Bug 9240 - After scaling UIImage.CGImage returns a CGImage with twice the width and height of the scaled UIImage
Summary: After scaling UIImage.CGImage returns a CGImage with twice the width and heig...
Alias: None
Product: iOS
Classification: Xamarin
Component: Xamarin.iOS.dll ()
Version: 6.0.x
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Bugzilla
Depends on:
Reported: 2013-01-03 13:57 UTC by Ben Holmes
Modified: 2013-01-14 09:27 UTC (History)
2 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Ben Holmes 2013-01-03 13:57:34 UTC
Scaling a UIImage with UIImage.Scale returns a UIImage with the correct size but when getting the CGImage reference from the object its size 2x larger in both width and height.


UIImage tempImg = UIImage.FromFile("textImg.jpg");

tempImg = tempImg.Scale (new SizeF (100f, 100f));


This example returns the following:


The same is true when this is done in-place like so:

UIImage tempImg = UIImage.FromFile("textImg.jpg");

Debug.Print(tempImg.Scale (new SizeF (100f, 100f).CGImage.Width.ToString());
Debug.Print(tempImg.Scale (new SizeF (100f, 100f).CGImage.Height.ToString());
Comment 1 Sebastien Pouliot 2013-01-03 21:16:57 UTC
I made a test using an existing (32x32) image and I cannot reproduce this.

			string file = Path.Combine (NSBundle.MainBundle.ResourcePath, "basn3p08.png");
			var original = UIImage.FromFile (file);
			Assert.That (original.CGImage.Width, Is.EqualTo (32f), "Width-a");
			Assert.That (original.CGImage.Height, Is.EqualTo (32f), "Height-a");
			SizeF half = new SizeF (16f, 16f);
			original = original.Scale (half);
			Assert.That (original.CGImage.Width, Is.EqualTo (16), "Width-b");
			Assert.That (original.CGImage.Height, Is.EqualTo (16), "Height-b");

Note that your 2nd code snippet won't compile (the parenthesis don't match).

> Debug.Print(tempImg.Scale (new SizeF (100f, 100f).CGImage.Width.ToString());
> Debug.Print(tempImg.Scale (new SizeF (100f, 100f).CGImage.Height.ToString());

So maybe your project was not rebuilded before being executed ? and you're seeing old values being printed (e.g. from a previous binary) ?

If not then please attach a small test case, including the textImg.jpg, file that you used so we can investigate this further.
Comment 2 Sebastien Pouliot 2013-01-14 09:27:05 UTC
This seems to happen only on devices and AFAICT it's an iOS optimization. IOW the backing image is not modified until (and if) it's needed.

Such optimization can save memory (and CPU time) on several scenarios, e.g.:
* several sizing/rotating operations can be merged into one;
* hardware acceleration (GPU) might not require the backing image to be resized in memory;

However that should not affect your code, i.e. if you scale an `UIImage` then you should look at _it's_ Size property (not the backing image size).