It would be neat to do something like this but by detecting high-frequency regions of the image (possibly using a wavelet transform) and placing smaller circles in areas with more high-frequency detail.

Here's a stippling implementation in Processing, inspired by the above:

https://github.com/jordanorelli/stippling

Something that might be really interesting: do it with moving video rather than a single image. There are several ways it might work, each of which could look very different.

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