Hey guys, I found another free program for image editing that helps a lot to clean the sketch in a drawing. First I'll show you one of my sketches.
Note I use blue marker for the sketch and black marker for the line art. This is important, at least for this tutorial. I'll show you how to remove the color blue using GIMP. You can get GIMP for free in the following link: https://www.gimp.org/downloads/
First, step. Scan the drawing and then open the file with GIMP.
Next, click Colors>Components>Decompose.
Check that the selected Color model is RGB and the Decompose to layers box is checked. Then click OK.
This will open a new window, the same image but separated in 3 layers (one for each color in RGB). These layers are all in grayscale.
You'll see a trash can in the small layers window. Select the layer named "red" and delete it by clicking the trash can. Do the same thing with the layer "green".
Now the layer "blue" is the only one left. You'll notice that the sketch is almost gone. The gray heads that are still visible were done with darker blue tones on purpose to show that you need the lightest blue marker you can get. But we can still get rid of them.
Go to Colors>Brightness-Contrast.
By adjusting the contrast level you'll get rid of gray lines (if they are not too close to black).
For some reason, you won't be able to save, but you can export. File>Export As...
Finally, you get your clean line art. You could color it with your favorite image editing software now. I personally use Inkscape to get a vector version of the drawing. You can download Inkscape for free in the following link: https://inkscape.org/en/download/
To do the vector version, first open the image file with Inkscape.
Click on the Image to select it, and then type Alt + Shift + B.
I usually use a Threshold of 0.6, but you can modify it to experiment or just leave the default value. Click OK.
This will trace a vector based on your drawing, you can then save it to SVG, or EMF in case you want to edit it with Adobe Flash. That’s all for now, have fun.