Chromakey

Chromakeying is the process of making a specific range of colors transparent. In contrast to transparency, this makes the pixel either completely opaque or completely transparent. A common use for this is in creating composite images, such as the image below of sea slugs in the Martian landscape. This image was created by making an image of the sea slug surrounded by a uniform dark color, then setting ``chromakey'' for that image to make that color transparent, creating a cutout. The cutout can then be positioned, rotated, or resized as desired, and then pasted onto the image behind it by clicking the `Paste' button in the information area at the left. This causes the opaque pixels in the currently-selected region to be pasted onto whatever is behind it (i.e., onto the background or the image behind the transparent image).

Left: Composite image created using chromakey feature
Right: Chromakey dialog

To change the chromakey settings, select ``Image..image properties..'' (or click the ``Prop'' button at left), check the ``Chromakeyed'' button, and click OK. A small dialog box will appear. If the image is color, select the RGB values for the minimum and maximum colors which should appear opaque. For grayscale images, select the minimum and maximum opaque pixel values. If ``Invert chromakey'' is checked, the opacity will be reversed. For example, if the minima and maxima RGB values were 20 30 40 and 123 123 123, normal chromakey would make a pixel opaque if its red is between 20 and 123, its green is between 30 and 123, or its blue is between 40 and 123, and transparent otherwise. Inverse chromakey would make the pixel transparent under those conditions and opaque otherwise.

To deactivate chromakey, uncheck the ``Chromakeyed'' button. All operations on chromakeyed images are significantly slower than with normal images. Also, chromakeying is performed recursively, so that the background will still be visible regardless of the number of chromakeyed images superimposed on it.