
Normal

Normal
mode is the default layer mode. The layer on top covers the layers
below it. If you want to see anything below the top layer when you
use this mode, the layer must have some transparent areas.
The equation is:
Equation 8.2.
Equation for layer mode “Normal”

Dissolve

Dissolve
mode dissolves the upper layer into the layer beneath it by drawing
a random pattern of pixels in areas of partial transparency. It is
useful as a layer mode, but it is also often useful as a painting
mode.
This is especially visible along the edges within an image. It is
easiest to see in an enlarged screenshot. The image on the left
illustrates “Normal” layer mode (enlarged) and the
image on the right shows the same two layers in
“Dissolve”
mode, where it can be clearly seen how the pixels are dispersed.

Multiply

Multiply
mode multiplies the pixel values of the upper layer with those of
the layer below it and then divides the result by 255. The result is
usually a darker image. If either layer is white, the resulting
image is the same as the other layer (1 * I = I). If either layer
is black, the resulting image is completely black (0 * I = 0).
The equation is:
Equation 8.3.
Equation for layer mode “Multiply”
The mode is commutative; the order of the two layers doesn't matter.

Divide

Divide
mode multiplies each pixel value in the lower layer by 256 and then
divides that by the corresponding pixel value of the upper layer
plus one. (Adding one to the denominator avoids dividing by zero.)
The resulting image is often lighter, and sometimes looks
“burned out”.
The equation is:
Equation 8.4.
Equation for layer mode “Divide”

Screen

Screen
mode inverts the values of each of the visible pixels in the two
layers of the image. (That is, it subtracts each of them from 255.)
Then it multiplies them together, divides by 255 and inverts
this value again. The resulting image is usually brighter, and
sometimes “washed out”
in appearance. The exceptions to this are a black layer, which does
not change the other layer, and a white layer, which results in a
white image. Darker colors in the image appear to be more
transparent.
The equation is:
Equation 8.5.
Equation for layer mode “Screen”
The mode is commutative; the order of the two layers doesn't matter.

Overlay

Overlay
mode inverts the pixel value of the lower layer, multiplies it by
two times the pixel value of the upper layer, adds that to the
original pixel value of the lower layer, divides by 255, and then
multiplies by the pixel value of the original lower layer and
divides by 255 again. It darkens the image, but not as much as with
“Multiply” mode.
The equation is:
^{[2]}
Equation 8.6.
Equation for layer mode “Overlay”

Dodge

Dodge
mode multiplies the pixel value of the lower layer by 256, then
divides that by the inverse of the pixel value of the top layer. The
resulting image is usually lighter, but some colors may be inverted.
In photography, dodging is a technique used in a darkroom to
decrease the exposure in particular areas of the image. This brings
out details in the shadows. When used for this purpose, dodge may
work best on Grayscale images and with a painting tool, rather than
as a layer mode.
The equation is:
Equation 8.7.
Equation for layer mode “Dodge”

Burn

Burn
mode inverts the pixel value of the lower layer, multiplies it by
256, divides that by one plus the pixel value of the upper layer,
then inverts the result. It tends to make the image darker, somewhat
similar to “Multiply” mode.
In photography, burning is a technique used in a darkroom to
increase the exposure in particular areas of the image. This brings
out details in the highlights. When used for this purpose, burn may
work best on Grayscale images and with a painting tool, rather than
as a layer mode.
The equation is:
Equation 8.8.
Equation for layer mode “Burn”

Hard light

Hard light
mode is rather complicated because the equation consists of two
parts, one for darker colors and one for brighter colors. If the
pixel color of the upper layer is greater than 128, the layers are
combined according to the first formula shown below. Otherwise, the
pixel values of the upper and lower layers are multiplied together
and multiplied by two, then divided by 256. You might use this mode
to combine two photographs and obtain bright colors and sharp edges.
The equation is complex and different according to
the value >128 or 128 :
Equation 8.9.
Equation for layer mode “Hard light”, M > 128
Equation 8.10.
Equation for layer mode “Hard light”, M 128

Soft light

Soft light is not related to
“Hard light” in anything but the name, but it
does tend to make the edges softer and the colors not so
bright. It is similar to “Overlay” mode. In some
versions of GIMP, “Overlay”
mode and “Soft light” mode are identical.
The equation is complicated. It needs Rs, the
result of Screen mode :
Equation 8.11.
Equation for layer mode “Screen”
Equation 8.12.
Equation for layer mode “Soft light”

Grain extract

Grain extract
mode is supposed to extract the “film grain”
from a layer to produce a new layer that is pure grain, but it can
also be useful for giving images an embossed appearance. It
subtracts the pixel value of the upper layer from that of the lower
layer and adds 128.
The equation is:
Equation 8.13.
Equation for layer mode “Grain extract”

Grain merge

There are two more layer modes, but these are available only for
painting tools. See
Painting Modes
for detailed information.
Grain merge mode merges a grain layer
(possibly one created from the “Grain extract”
mode) into the current layer, leaving a grainy version of the
original layer. It does just the opposite of
“Grain extract”. It adds the pixel values of the
upper and lower layers together and subtracts 128.
The equation is:
Equation 8.14.
Equation for layer mode “Grain merge”

Difference

Difference
mode subtracts the pixel value of the upper layer from that of the
lower layer and then takes the absolute value of the result. No
matter what the original two layers look like, the result looks
rather odd. You can use it to invert elements of an image.
The equation is:
Equation 8.15.
Equation for layer mode “Difference”
The mode is commutative; the order of the two layers doesn't matter.

Addition

Addition
mode is very simple. The pixel values of the upper and lower layers
are added to each other. The resulting image is usually lighter. The
equation can result in color values greater than 255, so some of the
light colors may be set to the maximum value of 255.
The equation is:
Equation 8.16.
Equation for layer mode “Addition”
The mode is commutative; the order of the two layers doesn't matter.

Subtract

Subtract
mode subtracts the pixel values of the upper layer from the pixel
values of the lower layer. The resulting image is normally darker.
You might get a lot of black or nearblack in the resulting image.
The equation can result in negative color values, so some of the
dark colors may be set to the minimum value of 0.
The equation is:
Equation 8.17.
Equation for layer mode “Subtraction”

Darken only

mode compares each component of each pixel in the upper layer with
the corresponding one in the lower layer and uses the smaller value
in the resulting image. Completely white layers have no effect on
the final image and completely black layers result in a black image.
The equation is:
Equation 8.18.
Equation for layer mode “Darken only”
The mode is commutative; the order of the two layers doesn't matter.

Lighten only

mode compares each component of each pixel in the upper layer with
the corresponding one in the lower layer and uses the larger value
in the resulting image. Completely black layers have no effect on
the final image and completely white layers result in a white image.
The equation is:
Equation 8.19.
Equation for layer mode “Lighten only”
The mode is commutative; the order of the two layers doesn't matter.

Hue

Hue
mode uses the hue of the upper layer and the saturation and value of
the lower layer to form the resulting image. However, if the
saturation of the upper layer is zero, the hue is taken from the
lower layer, too.

Saturation

Saturation
mode uses the saturation of the upper layer and the hue and value of
the lower layer to form the resulting image.

Color

Color
mode uses the hue and saturation of the upper layer and the value of
the lower layer to form the resulting image.

Value

Value
mode uses the value of the upper layer and the saturation and hue of
the lower layer to form the resulting image. You can use this mode
to reveal details in dark and light areas of an image without
changing the saturation.