Image Processing Reference
In the Decompose window, you can select the color model in which the
channels are to be created as layers. Select either RGB or RGBA (RGB with
alpha channel), and check the Decompose to layers box. Without this selection,
every single color channel will be depicted in a separate window.
• In the new portrait-RGB.xcf image, right-click to open the context menu
and duplicate the red layer in the Layers dialog.
• Make the new red-copy layer the only active layer and make it visible. Red
provides the best contrast values.
• Use the tonality correction ( Colors > Levels ) to increase the contrast.
The better you work here, the easier it is to do the following steps. The
method does not work with such sharp contrasts as the method using
the Threshold function does. The aim is to bring out the fine details.
• Access the Select > By Color menu item or use the corresponding selection
tool to create a selection in the image window. Click a white image area
so that all white tones in the image will be selected. I left the Threshold
settings in the tool options at 15. However, depending on the result from
the tonality correction, it may be necessary to experiment with different
threshold values until you find a value that provides the desired result—a
selection or mask, which is as accurate as possible. You may have to delete
the selection and start fresh until you find the appropriate value. You can
also try setting a small threshold in the Add to Selection tool option.
The CMYK option in this window
can be used to separate the color
channels as color extracts for
prepress (CMYK color model for four-
The tonality correction is used to increase the light-dark contrast on the image layer. It is
preferably optimized toward black and white.