Re: Experimental DSM-based terrain renderer with streamed orthophotos

I've also added initial support for the Czech Republic and done some 
performance optimizations. I've updated the page with screenshots. 
Downloading tiles seems to be a bit less reliable for CZ than for NL but 
it's great to have some more variation in terrain! It does appear I have 
some bugs in sampling the terrain for the simulation though, so if 
you've got problems with the runways I recommend you start in the air.

Switzerland would also be great to add - they do have the open data - 
but I haven't found a service where I can download the height data at 
lower resolution / bigger area. That's something that's required to have 
terrain streaming work well. If anyone knows of such a source let me 
know! Recommendations for other countries are welcome too, WMTS ortho 
combined with WCS/ArcGIS height would be easiest to use.

Regards,

Gerard

On 31-05-2026 14:13, Gerard Krol wrote:
> Hi all!
>
>
>
> This terrain renderer streams orthophotos and height data at runtime 
> and renders them using a quadtree-based terrain system with 
> displacement-mapped meshes.
>
> Screenshots are available at https://gerardkrol.nl/flightgear/
>
> Testing instructions:
> - clone https://gitlab.com/gckrol/flightgear and checkout the 
> `dsm-stable` branch
> - build as usual
> - select a location in The Netherlands (recommended: EHAM,EHSB)
> - start with the `--terrain-engine=dsm` commandline flag (put this in 
> Settings > Additional Settings in the launcher).
> - terrain will automatically be streamed in
> - happy flying!
>
> Performance
> Runs at 20-30 FPS on my 8 year old laptop at 1080p. I haven't measured 
> it in FlightGear but the terrain should take only about 150MB VRAM, 
> due to the wonders of BC1 texture compression.
>
> Licenses of used data
> - Aerial photo's: CC-BY 4.0 PDOK/Kadaster NL. Rate limits/etc. are not 
> published. They do request a Referer header (and I assume a User-Agent 
> one as well).
> https://www.pdok.nl/introductie/-/article/pdok-luchtfoto-rgb-open-
> - Heightmap data (AHN): Open data (no restrictions)
> https://www.pdok.nl/introductie/-/article/actueel-hoogtebestand-nederland-ahn 
>
>
> Limitations
> - The code is currently experimental and needs cleanup, documentation 
> and adaptation to FlightGear coding standards. Most of it is AI 
> written and has had only limited review by me.
> - NL only, but should be easily extended to different regions if data 
> is available in WMTS, WCS or comparable standards.
> - Sides of buildings sometimes look a bit weird on the ground or when 
> at low altitude - I've experimented with some shader tricks in a 
> separate project and this can probably be made less distracting.
> - Roofs of tall buildings may be misaligned due to the available data 
> not being 'true ortho'. In a separate project I've experimented with 
> manually/automatically aligning the data, and this improves 
> appearance. This is outside the scope for now.
> - This does not have any controls for cache management, LOD settings 
> etc. All could be easily added.
> - Always uses the default/basic fog, even when ALS is enabled.
> - Files are processed/compressed at runtime, even when loaded from the 
> cache.
> - Photo's are taken during the day - so it looks slightly weird to see 
> shadows under an overcast sky. I've experimented with a 'day for 
> night' filter though and a moonlit night still looks fine. Also - 
> shadows are baked in of course, so your nice sunrise/sunset will have 
> mismatched shadows.
> - I'm already doing some processing on the heightmap tiles, but 
> artefacts are still present in some locations, often in water. This 
> should be fixable with some better processing logic.
> - The branch is based on FG 'stable', so would have to be transposed 
> onto 'next'. As this is a quite isolated terrain system I'm not 
> expecting major problems.
>
> Hacking
> If you want to start hacking on this (which I would highly encourage), 
> here are some starting points:
> - The new code is in the terrain_dsm* files.
> - The WMTS url for the orthophotos is built in 
> terrain_dsm_tile_source.cxx:169
> - The WCS url for the height data is built in 
> terrain_dsm_height_source.cxx:248
> - Note: the renderer currently only supports the Dutch epsg:28992 
> coordinates, so you'll probably have to adjust the code.
> - Downloaded tiles are cached in the FG download folder under DSM 
> (default ~/.fgfs)
> - To load higher detail tiles in the distance, increase the 4.0 factor 
> in terrain_dsm_orthophoto.cxx:shouldSubdivide
>
> Next steps
> - Try it and discuss it!
> - Figure out if this has the potential to be merged, and what's needed 
> for it. Definitely needs settings and support for different regions (I 
> believe ortho/height data is available for a lot of countries).
>
> I'll keep working on this for a limited time, but I would definitely 
> welcome other contributors if people think the approach has potential.
>
> Let met know what you think!
>
> Regards,
>
> Gerard
>
>
>
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