Tamas Szekeres <[email protected]> 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 [email protected] | http://csbruce.com/ | http://www.cubewerx.com/ --------------------------+----------------------+-------------------------- "All the world's a player and we are merely stages." _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
