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From: "Stephan Assmus" <stippi@yellowbites.com>
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Subject: [Tiff] Alpha channel worries
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Date: Wed, 21 Jan 2004 00:17:12 +0100 CET
X-UIDL: 28abce53577da7e8974db4c45f44f944
Status: U

Hi all,

I have observed some weird effects when trying to save images with 
alpha channel and loading them later. The loading routine of the 
OpenBeOS TIFFTranslator just uses TIFFReadRGBAImage(). Images 
previously saved with that same translator, seem to have been modified 
to have the color channels associated with the alpha channel. This 
happens although I specify EXTRASAMPLE_UNASSALPHA for the alpha channel 
when saving. Is TIFFReadRGBAImage() specifically converting the RGB 
channels to be associated with the alpha channel (because libtiff 
assumes that's what the application wants)? Or might there be something 
wrong with me setting up the fields to specify EXTRASAMPLE_UNASSALPHA?

This is the code that sets up saving RGBA images:

	uint16 extraSamples[1];
	extraSamples[0] = EXTRASAMPLE_UNASSALPHA;
	TIFFSetField(tif, TIFFTAG_EXTRASAMPLES, 1, extraSamples);
	TIFFSetField(tif, TIFFTAG_SAMPLESPERPIXEL, 4);


Best regards,
-Stephan
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