High Pass Filter usage

1. With image file open, make all color and other changes first, then sharpen before printing or saving.

To sharpen:

Tools--Paint Tools--Blur / Sharpen
Filters--Generic--High Pass Filter

2. Go to Layers, Channels, Paths, Undo palette. If it's not accessible, save changes, exit Gimp, go to (show hidden folders) /root/.gimp-2.4 and delete sessionrc file. Then restart Gimp, reopen file and repeat step 1.

3. Go to Filters--Generic--High Pass Filter.
In Script-Fu: High Pass Filter dialog box, set:
Filter Radius to 10
Mode to Color
Check Keep Original Layer box
Then click OK
(Note: These values may already be preset.)

Image will turn gray with a slight tinge of color.

4. Then on Layers, Channels, Paths, Undo palette, go to Layers--Mode and change to Overlay. Image will return to full color, but sharper. Use Opacity slider to adjust sharpness. Then print or save.

These are the basics; feel free to experiment with other settings.

Black and White Filter usage

The Channel Mixer converts color images to black and white, then adds simulated color filters.

1. Open image file.

2. Go to Colors--Components--Channel Mixer.

3. In the Channel Mixer dialog box, click Open; this opens the Channel Mixer Settings dialog box.

4. Select one of the four color filters to use. This brings back the Channel Mixer dialog box, but now the preview will be in black and white. More information is available here.