I’ve finished implementing a filter in G’MIC for stereographic projection. This is basically a method that projects image data defined on a sphere onto a plane (see wikipedia page for more details). In photography, one common use of such a filter is the generation of so-called Mini planets, from equirectangular panoramas.
So, this new G’MIC filter is available under ‘Deformations / Stereographic projection’.
Suppose you have an equirectangular panorama like the one below (taken from the awesome Flickr gallery of Alexandre Duret-Lutz)
The G’MIC filter ‘Stereographic projection’ looks like this.
It has a few controls to scale/rotate/dilate the result, as well as a cool option to blur left/right sides of your image, in case it is not perfectly periodic along the X-axis (avoiding visible seams/discontinuities in the resulting image)
Here is the obtained result (fortunately, it is close to what the preview shows ).
This kind of projection is way better than the simple ‘Euclidean to polar’ transform, as you don’t get a singular point at the projection center, and the mapping is conformal, meaning angles are preserved locally.
Here is a quick video showing the G’MIC filter in action, using GIMP (this works the same on Krita of course).
Finally, note that the back-projection method is also implemented, so you can retrieve an equi-rectangular image from a tiny planet image (at least most of it), as this is shown in the video.
Does it look good ? Any suggestion for improvement ?