Legacy ImageMagick Discussions Archive

Hello, everybody! :)

I'm unable to get an FX-expression to work in combination with other commands, like "roll" or "crop".

For e.g., have a look at the following bash-script, where i try to roll an image of unknown(!) size 10% to the right:
As you can see above, theoretically I could declare a ROLL variable containing my FX-expression I got with "-format", but that would mean, that I call the convert process two times, which is not very efficient in my opinion.

Actually this posted script is a heavily stripped down version of my real problem, and in reality I need much more values than this ROLL thing. If I would create a single variable every time - which means an extra process, which reads the whole image again and again - isn't it a waste of resources? :)

So finally, my question is as follows: How can I for e.g. roll my image around 10% horizontally, using an FX expression?

Thank you very much for reading, and I hope, someone will find an answer to my question.

Have a nice day! :)

You have to do it in two commands. Most of IM 6 does not recognize inline fx expressions. The main ones that do are related to -distort. IM 7 will be more flexible in that regard.

You can do the equivalent of -roll with inline fx expressions if you use -distort SRT. See http://www.imagemagick.org/Usage/distorts/#srt. However, to make it wrap, you need to use -virtual-pixel HorizontalTile or VerticalTile see convert -list virtual-pixel.

To do a crop inline, you would use +distort SRT with -define distort:viewport. see http://www.imagemagick.org/Usage/distor ... t_viewport

For both use -filter point before -distort.

Thank you very much for your comments so far!

But isn't the -distort SRT much(!) more heavy weight internally? A simple roll should be much more efficient, right? Or is SRT optimized for this kind of situations?

Anyway I will try your solutions now, and maybe I'll compile the 7 alpha later.

Again, thank you! And have a nice day!

Edit: "alpha", not "beta"

Yes, you are right. distort SRT is not optimized like -roll or -crop. The -filter point does help to make it faster and avoid blurring.

Using two command lines is not really adding too much time, since you are just reading the width and height. You can add -ping to identify or convert so that it only looks at the headers and does not read the whole image. see http://www.imagemagick.org/Usage/basics/#controls

I am not sure why you are using page.width. Do you have a virtual canvas with offsets? If not, you can get the image width or height from just w or h in fx. Other image parameters can be achieved with string format, but would require another fx expression to do computations on them. See http://www.imagemagick.org/script/escape.php

P.S. -ping must come before the image in the command line

From Windows cmd, we can call "convert" once and read the image once, to set multiple environment variables from multiple escapes or fx calculations.

I suspect this is also possible from bash, but I haven't figured out how.

An example of generating 'calculated rolls' using Distort and FX expressions is in Animations

Animated Distorts - distorting multiple image based on image index
http://www.imagemagick.org/Usage/anim_mods/#distort

The key to making it roll is to use a tiling virtual pixel setting. That is defining what pixels outside the image proper should be.