If we record the video from our app and need to crop that video in the square shape on any other specific frame then use the following code-

In this code, we need the URL of the video that can be local URL because when we record any video it is stored locally on our iPhone. We are using the AVAsset here which is used by the AVFoundation framework.

First, we need to load our movie asset and create an avassetrack with our asset. Then create a video composition and pre-set some settings.

Next step requires to set the render size of video composition.this render size needs the height and width of your avassettrack and this refers to the required size after cropping the video.

Here the main thing is height of the render size. If we use the default navigation bar in our app then the height will be same as the avassettrack otherwise we need to subtract the height of custom view from natural height of avassettrack.

After that, we need to set some settings like portrait and the path where we export the cropped video.Here is the code-

- (void)cropVideoSquare:(NSURL*)outputUrl{
    
    //load our movie Asset
    AVAsset *asset = [AVAsset assetWithURL:outputUrl];
    
    //create an avassetrack with our asset
    AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    
    //create a video composition and preset some settings
    AVMutableVideoComposition* videoComposition = [AVMutableVideoComposition videoComposition];
    videoComposition.frameDuration = CMTimeMake(1, 30);
    //here we are setting its render size to its height x height (Square)
    videoComposition.renderSize = CGSizeMake(clipVideoTrack.naturalSize.height, clipVideoTrack.naturalSize.height-64);
    
    //create a video instruction
    AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    instruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(60, 30));
    
    AVMutableVideoCompositionLayerInstruction* transformer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:clipVideoTrack];
    
    //Here we shift the viewing square up to the TOP of the video so we only see the top
    CGAffineTransform t1 = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.height, 0 );
    t1.ty=0.0;
    //Use this code if you want the viewing square to be in the middle of the video
    //CGAffineTransform t1 = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.height, -(clipVideoTrack.naturalSize.width - clipVideoTrack.naturalSize.height) /2 );
    
    //Make sure the square is portrait
    CGAffineTransform t2 = CGAffineTransformRotate(t1, M_PI_2);
    
    CGAffineTransform finalTransform = t2;
    [transformer setTransform:finalTransform atTime:kCMTimeZero];
    
    //add the transformer layer instructions, then add to video composition
    instruction.layerInstructions = [NSArray arrayWithObject:transformer];
    videoComposition.instructions = [NSArray arrayWithObject: instruction];
    
    //Create an Export Path to store the cropped video
    NSString * documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    NSString *exportPath = [documentsPath stringByAppendingFormat:@"/CroppedVideo.mp4"];
    NSURL *exportUrl = [NSURL fileURLWithPath:exportPath];
    
    //Remove any prevouis videos at that path
    [[NSFileManager defaultManager]  removeItemAtURL:exportUrl error:nil];
    
    //Export
    exporter = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetHighestQuality] ;
    exporter.videoComposition = videoComposition;
    exporter.outputURL = exportUrl;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    
    [exporter exportAsynchronouslyWithCompletionHandler:^
     {
         dispatch_async(dispatch_get_main_queue(), ^{
             //Call when finished

             uploadedVideoPath=[exportUrl absoluteString];

         });
     }];
}