(Batch) resize images to fit within circle

Question

How can I automatically resize hundreds of images so that each fits within the same-sized circle?

I have images (icons, in PNG format) with various shapes. I will be compositing these each with an identical circle background. So, I need to resize the images such that all the axes, in every degree, of the shape fits within a circle of specified size.

Most image resizing tools will resize assuming a rectangular shape. With differing shapes, this can result in an image that has been made too small with too much padding to the circle's edge; or that is too large, and overlaps the circle's edge.

What tool (I might use Gimp or Inkscape, but general discussion is also fine) can I use to automatically resize these images to each entirely fit within a circle, with their largest axis in any direction fitting to a specified circle?

Answer 1

In general the process is this:

1. Loop over your set of PNG images, and for each one:
2. Get the current dimensions (width & height)
3. Calculate the scaling factor
4. Apply the scale transformation
5. Write the new image out to disk

For step 3, the formula involves the Pythagorean theorem.

Given W = width, H = height, and R = radius of the circle, the scaling factor would be: 2*R/sqrt(W^2+H^2)

To actually implement this, I would recommend trying ImageMagick or writing some Python code.

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Using ImageMagick 7, processing a single image to fit a circle of diameter 123 can be done with:

magick input.png -precision 15 -resize "%[fx:100*123/hypot(w,h)]\%" output.png

To batch process an entire folder on Windows, use a BAT file like:

set DIAMETER=123
for %%I in ( *.png ) do (
   magick "%%I" -precision 15 -resize "%%[fx:100*%DIAMETER%/hypot(w,h)]\%%" "output\%%I"
)