ImageMagick "Channel distortion: Undefined" message - windows

Question

I installed imagemagick for Windows from official site. I am trying to generate a diff image from two PNG images using ImageMagick. I am using following command:
compare file2.png file1.png -compose Src "diff.png"
But command exits with EXIT code "1". When run with -verbose flag, below message gets printed.
Image:file2.png Channel distortion: Undefined
I searched google for possible reason, but haven't found any answer. There is official ImageMagick documentation on distortion, but that is too wide and technical (and I am a novice).
Any idea why this message is thrown by imagemagick utility?

Answer 1

There is no real error. That message comes up when you do not specify -metric XXX with compare and take the default. The exit code 1 is what ImageMagick produces on success, I believe. See if you have an output file called diff.png. If you add -metric rmse, for example, you should not get that message. If I run the following without -metric XXX, I get:
compare -verbose lena.png lena.jpg -compose src diff.png
lena.png PNG 256x256 256x256+0+0 8-bit sRGB 118327B 0.010u 0:00.004
lena.jpg JPEG 256x256 256x256+0+0 8-bit sRGB 31640B 0.000u 0:00.002
Image: lena.png
Channel distortion: Undefined
lena.png=>diff.png PNG 256x256 256x256+0+0 8-bit sRGB 3c 1322B 0.130u 0:00.039
So it works fine. The message is telling you that it does not know which metric to use to give you difference statistics. I am not sure what it uses to generate the output. The term distortion may be a poor choice and perhaps should have been difference statistics.
If I add -metric rmse, then I get:
compare -verbose -metric rmse lena.png lena.jpg -compose src diff.png
lena.png PNG 256x256 256x256+0+0 8-bit sRGB 118327B 0.010u 0:00.004
lena.jpg JPEG 256x256 256x256+0+0 8-bit sRGB 31640B 0.000u 0:00.001
Image: lena.png
Channel distortion: RMSE
red: 810.821 (0.0123723)
green: 658.701 (0.0100511)
blue: 945.653 (0.0144297)
all: 813.547 (0.0124139)
Similar or same resulting image, but now the compare scores are reported since a specific metric is specified.
See the section about comparison statistics here