AVAssetTrack's preferredTransform sometimes seems to be wrong. How can this be fixed?

user_1821701 0
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I'm using AVAssetExportSession to export videos in an iOS app. To render the videos in their correct orientation, I'm using AVAssetTrack's preferredTransform. For some source videos, this property seems to have a wrong value, and the video appears offset or completely black in the result. How can I fix this?

avassetexportsession avfoundation ios
2 Answers

The preferredTransform is a CGAffineTransform. The properties a, b, c, d are concatenations of reflection and rotation matrices, and the properties tx and ty describe a translation. In all cases that I observed with an incorrect preferredTransform, the reflection/rotation part appeared to be correct, and only the translation part contained wrong values. A reliable fix seems to be to inspect a, b, c, d (eight cases in total, each corresponding to a case in UIImageOrientation) and update tx and ty accordingly:

extension AVAssetTrack {
  var fixedPreferredTransform: CGAffineTransform {
    var t = preferredTransform
    switch(t.a, t.b, t.c, t.d) {
    case (1, 0, 0, 1):
      t.tx = 0
      t.ty = 0
    case (1, 0, 0, -1):
      t.tx = 0
      t.ty = naturalSize.height
    case (-1, 0, 0, 1):
      t.tx = naturalSize.width
      t.ty = 0
    case (-1, 0, 0, -1):
      t.tx = naturalSize.width
      t.ty = naturalSize.height
    case (0, -1, 1, 0):
      t.tx = 0
      t.ty = naturalSize.width
    case (0, 1, -1, 0):
      t.tx = naturalSize.height
      t.ty = 0
    case (0, 1, 1, 0):
      t.tx = 0
      t.ty = 0
    case (0, -1, -1, 0):
      t.tx = naturalSize.height
      t.ty = naturalSize.width
    default:
      break
    }
    return t
  }
}
user_18217010

I ended up doing something slightly more robust I think, I nullified the transform based on where it would end up:

auto naturalFrame = CGRectMake(0, 0, naturalSize.width, naturalSize.height);
auto preferredFrame = CGRectApplyAffineTransform(naturalFrame, preferredTransform);
preferredTransform.tx -= preferredFrame.origin.x;
preferredTransform.ty -= preferredFrame.origin.y;

Note that you can't just apply the transform on (0, 0) since CGRect.origin takes into account things like flipping.

user_51146270
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