Swift - Rotate an image, then zoom in and crop to keep width/height aspect ratio intact

Question

If the title isn't clear, open Photos app on your iPhone and edit a picture. Try the rotation tool. The frame don't rotate, only the image does and zoom in to stay in the frame.

Here is a picture of this:

I would like to copy this comportment in my Swift project. It means rotating an image (it can be UIImage, CIImage or CGImage) but crop it keeping the same width/height ratio.

Where should I begin ? Thanks for your time.

Answer 1

There's an app a filter for that. CIStraightenFilter rotates an image by any amount while stretching it and cropping it to maintain the original dimensions:

let image: CIImage = /* my image 480x320 */
let rotated = image.applyingFilter("CIStraightenFilter", 
    withInputParameters: [ kCIInputAngleKey: Double.pi / 6 ]) // 30 degrees 
rotated.extent // still 480x320

Here's some examples (rotations of zero, 15°, 30°, and 45°):