Image Mask from Grayscale JPEG

Subject: Image Mask from Grayscale JPEG

From: "R. Scott Thompson" <email@hidden>

Date: Sun, 15 May 2005 00:15:19 -0500

Delivered-to: email@hidden

OK... I'm trying to do something I've never done before, draw with an image mask. It's turning out to be more challenging than I would have expected.

I have a grayscale JPEG (a lovely, blurry circle) and I want to use that as an Image Mask. Can load the file's pixel data into a CGImage, through ImageIO, but that creates an Image (i.e. a CGImage with a color space) and not an image mask (which I gather is a CGImage with no color space).

What I don't know is what is the best way to convert the Image into an Image Mask.

The most obvious way to do what I want, I believe, is going to be to create a grayscale bitmap context, draw the image I want to use as my mask into that bitmap, then use CGCreateImageMask to create a mask from the pixels. That seems like a very round-about way to create a mask from an image.

I tried using CGImageCreateWithMask and passed it the image I want to mask, and my masking image. Unfortunately the call always fails.

At one point, I thought it was because the file is tagged with the Generic Gray color space and the documentation says that CGImageCreateWithMask requires the image you use as a mask to be Device Gray. To work around this, I tried using CGImageCreateCopyWithColorSpace to create a copy of my mask data in the device gray space... but CGImageCreateWithMask still fails. (no explanation given... even if I use the debug library prefix version). :-(

Is there an easy way to transform a grayscale image into an image mask without having to draw it in a buffer first?

Scott

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Re: Image Mask from Grayscale JPEG
- From: Nick Nallick <email@hidden>