[gdal-dev] Tiff file with CMYK colors that appears much worse with GDAL than in Windows Photo Viewer

[Lars Schylberg]

Hi,

When I have more advanced needs in tiff-processing I usally use 
ImageMagick "convert" command.

It has more options to do advanced things with images.

Convert will complain that it doesn't understand the geotiff tags, but 
that is something You could ignore, it is just warnings.

Use the the tifflib utilities to reapply the the geo information to the 
tif-file when You are done.

     listgeo -no_norm original.tif > original.geo

     geotifcp -g original.geo modified.tif modified_geotiff.tif

The ImageMagick identify command could also be a useful tool.

Hope this will help You in the right direction

/Lars Schylberg, Saab


Den 2020-01-21 kl. 15:28, skrev Mikael Rittri:
> Hello, list. I need some advice.
>
> We have a customer with some Tiff files with CMYK colors. It is a rasterized vector
> map, not an orthophoto, so the assigned colors are somewhat arbitrary, of course.
>
> The file colors look pleasant enough when opened with Windows Photo Viewer, and
> these are the correct colors according to the customer. When I open them with QGIS
> or Carmenta Engine, I get worse colors (but identical in QGIS and Carmenta Engine).
> Another GIS program I tried displays even worse colors.
>
> The bad colors appear lighter and more saturated, almost fluorescent, and some hues
> are clearly different. For example, the original has fairly dark, blue (and slightly green)
> texts in water, while the water areas are a very light blue. In QGIS and Carmenta Engine,
> the texts in water become saturated cyan and the water areas become a nearly white
> cyan. The third GIS program I tried shows the same saturated cyan for texts in water
> but an almost-saturated cyan for water areas, which gives very bad contrast.
>
> I understand that to convert from CMYK to RGB one needs to know the CMYK color
> profile (and the RGB color profile too, I guess).
>
> Window Properties says that the these Tiff files have "Color Representation: Uncalibrated".
>
> gdalinfo says, among other things, that
>
>    TIFFTAG_SOFTWARE=Adobe Photoshop CC (Windows)
>    SOURCE_COLOR_SPACE=CMYK
>
> and that the bands are Red/Green/Blue/Alpha. But I guess these band colors mean
> after automatic color conversion to RGB, since gdalinfo with "GTIFF_RAW:" prefixed
> to the file name says that bands are Cyan/Magenta/Yellow/Black.
>
> tiffinfo gives, among other things, a long XML text, x:xmpmeta, but the only color
> information I could find in it was
>
>      <photoshop:ColorMode>4</photoshop:ColorMode>
>
> Finally, I noticed a section about "Color Profile Metadata" at https://gdal.org/drivers/raster/gtiff.html,
> but I didn't understand how to use it.
>
> I guess my first question should be: how one would make the GDAL API respect the
> Color Profile Metadata in our tiff files? It seems the default setting does not respect some
> embedded CMYK profile that at least Windows Photo Viewer can find? Or is the problem
> that Windows Photo Viewer can pick up enough information from the XML line about
> photoshop:ColorMode = 4, while GDAL cannot find the CMYK profile from this kind of format?
>
> My understanding of color spaces is perhaps only halfway along https://xkcd.com/1882,
> so I probably need some pretty basic hints on where to learn more!
>
> Regards,
>
> Mikael Rittri
> Carmenta Geospatial Technologies
> www.carmenta.com
>
>
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