approach would be a lot faster because it would avoid these unneeded steps.