[gdal-dev] GDAL alpha channels

[Frank Warmerdam]

Craig,

My opinion is that applications should assume alpha is not pre-multiplied.
I have been hesitant to "un-premultiply" premultiplied TIFF images - partly
because it is hard to hit all the appropriate code paths safely and partly
because the result will be lossy.

Going forward GDAL should do one of two things:

1) provide a metadata item so applications can know those rare
circumstances when alpha is pre-multiplied.

2) un-premultiply premultiplied alpha when encountered.

As you note it doesn't really matter when alpha is primarily used
for on/off masking so we have gotten away over the years with
only a few complaints about the topic.


On Tue, May 29, 2012 at 2:00 PM, Craig Bruce <csbruce at cubewerx.com> wrote:
> Tamas Szekeres <szekerest at gmail.com> wrote:
>
>> I have added a ticked for such an issue long ago, but I don't remember
>> what would be the expected solution.
>> http://trac.osgeo.org/gdal/ticket/2279
>>
>> There should be an external config option to instruct the driver which
>> representation should be used.
>
> I think it would be better for GDAL to present a single alpha model through
> its interface, as it does with the pixel-is-area vs. pixel-is-point
> issue.  As far as I am aware, TIFF is the only common format that uses
> a pre-multiplied alpha.  However, pre-multiplied alphas present a number
> of problems, including strange or non-integration with color models other
> than RGB, and throwing away useful information.  For example, if you take
> a pre-multiplied RGBA TIFF and need to write it out to a nodata-valued
> output format (effectively, a 1-bit alpha channel), you need to recreate
> the information that pre-multiplication threw away, and this can only be
> done approximately.  The better fixed model to use is non-pre-multiplied,
> like PNG uses.
>
> At the moment, all GDAL applications process alpha values incorrectly
> (since it's not possible _to_ process them correctly).  I'm not sure
> if many people have noticed this or if or what they've done about it,
> if anything.  The problem may be obscured because for sources with
> alpha values of only 0 and MAX, there really isn't a problem.  Do any
> applications expect pre-multiplied alphas?  If so, they're only getting
> them from some TIFF images and are processing other sources incorrectly.
>
> --------------------------+----------------------+--------------------------
> Dr. Craig S. Bruce        | Ph 819-771-8303 x205 |             CubeWerx Inc.
> Senior Software Developer |   Fax 819-771-8388   |  Gatineau, Québec, Canada
> csbruce at cubewerx.com      |  http://csbruce.com/ |  http://www.cubewerx.com/
> --------------------------+----------------------+--------------------------
>            "All the world's a player and we are merely stages."
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