There are two different approaches used by graphic file formats
for supporting transparent image areas: simple binary transparency
and alpha transparency. Simple binary transparency is supported
in GIF format. Here, one color
from the indexed color palette is marked as the transparent color.
Alpha transparency is supported in PNG format. Here, the
transparency information is stored in a separate channel, the
There is usually no need to save images in GIF format any more,
because PNG supports all the features of GIF and offers additional
features (e.g., alpha transparency). Nevertheless, this format is
still used for animations.
Creating an image with transparent areas (alpha transparency)
First of all, we will use the same image as in the previous
tutorials, Wilber the GIMP mascot.
The Wilber image opened in RGBA mode.
To save an image with alpha transparency, you must have an alpha
channel. To check if the image has an alpha channel, go to the
channel dialog and
verify that an entry for “Alpha” exists, besides
Red, Green and Blue. If this is not the case, add a new alpha channel
from the layers menu.
You can now remove the background layer to get a completely
transparent background, or create a gradient from color to
transparency. You are only limited by your imagination. To
demonstrate the cabilities of alpha transparency, we'll make a soft
glow in the background around our Wilber.