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Making Tilt-Shift Photo in GIMP
There is an interesting photographic effect called Tilt-Shift. It makes a photo of city panorama looking like a miniature due to small depth of field. Wikipedia says it can be obtained optically with some advanced techniques, but it can also be approximated with postprocessing.
Yesterday I visited St. Dominic's Fair in Gdańsk, where I had an opportunity to enter a Ferris wheel and take a photo of my city from some height. Here is my experiment with tilt-shift. I've made it GIMP.
To do it, top and bottom of the photo needs to be blurred. But an out of focus photo is not the same thing as standard Gaussian blur. That's why a special kind of blur is needed. There is a GIMP plugin for it: Focus Blur (Windows binary can be found HERE). In Photoshop, the effect is available as Lens Blur.
Image needs to be blurred more the closer a pixel is to the top or bottom edge of the image. But I have no idea how to do blur (or any other effect) with intensity varying over image location, so here is the trick: We can use only two layers - normal and heavily blurred - and blend between them using layer mask.
So to add tilt-shift effect to your photo using GIMP:
If you've done everything right, you should now already have blending between layers applied so that top and bottom of the image looks like out of focus. Now you can: