Re: Has My Image Been Premultiplied?
Re: Has My Image Been Premultiplied?
- Subject: Re: Has My Image Been Premultiplied?
- From: Jason Harris <email@hidden>
- Date: Fri, 21 Feb 2003 21:18:34 -0700
Scott Anguish Tried to Tell Me:
>
> Here's the problem: sometimes it doesn't. I tested this with about 50
>
> different images and found that about 90% of the time, the final image
>
> was
>
> premultiplied, but that 10% that isn't is the kicker.
>
>
No, NSBitmapImageRep always premultiplies. If you need to work with
>
unpremultiplied data, you have to use Quartz, specifically
>
CGImageCreate with kCGImageAlphaLast.
>
>
If you're seeing something other than that behavior, please send me
>
some example code.
Actually, I can demonstrate this without using any code at all. Create a
new NIB that contains two image wells (both editable). Run the NIB - you
can now drag TIFF images into the image wells.
I dragged one of my problem-images into the left image well and used Pixie
to snap a blowup screenshot of it. Into the right image well, I dragged the
same image after having added a single pixel of fully masked red color.
Again, I snapped a blown up screenshot using Pixie.
Result - one premultiplied screenshot, one unpremultiplied screenshot. The
runtime is not premultiplying all incoming images.
All of the images and my test NIB are attached. I hope someone can shed
some light on this. I'd hate to have to override private API to fix this.
Jason Harris
Geekspiff
[demime 0.98b removed an attachment of type application/x-stuffit which had a name of Bad Premultiplication.sit"; x-mac-creator="53495421"; x-mac-type="53495435]
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