I frequently do things like draw a diagram in a presentation app and take screenshots of it to include into other programs. I’ll create a diagram and then take screenshots of steps of its evolution to show how I built it, step by step. Here’s how I crop to the same location and size every time, perfectly and easily.

Here’s the scenario. Suppose I want to take screenshots of something, say, a diagram for a presentation. I’m going to evolve this diagram as I narrate the presentation. First I’ll show a simple version, then a more complicated version, on subsequent slides. It’ll look like this, slide by slide:

Three Steps

I want the picture to be in the same place and the same size, not shift around as I flip between slides. How can I do this?

My secret weapon is the imagemagick suite of command-line tools and a thick black border. I’ll draw the first version of the diagram, inside a thick black box. Like this:

With Border

Then I start changing the image. Every time I get it to a state I want to snapshot, I screenshot it by pressing Cmd-Shift-4 (I use a Mac) and clicking and dragging the screenshot region within the border. It doesn’t matter exactly where inside the border; I make the border thick to make it easy. All that matters is that the edge of the resulting screenshot is totally black, that it doesn’t have any margins that are different-colored.

When I’m done, I’ll have a series of screenshots of different images, of slightly different sizes, with irregular black borders around the edges. Now I’m going to trim the black borders off with ImageMagick’s -trim command, leaving the images perfectly sized and located within the area inside the border:

for img in Screen*; do convert $img -trim $img; done

And voila, the border is gone. The -trim command works by deleting the color of the corner pixels, shrinking the image perfectly.

In general, if you wonder whether ImageMagick can do something, the answer is usually yes. For example, to make this post, I took three images of the diagram and then created the final product with convert Screen* +append step-N.jpg. You can do pretty much anything with ImageMagick.

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