Today we will learn how to make halftones using GIMP. For the most part, the concept is similar to those posted by PatrickSpens and RamyB. GIMP goes about things a little differently, however.
In keeping with the anthro-fruit theme, here is a Terrible Tangerine.
Start with all colors separated into their own layers. In this example there is a "glossy" layer and a shadow on the red layer, both at about 35% opacity.
Copy-paste the LOI (layer of interest) as well as a new background layer positioned just under the LOI (I'll explain why in a bit). To make things a bit easier to see and work with, turn off the layers not currently in use. Note that alpha channels are used extensively here.
Next, convert the red to black, similar to the color overlay technique used in the Photoshop tutorials. Use the color exchange function for that operation.
Here is the color exchange dialog for reference. One caveat, if you use the color picker (eye dropper) you can't leave the dialog box to pick your color, but you can pick from the preview box.
With the color converted, you must merge the LOI with the copied background layer to remove the alpha.
This is because the filter we are about to use ignores alpha.
Now it's business time.
Select the newsprint filter from Filters/distorts
to get the dialog box below.
The input SPI comes from the drawing.
The output SPI and Cell size are tied together, if you change one, the other changes inversely. Like RamyB's, this example uses a low res picture, so the cell size is set to 6. Other sizes can add unwanted artifacts to the lines. Play around to get a good idea of what it will look like. For a high res 300 DPI drawing, 30 is the lowest setting per Woot guidelines.
Leave the Screen setting on RGB.
You might have noticed by now that there was no copy/paste into a new drawing for converting the image mode to grayscale, as seen in the Photoshop tutorial. That's because it isn't needed. This filter is designed to work on full color, and you can set each channel's halftone independently as per printed halftone theory.
Good reading, that, so read it, but it's not germane to our current needs.
To work in black/white, set the sliders for each channel at the same angle (set them full stop either direction for a quick fix), then check the lock channel box to tie them together. This give the desired monocolor halftones.
The spot functions and angle are similar to what is seen in Photoshop. Here it is set to 67.5° lines, but later it was changed to 22.5°. Play with what looks best for your design.
Now change the color back to the original for that channel, in this case red. Use the color Exchange tool again. You can pick the "to" color from the "from" color that appears when the dialog is opened, to make sure you get the right one without remembering what the values were, then pick black for the from color. Easy peasy.
The white background is no longer needed, so eliminate it with the Color to Alpha menu item,
selecting white as the target color.
Here is the result. You can see the angle is set to 22.5° vs. 67.5° originally selected
The process for halftoning white is a bit different, because white is special. Select your "shiny" white layer and duplicate it, then put a new background layer directly under it.
The new background layer should be black.
The rest of the steps are the same as before.
Merge the layers.
No need to swap colors here, since white was used throughout, but the black does need to be converted to alpha to get the transparency back.
And here's the finished product.
It looks kind of crappy, but not too bad when zoomed out.
note: I'm sure there are many many other ways to do this more elegantly using fancy alpha channel blending and layer masks, but this technique is quick and mostly effective.
note2: had to edit a few times, sorry. Should be good now,