Smooth edges on gdal_trace_outline about dans-gdal-scripts HOT 1 CLOSED

Richard,

Is the ugliness only apparent when you zoom in to the scale of
individual pixels? It is my guess that this is the case. There is
currently no option in gdal_trace_outline to do this directly. If your
goal is to have a polygon that looks nice at this level of detail, then
here is a proposed solution (untested):

1. convert your image to a bitmap image (black and white, no gray)
2. expand the resolution of this image by a factor of 10 or more (if
   this will fit in memory!) using your favorite type of interpolation
   (probably cubic or lanczos, definitely not nearest neighbor or linear).
   This can be done using gdalwarp, or your favorite image editor
   application. Optionally hit it with a Gaussian blur.
3. threshold this image to again be black and white (no gray). The
   outline should hopefully be smooth.
4. run gdal_trace_outline, trying various settings for the "-dp-toler"
   flag. Maybe set dp-toler to be equal to the amount you expanded the
   resolution by, or perhaps twice this amount.

If the ugliness is apparent at larger scales (not just at the scale of a
couple of pixels) then something is wrong and you should send me a
portion of your image that reproduces the problem.

There is a program called "potrace" (http://potrace.sourceforge.net/)
that creates artistically nice looking vector images from bitmaps. I
have never used it, but there is a possibility that it may work well for
your purposes.

• Dan

On 07/15/2013 10:28 AM, Richard Fairhurst wrote:

gdal_trace_outline is a godsend but tends to produce very jagged edges
from SRTM data. Applying the Douglas-Peucker simplification results in
sharp bends at corners and unnatural-looking shapes. Turning this off
produces sawtooth edges.

It would be good to have more 'natural'-looking smoothing, similar to
the output produced (in raster form) by gdaldem.

There are some ideas in this thread
http://osgeo-org.1560.x6.nabble.com/gdal-dev-gdal-polygonize-and-polygon-edges-td5035292.html
(which is how I found gdal_trace_outline!). Right now I'm using a
combination of ogr2ogr and Mapnik's line-smooth, which produces
reasonable results, but it would be better to have it in
gdal_trace_outline.

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