-convolve with a zero sum (edge extraction) kernel is not behaving as it used to. The problem started at IM 6.7.6.6 from my tests. Anthony thinks it may have reverted to the GPU version rather than his -morphology version.
Input:
Filter:
filt="
-1 -2 -1 \
-2 12 -2 \
-1 -2 -1 \
"
Recent versions after IM 6.7.6.5 including IM 6.7.8.7 produce the following:
convert lenag.jpg -convolve "$filt" lenag_filt_imb.jpg
Even adding color management does not help
convert lenag.jpg -set colorspace RGB -convolve "$filt" lenag_filt_imc.jpg
Older version of IM (before 6.7.6.6) with -convolve produce:
im6 convert lenag.jpg -convolve "$filt" lenag_filt_im6.jpg
IM 6.7.8.7 using -morphology convolve or correlate produce:
convert lenag.jpg -morphology correlate "$filt" lenag_filt_morph.jpg
