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I want to use the imagemagick convert command to draw on a image file. I have the image data as raw-text in a buffer and now I would like to run convert - -draw 'rectangle 0,0,100,100' - on it (the dashes tell convert to use stdin for input and print the data to stdout). If I leave out the -draw 'rectangle 0,0,100,100' then I can just use (call-process-region (point-min) (point-max) "convert" t t nil "-" "-") to pass the buffer contents as stdin to the convert program and insert the from convert returned image data in the buffer (this does not do anything but it works, e.g. I could replace the last - with a filename to write the resulting image data to a file). However now I want to add the -draw 'rectangle 0,0,100,100' as an extra argument to call-process-region but I really can not find (out) how to do this properly. For example I tried (call-process-region (point-min) (point-max) "convert" t t nil "-" "-draw" "'rectangle 0,0,50,50'" "-") but then I get message convert: non-conforming drawing primitive definition rectangle 0,0,50,50' @ error/draw.c/RenderMVGContent/4473.`

I tried to run it as a shell-command (i.e. using /bin/bash -c)? But then I don't know how to pass the buffer contents as input.

So I would like to know how to properly pass the single-quotes string 'rectangle 0,0,100,100' to the command.

(FYI I would like to extend djvu.el to show annotations)

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I would like to run convert - -draw 'rectangle 0,0,100,100' -

Note that you're quoting for the shell there, so that it will not break that argument into two, on account of the space.

For example I tried (call-process-region (point-min) (point-max) "convert" t t nil "-" "-draw" "'rectangle 0,0,50,50'" "-")

Note that you've included the quotes which are for the shell in a call which is not being processed by a shell, which means that the convert process is receiving an argument containing those ' surrounding characters.

That is like running this shell command:

convert - -draw "'rectangle 0,0,100,100'" -

Try:

(call-process-region (point-min) (point-max) "convert" t t nil
                     "-" "-draw" "rectangle 0,0,50,50" "-")
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  • Thanks for the quick answer and nice explanation. It took me a long way to get to this point (trying many things before) and because I really spent too much time already I thought I should just ask here. Then, just after I posted this question I found the function call-shell-region which worked for me also. But anyway, your answer helps me to understand why things work how they work, which is very important to me. Great!
    – dalanicolai
    Commented Sep 15, 2020 at 13:42

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