To create the 512x512 texture, did the following:

% unu resample -k box --size 512 512 -i ddro-80.nrrd -o ddr-512.nrrd
% unu minmax ddr-512.nrrd
min: -7.470736
max: 64.226639
% unu affine -7.470736 ddr-512.nrrd 64.226639 0 1 \
  | unu quantize -i - -b 8 \
  | unu save -i - -f png -o ddro-tx.png