Using GIMP I'm trying to cut out all white background from a picture of a pen and ink drawing in order put it on a colored T shirt, like a silk screen. Thanks for your help!
Color>Color to alpha to remove the white (makes all the white transparent). And save the image as a PNG, JPEG doesn't support transparency.
But usually you want to clean things a bit first. If the image is a scan (lighting is regular), just using the Brightness/Contrast tool to blow out the white (increase contrast (much) and brightness (a bit)) is often enough. But don't overdo it, watch that the edges don't get pixellated.
If it's a photo (likely with uneven lighting), try this:
- Duplicate the image
- Apply a Gaussian blur to the top layer. This blur should be sufficient to make the lines disappear entirely (I typically use about 1/3 of the image)
- Put the top layer in 'Grain merge' mode
Layer>New from visibleto finalize things.
You don't need to do that for printing. White is not reproduced as a separate colour in most printing processes, including regular screen printing, unless you have a specific need for it.
If the pen and ink drawing is monochrome, a screen can be made that will print in black ink, or any other single colour of ink. The white areas will be transparent.
If the drawing is coloured, it can be reproduced in CMYK printing - again no white ink is involved in the process.
As to answer your question directly, it is possible to remove a white background should the need arise. In GIMP click
Colour to Alpha >
OK - then export in a format that supports alpha transparency, such as PNG, GIF, PSD, TIFF, XCF.