Turn images into 1-bit dithered black and white

Dithering occurs when you have a limited number of colors, say 256 for GIF's or 2 for black and white, and want to represent subtle gradations between colors without creating bands of color. The resulting images take up far less space than those with colorspaces of thousands or millions of colors. GIMP can handle dithering, although it's not obvious how to do so.

Goal:

Although dithering is a sign of low quality, I personally like its effects. And the continued popularity of 256-color GIFs supports me. I wanted to turn a full-color image into a black-and-white image, and also see how many kilobytes smaller it became.

Walkthrough

1. I chose this photo of the Cat Eye Nebula, from NASA's Hubble site. I shrank it down to 500 by 400 pixels, to more clearly see the effects of dithering, and exported it as a PNG, default settings. It weighs in at 292 kB.
2. To start dithering, go to Image > Mode > Indexed... in the menubar. It'll pop up a window like the one below, where you can choose the colormap. You have the option to map it to several palettes, or a web-optimized palette, but why use any colors at all? Choose "Use black and white (1-bit) palette" to get the lowest quality possible. Then next to "Color dithering", choose any of the options.
   
   
   


3. Save and export, if you choose. The PNG format may yield better results than JPEG because it's a lossless compression algorithm.

Result

These are the results of the 4 choices of dithering mode: "None" gives you featureless blotches, while the other choices of dithering cleverly intersperse white and black pixels to give the appearance of different gray tones.

None (no dithering): 4 kB

Floyd-Steinberg (normal) (my preference): 19 kB

Floyd-Steinberg (reduced color bleeding): 9 kB

Positioned: 7 kB

Learn More

Here is the GIMP documentation for this feature.

Use Resynthesizer to quickly remove something

The Resynthesizer plugin for GIMP has been in development since 2000, according to the documentation, though it closely resembles a feature Photoshop introduced back in 2011. Both tools can remove an object from an image by covering it with nearby textures. Resynthesizer can also take one sample of a texture and seamlessly fill a whole image with it.

Goal

Remove the two hiking boots from this image:

Walkthrough

1. Select one of the boots somehow. I used the Scissors Select tool, which automatically aligns your selection to the edge of an object. You start by clicking anywhere on the boot's outline, and then continue to click around its outline. The line of your selection will fit itself to the best path between every two points, but if it missed, click the middle of the misplaced line and drag it to where it should be. Go in a circle and close the loop by clicking the first point you placed. Then hit Return to make it into a selection.
   
   
2. Optional, helpful with shadows: Go to Select > Grow... in the menubar, and in the resulting window, type in a number over 50, click OK, and see how much bigger the selection gets. I wanted to get the selection boundary outside the shadow of the boot, so the next step wouldn't fill it with a dark texture. If 50 is too far or too short, go to Edit > Undo or hit the shortcut Command-Z, and try step 2 again. I grew the selection by 100 pixels.
3. When you have the selection you want, go to Filters > Enhance > Heal selection... and in the window that pops up, type in a generous radius to take texture from. 100 was ideal for this image. This is the number of pixels outside your selection the program will go to search for textures. In the picture below, I didn't grow the selection or choose a large enough radius, and it filled the boot with a conspicuous dirt-like texture. Click OK and let it do its thing.

4. Resynthesizer is a CPU-hungry process, GIMP is a slow program, and both these factors will make this process take a long time. 1 or 2 mysterious icons should pop up in your Dock, with names like "script-fu." These are processes GIMP is calling on to get the job done. If you force quit one of them, and I have done so before, options will disappear from your Filters menu, and you'll have to restart GIMP to get them back.
5. When the loading screen disappears, GIMP may still take several seconds to draw your updated image. Be patient. If it doesn't look how you wanted, hit Undo and play around with the size of the selection or the radius to take the texture from.

Result

After applying the tool to both boots, they are effectively removed from the picture.

Learn More

A DuckDuckGo search turned up this lengthy post by Pat David, which gives a much longer explanation of what the plugin will do. I didn't base my walkthrough on his post, but I'll probably refer back to it in the future.

Make a Mini Planet

Goal

Turn a panoramic or regular landscape photo into a mini planet, like this one:

by nate bolt, on Flickr

Walkthrough

1. I chose a picture of some sheep in a field. This picture has a fairly flat horizon, featureless sky, and a bottom section without structures, which is good because the bottom will get distorted a lot in order to make it the center of my planet. I'm pretty sure that a picture without an uninterrupted band of sky at the top won't look good, but I can't definitively say so. Open this picture in GIMP.

2. Duplicate the layer so you have the option of starting over at any time.
3. To keep a version of the picture you can edit again later, save it in xcf format, GIMP's native format that preserves all the image's layers. Go to File > Save and in the popup window, choose a folder and set the name to planet.xcf (only the .xcf part matters). Click File > Save often during every project, in case GIMP or a plugin crashes.
4. Click Filters > Distorts > Polar Coordinates... in the menu bar. Expect this plugin to take longer than expected to load. It brings you to a window with a preview and five configuration options. Set the circle depth at 100% to give the planet a circular border, offset angle at 278.98 to put the flagpole at the top, Map backwards unchecked, Map from top unchecked, To polar checked. Then click OK to apply the filter.
   

Use Liquid Rescale to make an image fit your desktop

My camera takes pictures with an aspect ratio of 4:3, meaning if you shrank the picture so it was 4 inches across, it would be 3 inches high. Unfortunately, that means if I used it as the desktop background on a widescreen, 1080p monitor, either the top and bottom of the picture would get cut off or there would be blank space on the sides.

Goal

I want to make the picture fit the display perfectly without cropping it. I have two options in GIMP: use the traditional scale tool, which will stretch the image horizontally, or the Liquid Rescale plugin, which is designed to leave a photo's important features intact while stretching the less important areas. The scale tool causes a lot of distortion, though, while Liquid Rescale keeps things like faces, letters and buildings their original shape. Liquid Rescale comes with lisanet's version of GIMP.

Walkthrough

1. I found a picture of a damselfly that I wanted to resize. Find your picture, check that it's a different shape from your display, right-click it in the Finder, and select Open With > GIMP.
2. Go to the list of layers on the right side of the GIMP window, and click the Duplicate Layer button. If you keep making changes and then decide you don't like it anymore, you can delete the layer you modified and revert to your original photo.