Cheap Small Planets

1Select the right shot

You will need a photo with a few characteristics to get a nice result:

you will need a photo that can be easily “simetrised”, you’ll see why later, but it is important that the photo has one main (or a block of) suject, ideally in the centre,
you need a photo with a good amount of sky and of land,
you need at least one side of your shot to be made only of sky and land, without any complex objects.

For this tutorial, we are beginning with a simple seaside shot, that has the right characteristics:

The photo is then cropped so that the main subjects are in the middle and the photo is already quite symmetrical with more sky and less ground.

You could apply the polar transformation filter now. In Gimp, go to Filters->Distorts->Polar Coords. Unselect the "Map from Top" option and push the slider "Circle depth in percent" up to 100. You should see what the problem is and why I was talking about “simetrisation”: the borders don’t match. Cancel that.

2Making border match

We need to tweak a bit the photo so that, when mapped to a circle, the borders match.

This is quite easy if you selected the right photo.

First you need to duplicate half of the image and mirror it to the other side:

select and copy/paste one half of the image: ideally, you should copy the half that has the "cleaner" vertical border, for example, here I copied the one with all the sea and a little of the land,
move the new layer to the other side of the photo and select Layer->Transform->Flip Horizontally

Now, this new layer will be hiding the central subject in the photo, so you need to erase some part of the layer.

right click on the layer (on OSX, command-click) and choose "add layer mask", select "white (full opacity)"
with a black paintbrush, start painting in the layer mask, over the parts where you should see the subject form the main layer: In my example, I am not very precise as the background on the left and the right of the subject are very similar.

When you are done, just merge the layer down and you’ll be ready to apply the polar mapping.

Go back to Filters->Distorts->Polar Coords. Unselect the "Map from Top" option and push the slider "Circle depth in percent" up to 100. Check that everything is right and apply.