[Image-SIG] Question on TIFF EXTRASAMPLES tag support
Frank Warmerdam
warmerdam at pobox.com
Wed Sep 10 16:16:35 EDT 2003
Tom Bridgman wrote:
...
> Apparently, this file has an incompatible 'EXTRASAMPLES'=1 flag but this
> appears to be indicating the presence of an alpha channel.
>
> After checking some additional references, I'm wondering what's the
> difference between the EXTRASAMPLES=1 and RGBA? If there is no
> significant difference, couldn't support for this format be installed by
> just adding:
>
> (2, 1, (8,8,8,8), (1,)): ("RGBA", "RGBA"),
>
> to the OPEN_INFO list in TiffImagePlugin.py?
>
> Can anyone enlighten me?
>
> Should this be installed in the distribution?
>
> Other comments/suggestions? I'm no expert on PIL - I'm making this
> determination based on one afternoon of study...
Tom,
extrasamples=1 is defined in the specification as being an associated alpha
data with pre-multiplied color components. The text reads:
This field must have a value of 1 (associated alpha data with pre-multiplied color
components). The associated alpha data stored in component SamplesPerPixel-1
of each pixel contains the opacity of that pixel, and the color information is pre-
multiplied by alpha.
I gather this means that the RGB color values have already been multiplied
with the alpha value before storing in the file though this seems a bit
weird to me.
The specification is at:
ftp://ftp.remotesensing.org/pub/libtiff/TIFF6.pdf
Page 77 is what I was looking at. There is lots more discussion.
Best regards,
--
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I set the clouds in motion - turn up | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for Rent
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