How to center a PNG with a transparent background using the CLI

command linegraphicsimage editing

I'd like to come up with a technique for properly posting images on this site on my Macbook using a CLI tool to do the conversion.

This is a sample image that I'd like to make it so that the image is properly sized with a background that's transparent and makes it so that when the image is posted to AskDifferent or other StackExchange sites, that the image will be large enough with a transparent background that no additional tricks are required to center images within posts on these sites.

For reference this is the question that started this:
– Have DHCP for Wi-Fi and static IP for Ethernet?

Original

This is the image that without any tricks shows up as left justified

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Goal

Another user helped me out and posted a improved version of this image that he used GIMP to merge a transparent background which resulted in this:

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NOTE: To be clear, I'm familiar with the techniques covered in posts such as this mSO post on the topic, titled: Is it possible to horizontally center an image on Stack Exchange sites?.

Best Answer

Background

I've used ImageMagick in the past to do pretty extensive image manipulations from the CLI. I've written about it extensively here on the Unix & Linux StackExchange site.

Specifically, here's a method I highlighted in this question titled: How to convert, resize and center image with ImageMagick.

Approach

The approach discussed there could be adapted to achieve a solution to this question like so:

$ convert 807634040.png \
    -gravity center \
    -background transparent \
    -extent 678x 807634040-new.png

Resulting in an image like this:

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How it works

The convert command is expanding the original image, 807634040.png, so that its width is now 678 pixels. This looks to be the optimal size to use when posting images on SE sites. The -extents 678x means to use the width of 678px but leave the horizontal as whatever it was originally set.

The other options mean to center the image (-gravity center) and to use a transparent background when extending the original.

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