[gdal-dev] can GDAL render two images as if over-plotted?

[Even Rouault]

Michael,

I'm not totally sure if image composition really belongs to gdaldem. It 
feels it is more generic than that, although making nice looking DEM 
rendering is definitely a common use case. There's this long existing 
Python script at 
https://github.com/OSGeo/gdal/blob/master/swig/python/gdal-utils/osgeo_utils/samples/hsv_merge.py 
that combines the hillshade and color-relief together.

In the ecosystem at large, ImageMagick has also a lot of compositing 
operators: https://imagemagick.org/Usage/compose/

If we added a utility in GDAL, it would have to be decided if it allows 
different georeferencing/extent of images to be composed together. That 
would likely brings a bit of complexity, although using on-the-fly VRT 
underneath could help making images with the same size, resolution, extent.

There's also the question of if we would need this to be able to operate 
in a VRT-ish way (a new subset of VRT would then be needed), because 
that would have a strong impact on the design, compared to having a 
utility that materializes the output in a regular file format.

Even


Le 09/07/2024 à 03:12, Michael Sumner via gdal-dev a écrit :
> Just an update, I have found out how to do this numerically.    Will 
> consider a PR for gdaldem if I can make it work at some point.
>
> Thanks, Mike
>
>
>
> On Tue, Jul 9, 2024 at 7:31 AM Michael Sumner <mdsumner at gmail.com> wrote:
>
>     Is it possible to "layer" one image over another, the first at
>     full transparency and the second with partial transparency?
>
>     I'm thinking of combining a hillshade and a (partially)
>     transparent color-relief image, a visual enhancement technique. 
>      (I know how to do this downstream in R and Python or similar, I'd
>     like to stay in C++'s warm embrace. )
>
>     I can go and find out what "over plot" means when it comes to
>     colour imagery, just wondering if there's anything in-built
>     already.   This might be a nice additional mode to gdaldem (and is
>     what I thought "color-relief" meant,  at first).
>
>     Cheers, Mike
>
>
>     -- 
>     Michael Sumner
>     Research Software Engineer
>     Australian Antarctic Division
>     Hobart, Australia
>     e-mail: mdsumner at gmail.com
>
>
>
> -- 
> Michael Sumner
> Research Software Engineer
> Australian Antarctic Division
> Hobart, Australia
> e-mail: mdsumner at gmail.com
>
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