Re: Reading RGBA pixel values from image on disk
Re: Reading RGBA pixel values from image on disk
- Subject: Re: Reading RGBA pixel values from image on disk
- From: Leonardo <email@hidden>
- Date: Sat, 12 Mar 2011 03:03:31 +0100
- Thread-topic: Reading RGBA pixel values from image on disk
The tif file preserves the transparency. I can quit re-open with Photoshop
and the transparent pixels are always there. Anyway,
I have found a workaround: I transform the TIF representation to a PNG one:
pngData = [tifRep representationUsingType:NSPNGFileType
pngRep = [NSBitmapImageRep imageRepWithData:pngData];
Then I get the right RGBA values. It takes a longer time but it's great.
Thank you to everybody.
Regards
-- Leonardo
> Da: Quincey Morris <email@hidden>
> Data: Thu, 10 Mar 2011 11:07:30 -0800
> A: Leonardo <email@hidden>
> Cc: <email@hidden>
> Oggetto: Re: Reading RGBA pixel values from image on disk
>
> On Mar 10, 2011, at 10:25, Leonardo wrote:
>
>> representations count 1
>> colorSpaceA sRGB IEC61966-2.1 colorspace
>> isPlanar 0
>> numberOfPlanes 1
>> NSAlphaNonpremultipliedBitmapFormat 0
>> hasAlpha 0
>> bitsPPixel 24
>> bytesPerPixel 3
>
> Yes, but what is samplesPerPixel? 3 or 4?
>
>> The pixel I have set on Photoshop as RGBA
>> 0.00, 1.00, 0.00, 0.20
>> becomes in my App as
>> 0.80, 1.00, 0.80, 1.00
>> which clearly means it has been pre-multiplied by a white opaque color
>
> No, if it had been *premultiplied*, the alpha value would still be 0.2. It's
> been *composited* onto a white opaque background.
>
>> 1.00, 1.00, 1.00, 1.00
>> final = (src * 0.2) + (dst * (1.0 - 0.2))
>>
>> I would like to know where the alpha info has gone within the file
>
> It sure looks like Photoshop flattened the file and output it without
> transparency.
>
>
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