Legacy ImageMagick Discussions Archive

Hi, sorry, this may be a simple question, but I cannot find the answer after lots of searching!

Is it possible to use a mathematical expression that would be evaluated and replaced by a number within a command? I see all the math operators available for the FX operator, but I don’t think that’s what I should be using here…

For example, if I wanted to resize an image to a width that is equal to 16 to the 3rd power: This obviously doesn’t work, not only since the ^ character has a different meaning in the context of image size. Would this, or other math expressions, be possible via a variable of some sort? Or by using some alternate syntax or enclosure?

I need this for passing a command line argument to a shell alias—resizing to $1^3, for example.

Thank you for any help!

What version of IM, on what platform?

If you use IM V7, you can use "%[fx:...]" expressions such as: This is Windows syntax, where caret ^ is a special character so must be escaped by ^. For bash, don't double the ^.

FX expressions can use variables defined with IM, such as w for image width.

snibgo shows how to do it in IM 7, which has more capabilities. In IM 6, you need to compute the final variable first, then use it in the resize such as
or

Thank you snibgo and fmw42!

Sorry for not including it—I'm using version 7.0.8-3 via Mac Terminal. I think the fx expression plus percent escape is what I want, but IM returns an "invalid argument" when I try %[fx:20^2], %[fx:20^^2], "%[fx:20^2]", "%[fx:20^^2]", or %\[fx:20^2\]… How should this be correctly formatted?

Please show the exact command you used, and the exact error message.

returns: "convert: invalid argument for option '-resize': %[fx:20^2] @ error/convert.c/ConvertImageCommand/2609"

Using double carets or quotes around it returns a similar error.

In Unix, you must enclose %[fx:20^2] in double quotes as per my example above.

convert input.jpg -resize "%[fx:20^2]" output.jpg

I did try that, but it returns the exact same error as before (the double quotes are not regurgitated in the error message).

Also don't work: single quotes; escaping the brackets and/or caret with backslashes

Sorry my typo, you can only do that in IM 7 as described above. But you do need the double quotes. In IM 6, you have to compute a variable, then use the variable in the -resize.

IM 7 IM6

Thanks fmw42! Your code for IM6 works for me if I omit the usual backslash escapes for parentheses that I have to use within IM commands. The IM7 code won't work despite my version being 7.0.8-3.

I do not have \ on those parentheses. They are Unix parentheses not ImageMagick parentheses. In Unix for a computation using ImageMagick, you can use $(...) or backticks `...`

Imagemagick parentheses must include \( and \) and have spaces on each side.

If you are not using Unix as in Linux or Mac OSX, then for example in Windows, parentheses should not be escaped by \

WorldOfDepth wrote:convert input.jpg -resize %[fx:20^2] output.jpg

I used "magick", not "convert". Try again with "magick".

Snibgo, wow, I overlooked that, and indeed it works with magick, and even without double quotes! I'm just so used to convert and composite as my defaults. Thanks so much, and thanks fmw42 again for the option via a variable.

I do not understand. You need the double quotes on a Mac. Are you sure you are using Unix on a Mac and not some Windows emulator on the Mac.

Fmw42, I'm using the utility program Terminal in OSX Yosemite. The window is labeled bash. I believe it's Unix.

The %[fx:20^2] appears to works within a 'magick' command regardless of whether or not it's enclosed in double quotes.