From left to right: original image, after applying Channel Mixer
You can find this filter through
This filter combines values of the RGB channels.
It works with images with or without an alpha channel.
It has monochrome mode and a preview.
“Channel Mixer” filter options
From this menu you select the channel to mix to. Choices are
Red, Green, or Blue. It is insensitive when
Monochrome option is checked.
Red, Green, Blue
These three sliders set the contribution of red, green or blue
channel to output. Can be negative. These sliders are graduated
from -200 to 200. They represent the percentage which will be
attributed to the output channel. 100% corresponds to the
value of the channel of the studied pixel in the image.
This option converts the RGB image into a gray-scale RGB image.
The Channel Mixer filter is often used with this aim in view,
because it often provides a better result than the other ways
(see Grayscale in
Glossary). Makes the Output Channel menu
Calculations may result in too high values and an image too
much clear. This option lessens luminosities while keeping a
good visual ratio between them.
Load settings from a file.
Save settings to a file.
How does Channel Mixer work?
In RGB mode
In this mode, you have to select an Output Channel
. This channel is the one which will be modified. In
the dialog window, its default value is 100%, corresponding to
the value of the channel in the original image. It can be
increased or decreased. That's why slider ends are -200 and 200.
Three RGB sliders let you give a percentage to every channel.
For every pixel in the image, the sum of the calculated values
for every channel from these percentages will be given to the
Output Channel. Here is an example:
Figure 11.40. The original image and its channels
RGB values of the pixels in red, green, blue, gray squares
are displayed. The black rectangle is special, because
black (0;0;0) is not concerned by filter (0 multiplied by
any percentage always gives 0). The result cann't exceed
255 nor be negative.
Figure 11.41. Output channel is red. Green Channel +50
In the red square, the pixel values are 230;10;10.
Percentages are 1;0.5;0. The claculation result is
230*1 + 10*0,5 + 10*0 =235. The same reasoning is valid
for the green and the blue squares.
In the gray square, which contains red color, the
calculation result is above 255. It is reduced to 255. A
negative value would be reduced to 0.
Output channel is red. Green Channel +50%.
The Preserve Luminosity option is checked.
The values attributed to the Red Output channel are lower,
preventing a too much clear image.
In Monochrome mode
When this option is checked, the image preview turns to
grayscale, but the image is still a RGB image with three
channels, until the filter action is validated.
In every square, pixels have been converted into a gray
level equal to the value of the Red channel in the
original image (The background has been painted with pink
afterwards to make white squares visible).
Here is how the Preserve Luminosity works
in the monochrome mode:
“For example, suppose the sliders were Red:75%, Green:75%,
Blue:0%. With Monochrome on and the
option off, the resulting picture would be at 75%+75%+0% =150%,
very bright indeed. A pixel with a value of, say,
R,G,B=127,100,80 would map to 127*0.75+100*0.75+80*0=170 for
each channel. With the Preserve Luminosity option on, the
sliders will be scaled so they always add up to 100%. In this
example, that scale value is 1/(75%+75%+0%) or 0.667. So the
pixel values would be about 113. The Preserve Luminosity
option just assures that the scale values from the sliders
always adds up to 100%. Of course, strange things happen when
any of the sliders have large negative values” (from the
plug-in author himself).
Which channel will you modify?:
This depends on what you want to do. In principle, the Red
channel suits contrast modifications well. The Green channel is
well adapted to details changes and the Blue channel to noise,
grain changes. You can use the
The 30%, 59%, 11% gives you the same gray levels as the
Grayscale command in Image/Mode. This wil no longer be valid