[Image-SIG] Question on TIFF EXTRASAMPLES tag support

[Tom Bridgman]

Frank Warmerdam wrote:
> > 
> 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.
> 
This makes sense for some types of compositing.

Reading this, it looks like the flag is set more as information to any
software that reads the file and does additional processing - the image
data itself is unaffected.  In this case, trying Chris Cogdon's
suggestion is probably the best possible suggestion.

Thanks,
Tom
--
Dr. William T."Tom" Bridgman           Scientific Visualization Studio
Global Science & Technology, Inc.      NASA/Goddard Space Flight Center
Email: bridgman at wyeth.gsfc.nasa.gov    Code 935
Phone: 301-286-1346                    Greenbelt, MD 20771
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