Rendering artifacts with linear interpolation in gdal_grid

[moorglade]

Expected behavior and actual behavior.

I use gdal_grid and gdaldem to create a colorized GeoTIFF from simple .csv data files. In some cases, I observe strange rendering artifacts when using the linear interpolation algorithm.

For example, this is a correct (expected) rendering of a sample file using the nearest algorithm:

When I switch to the linear algorithm, I get unexpected rendering artifacts (notice the red bars at the top):

Steps to reproduce the problem.

I uploaded example input files here.

1. Generate GeoTIFF from samples.csv & samples.vrt:

   gdal_grid \
     -ot Float64 \
     -of GTiff \
     -txe 16.870876 19.691624 \
     -tye 54.980014 53.746311 \
     -outsize 682 512 \
     -a linear:radius=0.0:nodata=-9999.0 \
     samples.vrt samples.tiff

2. Colorize the GeoTIFF file:

   gdaldem \
     color-relief \
     samples.tiff colormap samples-colored.tiff

Some other things I noticed:

• this problem only happens for some specific map areas / input data sets
• the artifacts are not present if I flip the output GeoTIFF by providing -tye parameters in the reverse order, i.e. -tye 53.746311 54.980014 instead of -tye 54.980014 53.746311 - so maybe the issue is somehow related to negative pixel dimensions

Operating system

Ubuntu 18.04 64 bit

I also replicated the problem inside a CentOS 7.4 Docker container.

GDAL version and provenance

GDAL 2.3.0 compiled from source.

[rouault]

Attaching input dataset here as bug638.zip

[moorglade]

I don't know if it helps, but I did some debugging, and it seems the problem occurs when computing Delaunay triangulation coefficients in GDALTriangulationComputeBarycentricCoefficients.

Basically, there is a single triangle for which dfDenom is 0.0. The points of the triangle appear to be collinear:

• P1 = (19.1864624, 54.9262161)
• P2 = (19.2020874, 54.9262695)
• P3 = (19.1942749, 54.9262428)