With this option you can choose which component of the image
GIMP shall use to calculate the similarity and to determine
the borders of filling.
The components you can choose from are
This option is not easy to understand. You have chosen,
for example, the Red channel. When you click on any pixel,
the tool searches for contiguous pixels similar for
the red channel to the clicked pixel,
according to the set threshold. Here is an example:
Original image: three strips with gradients of pure
colors. Red (255;0;0), Green (0;255;0), Blue
(0;0;255). We are going to use the Bucket-fill tool
with the magenta color and a Threshold set to 15.
Image 1: Fill By = Composite. We successively clicked in the
three color strips. Every strip is filled according
to the threshold.
Image 2: Fill By = Red. We clicked in the red strip. The
tool searches for contiguous pixels which have a similar
value in the red channel, according to the set
threshold. Only a narrow area corresponds to these
standards. In the green and the blue strip, the value
of pixels in the red channel is 0, very much different from
the red channel value of the clicked pixel: the color
doesn't spread to them.
Image 3: Fill By = Red. We clicked in the green strip.
There, the value of the clicked pixel in the red channel is
0. All pixels in the green and the blue strips have the
same red channel value (0): they are all painted.
Example for “Fill By”