Re: Problem with Image mask

Subject: Re: Problem with Image mask

From: David Duncan <email@hidden>

Date: Sat, 11 Sep 2010 17:50:42 -0700

Delivered-to: email@hidden

On Sep 11, 2010, at 4:45 PM, Paul Reilly wrote:

I have an app which cuts out shapes from a picture. It works great - I can cut the shape using a mask, with the code shown below. My problem is, if I save the cut out image to disk, it has to go through NSData with UIImagePNGRepresentation and this seems to be reversing the effect of my mask. You can see the result in the following picture.

https://sites.google.com/site/cocoacodepaul/

Can someone tell me where I am going wrong? Why does saving the UIImage to disk (going through NSData) reverse the effect of my image mask?

CGImageCreateMask and CGImageCreate generate CGImages that when used for masking are treated as inverses of each other (this is described in the documentation for CGImageClipToMask). For consistency I would recommend adopting the the "inverse" definition used by creating an image via CGImageCreate, which basically amounts to using the image's alpha as a multiplier against the incoming pixels (vs CGImageCreateMask which first inverts the alpha value).

Reality is what, when you stop believing in it, doesn't go away.

Failure is not an option. It is a privilege reserved for those who try.

David Duncan

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>Problem with Image mask (From: Paul Reilly <email@hidden>)