Legacy ImageMagick Discussions Archive

IM 6.7.3.1 Q16 Mac OSX Tiger

One of my scripts, nearestcolor, is failing now. I think I have traced the issue down to compare not working properly. It seems to always return the first pixel in a very simple compare operation. I am testing one color to see if compare can find it in a 3 color image with one pixel for each color.

convert -size 1x1 xc:white xc:red xc:black +append tmp.gif
compare -metric rmse -dissimilarity-threshold 100% -subimage-search \
tmp.gif \( -size 1x1 xc:red \) null:
30893.5 (0.471405) @ 0,0

result should be a perfect match to pixel 0,1


Similarly if I switch the first and last (white and black pixels), it still returns the first pixel

convert -size 1x1 xc:black xc:red xc:white +append tmp.gif
compare -metric rmse -dissimilarity-threshold 100% -subimage-search \
tmp.gif \( -size 1x1 xc:red \) null:
21845 (0.333333) @ 0,0


This also fails:
convert -size 1x1 xc:black xc:red xc:white +append brw.png
convert -size 1x1 xc:red red1.png
compare -metric rmse -dissimilarity-threshold 100% -subimage-search brw.png red1.png null:
21845 (0.333333) @ 0,0

As does this:

compare -metric rmse -dissimilarity-threshold 1 -subimage-search brw.png red1.png null:
21845 (0.333333) @ 0,0


My suspicion has to do with a 1D search. Note this works fine:

convert rose: rose.png
convert rose: -gravity center -crop 20x20+0+0 +repage rose_c20.png
compare -metric rmse -dissimilarity-threshold 100% -subimage-search rose.png rose_c20.png null:
0 (0) @ 25,13



A second issue is a complete failure, if I try to create the 3x1 3 color image in a parenthesis (yet the one parenthesis works fine as above)

compare -metric rmse -dissimilarity-threshold 100% -subimage-search \
\( -size 1x1 xc:white xc:red xc:black -append \) \( -size 1x1 xc:red \) null:
compare: unrecognized option `-append' @ error/compare.c/CompareImageCommand/371.

Any information or confirmation of this issue, yet?

This also fails. I had thought perhaps it was a 1D issue, but this shows it is not.


convert -size 10x10 xc:black xc:red xc:white +append brw10.png
convert -size 10x10 xc:red red10.png
compare -metric rmse -dissimilarity-threshold 100% -subimage-search brw10.png red10.png null:
21845 (0.333333) @ 0,0

The correct result should be at 11,0.

ImageMagick uses the normalized correlation coefficient for template matching. NCC fails for images of constant color. We'll need to check for constant color and revert to distance squared. Look for a patch in ImageMagick 6.7.3-2 Beta within a few days.

This used to work in my nearestcolor script when I first created it in 4/2/2009 (about IM 6.5.1.x). So I take it you made change to use NCC at some point from rms dist. So does that mean it is ignoring -metric rmse or just reporting the metric even though the match is done by ncc?

Why don't you make the compare use the -metric for the type of matching. I thought that was what it was for. So if one wants to use ncc

compare -metric ncc -subimage-match image1 image2 result

or if one wants to use rms distance

compare -metric rmse -subimage-match image1 image2 result

Is that practical?


I don't understand how you will know from the images whether it is constant color, unless you only look at the small image to see if it is constant color. I guess that is what you mean.


Any way, thank for looking into this and fixing it.

P.S

You must have changed it at about:

2010-11-23 6.6.6-0 Cristy <quetzlzacatenango@image...>
Support NCC (normalized cross correlation) compare metric.

We considered -metric as a solution to this problem and it seems you concur. Give us a few days and we'll get a patch in ImageMagick 6.7.3-2 Beta. Thanks.

I also always thought that "compare" always did vector color difference comparisons for images, and sub-image searches. If it is not what I have been telling people on the forums has been wrong.

I would also prefer to have it still do default searches using the defined metric (and its default).
As such a "ncc" metric would be an acceptable solution.

Though if "ncc" is only for sub-image searching then a warning should be produced when it isn't a sub-image search. The reference manual also needs to be updated.

As part of IMv7 I will be looking at methods to merge the special uses of "compare", "montage", etc as operations in "magick" (yet to be created) This however is still a long way off, as I am still working though existing options, modifying them so I can apply them in the strict 'as I see them' order. Also making lots of notes about where/how various settings set stored/used, and making adjustments. Eventually this should make for a much more exactly reference manual.