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Feathering and interpolation advice.

ozbigbenozbigben Global Mapper UserPosts: 9
edited July 2014 in Elevation Data
I'm working on merging multiple data sources together for eventual use in a CG rendering application and could use some advice on getting the best possible exports from GM. Whilst accuracy is nice, producing smoother terrains for later use is the prime consideration. I'm working with GM14.2

What I have so far:
  • Load all of the DTMs in GM and sort by resolution.
  • To remove sharp edges from the borders of DTMs with very different resolutions (e.g. 75m and 1.9m) I feather the edges, masked to available data (sinusoidal input, lat/long output)
  • Export multiple geotiffs at different zoom levels using current resolution and bounds of the window size

Some challenges:
1. Overlapping feathered borders get pixellated compared to their appearance on screen. To workaround this I hide terrains until there is no overlap between any DTMs, export the data at the resolution of the finest DTM, import that into GM, hide the layers used to create that and display the remaining layers (again so there is no overlap), rinse and repeat until all of the data has been blended. This is a bit fiddly, but I can put up with it. If there's a better way I'm happy to take suggestions.

2. I'm getting a bit of stepping/artefacts in the feathered regions The default export uses bilinear and I tried bicubic and it was different, although still obvious. Is this possibly a limitation of the feathering process or would one of the other resamplng methods be more suitable? (I'll keep trying) I don't mind a slight loss in resolution if the terrain is smooth.

Some samples: 2000px feather, exported as 32bit geotiff, 1.9m Hirise on the left, MOLA128 on the right

bilnear-feather.jpg
bilinear

bicubic-feather.jpg
bicubic

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2014
    The stepping is a strange artifact. If you use the Path Profile Tool and draw a path over the bumpy area does it step up and down or is it smooth there? I'm wondering if there is some small precision stepping in the GeoTIFF (or source data), resulting in tiny steps that are insignificant in terms of the actual elevation values, but that result in shadowing effects as the hill shading only considers the surface normal at each sample location and doesn't do full (i.e. very slow) raw tracing.

    There is a new option in v15.2 on the Vertical Options tab of the Configuration dialog to perform hill shading from multiple different light directions and take the min/max/average of the calculated shadow intensity. Trying that option may get rid of some of the strange visual effects caused by small stair-steps in the actual elevation data. You can get v15.2 from Global Mapper Downloads and get it a try.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • ozbigbenozbigben Global Mapper User Posts: 9
    edited July 2014
    Looks like there is some small stepping in the data. That's saved me a lot of time investigating :).

    I have a workaround for masking layers of data in Terragen but I'll give 15.2 a try as this would reduce the amount of redundant data I'd have to load.

    Thanks

    Ben
  • ozbigbenozbigben Global Mapper User Posts: 9
    edited July 2014
    The profiles looked smoother in v15 and the improved performance was enough to convince me to upgrade anyway. I have a large render running at the moment, but I'll test it out during the week.
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