GIMP tools - BDA

GIMP 60
Value adjusts the general colour in your image;
Red, Green, Blue work on a particular colour channel;
Alpha works on transparent masks.
In the dialog box opened you can see a histogram with the distribution of pixels in the
whole image or its selection. Adjusting the settings in this window changes the intensity
of the image. It usually helps and makes colours brighter.
All tones in a balanced image are distributed evenly. For example, an image with a blue
predominant colour will produce a histogram shifted to the left in Green and Red
channels, signified by green and red lacking on highlights.
The key settings are adjusted in Input and Output levels.
Input Levels
Moving input sliders (A) adjusts dark tones, undertones and light tones individually:
triangular slider B adjusts shadows;
triangular slider C adjusts undertones;
triangular slider D adjusts light tones.
When adjusting images, triangular sliders B and D are moved to the group of points
signifying the beginning and end of the histogram. When working on undertones, move
triangular slider C to the right to darken them or to the left to lighten them. The position
of triangular slider B determines the value of the black point, and all points with the
lower value will be black. This applies to triangular slider D as it determines the white
point. The middle triangle (C) determines the mid point. The closer this triangle is to B,
the lighter the image you get; and the closer it is to D, the darker the image is.
Input Levels are often used to lighten bright tones (usually highlights), darken shadows,
change the balance of bright and dark tones.
Output Levels
At first choose a Channel.
In the Value channel, points are compressed and look more alike, shadows are made
lighter, and highlights darker.
The following example explains the colour (red, blue, green) channel the best: if you use
the red channel and set the output level at 100–140, all points will be red.
Working on the Alpha channel, all values will be shifted to the range you have set.
Slider E is for adjusting the contrast in an image. To lighten an image, move triangular
slider F to the right; to darken an image, move triangular slider G to the left. Where
required, you can enter the exact values into input boxes that are below the output level
scale.