Legacy ImageMagick Discussions Archive

Hi,

I'm curious to create a 'difference' between 2 images that can be applied to turn one into the other. I believe XOR should do this (?) and so "composite -compose difference" should do the trick (?)

In particular I'm curious whether such differences can be JPEG compressed and used to update a webcam server a bit more efficiently than just sending regular JPEGs - kind of like a rudimentary MPEG perhaps.

It seems to get almost there, but leaves a 'halo' around image elements which spoils the effect. Also some 'ghost' effects. I tried creating these simple example PNGs

Image1

Image2 - just moved down-right a little

difference (XOR)

Image1 XOR difference -> should be Image 2 ? But get artifacts.

Commands used -
I've tried using -quality 100 in case that helps too. Any suggestions appreciated

If I've got the wrong end of the stick somewhere, just let me know too Cheers!

"-compose" is a setting used by the "-composite" operation. The setting must come first. If there is no operation, the setting has no effect. So your commands are wrong.

"-compose Xor -composite" is different to "-compose Difference -composite".

Despite the name, "-compose Xor" does not simply apply the mathematical XOR operator to the channels, so the operation is not reversible in the way that the mathematical XOR is. See http://www.imagemagick.org/Usage/compose/#xor

Hi snibgo,

Thanks for your reply, though I am using '-difference' which I gather is the mathematical XOR rather than the other one you mention.

https://www.imagemagick.org/Usage/compose/#set_theory

I'm using the 'composite' command line on Raspberry Pi, which presumably fires off -composite as you mention.

I tried changing the order of commands as you suggested, but came out with the exact same results

I've just found this old post, which I'll read through about XOR - https://www.imagemagick.org/discourse-s ... hp?t=19995

Cheers

Alex

If the two images are black and white only, then "-compose Difference -composite" gives the same as a mathematical XOR. But "Difference" is actually the absolute value of one image minus the other, so for pixel values that are neither 0 nor 100%, the "Difference" is different to XOR. Your inputs have almost all pixel values 0 or 100%, but the anti-aliased pixels are neither.

If you really want XOR, then use "-evaluate-sequence Xor". Using your inputs:


As expected, xor_y.png is identical to your second input.

Thanks very much snibgo - that did indeed work!

Unfortunately trying it on a couple of similar photos from a series, generated a surprisingly complex and psychedelic 'difference' which took up more space than the original image to compress as a JPEG. Also the JPEG artifacts at quite high compression turned the final image into a bit of a surrealist painting Not quite what I was hoping, but a fun experiment thanks.

Photo 1 - compressed with mozjpeg -quality 40 -tune-ms-ssim

High Quality XOR of photo 1 and 2

XOR JPEG compressed with mozjpeg ms-ssim -quality 40

Re-created image for photo 2 using JPEG XOR

In the end, it will be simpler and more efficient just to send highly compressed JPEGs from my webcam

Thanks again!