Getting the 'scale' from a CATransform3D

Question

I am creating an iPad app and want to update the user with the current size of a view. I used to be doing this by calculating the scale of a CGAffineTransform and multiplying the xScale by the width (of the related view) and the yScale by the height. I'd like to keep doing this, but I'm no longer doing 2d transformations, and I'm not sure what equation to use to extract the scale information from CATransform3D.

Can you help?

score 22 · Accepted Answer · edited Sep 16 '13 at 12:59

22

To get the current scale of the layer you just perform a valueForKeyPath: on the layer:

CGFloat currentScale = [[layer valueForKeyPath: @"transform.scale"] floatValue];

Other keys can be found in Apples Core Animation Programming Guide

edited Sep 16 '13 at 12:59

Matthias Bauch

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answered Aug 22 '11 at 04:15

jlalvarez18

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Funny, always returns nil [[self presentationLayer] valueForKey:@"transform.scale"] – pronebird Nov 01 '13 at 12:30
2

Andy, that's because you are calling `valueForKey` and not `valueForKeyPath`. – Mark May 20 '17 at 16:41

score 8 · Answer 2 · answered Dec 14 '10 at 22:56

I'm not familiar with the API but CATransform3D looks like a regular 4x4 transformation matrix for doing 3D transformations.

Assuming that it represents nothing more than a combination of scale, rotation and translation, the scale factors can be extracted by calculating the magnitudes of either the rows or columns of the upper left 3x3 depending on whether CATransform3D is row or column major respectively.

For example, if it is row-major, the scale in the x-direction is the square root of ( m11 * m11 ) + ( m12 * m12 ) + ( m13 * m13 ). The y and z scales would similarly be the magnitudes of the second and third rows.

From the documentation for CATransform3DMakeTranslation it appears that CATransform3D is indeed row-major.

score 0 · Answer 3 · answered Nov 04 '19 at 16:07

Here's an update answer for swift 4+

guard let currentScaleX = view.layer.value(forKeyPath: "transform.scale.x") as? CGFloat {
    return 
}
print ("the scale x is \(currentScaleX)")
guard let currentScaleY = view.layer.value(forKeyPath: "transform.scale.y") as? CGFloat {
    return 
}
print ("the scale y is \(currentScaleY)")

Getting the 'scale' from a CATransform3D

3 Answers3