Make Image Transparent in Gimp
Make your paper drawing or photo background transparent for use in 2D or 3D animation
This can be done in several ways. The method discussed here work best for high contrast images with distinct contours, like black drawing on white paper or a solid shape against an even coloured background. The first example down below shows a greyblack pencil drawing on white paper . (Tips for more complex backgrounds will be added later.) If you have a lot of images, more than 10 or so, the method for Blender is recommended (see Blender section), with that method you can save multiple images at once. With this Gimp method you need to save each image separately.
Preparing: If you like to have a standard size or even size of your image, like 400x400 or 1024x1024, then create such an image first and then drag or open the images to this base image. This might be a good idea if you have paper drawings that a scanner did not save exactly with the same size. To create a new image, choose File - New, enter the resolution size you’d like.
If your images already have a nice size you can just open them with the usual File - Open.
If you would like to treat them as layers, open or create one image, then select File - Open as Layers. Select all of the images.
To Open as layers will make editing quicker if the images are similar.
If you have large images with a lot of empty white space around the motive, you can start by creating a bit smaller image size and then open or drag in the images as layers and only the important part of them will show.
First, as a help, create a completely black layer and a completely white layer and move them to the bottom, you can turn these layers on and off later on to view spots that you’d like to get rid of. To do a black layer, make the active foreground Color black, click on the color icon to the upper left, drag the color so all the values show 0.0 or HTML notation says 000000.
Then select New Layer from the Layer menu, at “Fill with” choose Foreground color. With the small arrows at the bottom of the layer section you can move down the black layer to the bottom. To create a White layer do the same but choose White next to “Fill with”.
Uncheck the visibility of all the image layers except one by clicking the “eye” to the left of the layer.
Make sure the visible layer is the one selected.
Before we try to remove a color, we may need to enhance the contrast of the colors.
To do so, choose Colors - Levels.
In this case we are going to drag the bottom color stop to the right, to increase the blackness of the Stickman. It is just a thin grey pencil line so it needs to stand out against the white more.
The quickest way to make the images transparent is by choosing from the menu Colors - Color to Alpha.
Depending on the color of your paper and the characteristics of your drawings, you will need to adjust the Transparency and Opacity threshold.
Turning on and off the Black or White layers will help you decide what levels will work best.
You can save the settings as a preset if you think your images are of the same kind. (Click on the plus sign to the left of “Presets”.)
One thing you may notice if your scanning drawings, the size may differ between them, this could be fixed by clicking on the layer that seems to have a different border and choose Layer - Layer to Image size. This will make the borders the same size as the base image.
Another Method to remove the background is to choose the Fuzzy Select Tool or the similar “Select by Color”, both from Menu Tools - Selection Tools,. These are good tools for removing more complex, difficult backgrounds. There are a lot of settings you can try here but try to start with "Select by Composite". Make sure your image has an alpha layer. If not, click on Layer - Transparency - Add Alpha Channel. (If no alpha channel exists the area you remove will get a solid background colour.) Click on the color you want to remove. An area will be selected. Click on delete to remove. Experiment with Threshold. You may need to repeat the process several times and click on different nuances of the image to clean it up.
After you have removed most of the background, look for speckles, turn either black or white layer on as a background to look for leftover spots. Use Eraser to remove it. Be sure that the layer you want to work with is the one that is selected. (It is easy to make changes on the wrong layer by mistake when they are covered by each other.)
When you're done you can export your layers as .png to keep the transparency.
Make one layer visible at a time, uncheck any colored backgrounds (black and white layer) so that only your selected transparent layer is visible, Choose File Export As, name your image, recommended with a nr at the end and .png as file format, like Image001.png, Image002.png and so on.
Press Export, Gimp will ask some standard questions, you don’t need to change anything, press Export again in that little Window. Do so for all of the layers, don’t forget to name them with different numbers or they will be overwritten.