A reader recently commented that he didn't understand why there are so many layer modes in GIMP and he wondered just what one is supposed to do with all of them. Since I wasn't able to help right off the bat, I decided to do some research and experiment. Figuring stuff out for yourself is fun and I find that it burns in the knowledge. So what follows is the process I went through to try and understand more about layer modes.

First, I created a 900 x 1200 canvas in GIMP divided into six sections: light red, dark red, light blue, dark blue, light green, dark green. I varied the intensities of the colors a bit to make it easier to notice possible differences. The HSV and RGB numbers in each large block indicate the actual color values. On top of each color I put a grayscale gradient with a block of black on the left and a block of white on the right. I merged all of the small color blocks onto a transparent layer to make a test pattern layer above the six-color background. I thought this example would be useful for artists to see how layer modes affect colors.

Color Patterns on Color Blocks

Next, I created a 900 x 1200 canvas with a photo of a scarlet macaw on the lower layer and a photo of some roses that I cut out and put on a transparent background on the upper layer. I thought this example would be useful for photographers to see how layer modes affect images.

Roses on Macaw

Then I created a copy of the scarlet macaw photo with a grayscale gradient layer above it running from HSV 0, 0, 20 at the top to HSV 0, 0, 80 at the bottom to show the effects of layer modes on grayscales over color images. I also created a duplicate layer of the scarlet macaw to show the effects of layer modes on images over themselves.

Macaw Grayscale Gradient

For each of the test images, I changed the layer mode of the upper layer to each mode and saved the result. Since pictures are worth more than any number of words, especially (I'm sure) to the types of people who use GIMP, I've included images in each section below as references. What follows are my observations and thoughts about how to use each layer mode. Feel free to copy any of these images to your desktop so you can load them into GIMP for a closer look.

Normal is the default layer mode. It simply shows the topmost visible layer in the Layers list. If there is any transparency on that layer, you will also see anything below it through the transparent areas. You can blend the topmost visible layer with what's below it by adjusting the layer's opacity.

Color Pattern; Mode Normal; Opacity 50% Photo Overlay; Mode Normal; Opacity 50%

The purpose of the Dissolve layer mode is to dissolve the upper image into the lower one by creating variable-sized and random-looking holes in the upper image. The frequency of the occurrence of these holes is based on the upper layer's opacity (which can be thought of roughly as the percentage of dissolving), each upper pixel's original opacity, and the effect of the layer's mask on each pixel.

If the layer on which Dissolve is applied has no transparent or semi-transparent pixels, you won't see a change in the image until you move the layer's opacity off of 100%. This was true of my two test images.

Color Pattern; Mode Dissolve; Opacity 90% Photo Overlay; Mode Dissolve; Opacity 80%

Dissolve well. The bottom layer is pure white. The top layer is a gradient of red that goes from opaque at the top to transparent at the bottom. The mask on the red layer is a gradient that goes from opaque at the left to transparent at the right. Examine the images to see the differences among Normal mode, Dissolve mode at 100% opacity, and Dissolve mode at 50% opacity. Saul Goode at Gimp Chat provided the following example which illustrateswell. The bottom layer is pure white. The top layer is a gradient of red that goes from opaque at the top to transparent at the bottom. The mask on the red layer is a gradient that goes from opaque at the left to transparent at the right. Examine the images to see the differences amongmode,mode at 100% opacity, andmode at 50% opacity.

Dissolve Example Layer Dialog Red Gradient in Normal Mode Red Gradient; Dissolve Mode; 100% Opacity Red Gradient; Dissolve Mode; 50% Opacity

Thoughts:

I find the effects of Dissolve to be rather jarring because of the sharp transitions between the upper and lower images. Though there may be interesting artistic uses of this method, you might want to further process the image to reduce the sharpness and blend the images better.

However, it occurs to me that the Dissolve layer mode could be used to create an interesting texture effect of one color over another. Since there is no anti-aliasing or feathering of the holes, you could create the density of stippling you like, then create a new layer from what's visible, select the color of the holes, use Select > Feather with a small number, then either hit Delete and put a colored layer under it or pour a color into the selection.

Example:

To test my texture effect idea, I created a 900 x 1200 canvas with a blue layer over a green layer. I changed the layer mode of the blue layer to Dissolve and set the opacity to 55%. The image on the left is a 50 x 50 cutout scaled up to 1000 x 1000 to show the grain.

Then I created a new layer from visible, used the Select by Color tool to select all green pixels, used Select > Feather with a setting of 3 px, then filled the selection with yellow. The image on the right is a 50 x 50 cutout scaled up to 1000 x 1000 to show the grain.

After comparing the HSV and RGB values of the original colors and the final results of using Lighten only, I found that the color of each result is determined by taking the largest value of each of red, green, and blue from the initial colors. For example, if an RGB of 134, 204, 134 is combined with an RGB of 78, 78, 179, the result is RGB 134, 204, 179. Since pure black is 0, 0, 0 and pure white is 255, 255, 255, taking the highest numbers makes the resulting color lighter.

Color Pattern; Mode Lighten Only Photo Overlay; Mode Lighten Only

Observations:

It does not matter which layer is on top.

Black has no effect on the final result.

White results in white.

A color lightened against itself has no effect on the final result.

Shades of gray serve to lighten the other color. Darker shades of gray lighten less. Lighter shades of gray lighten more.

If one layer is grayscale and the other is color, the effect of Lighten only is to desaturate the color layer. That is, if you look at the HSV numbers, only the saturation changes.

When two non-grayscale colors are involved, the result is a lighter version of a combination of the two. Since there is no attempt to maintain the hues of the two initial colors, the resulting hue can be quite different from either.

The lightening power of Lighten only is not as intense as either Screen, Addition, or Dodge.

Thoughts:

Use Lighten only with a grayscale layer and a color layer for lightening (desaturating) the color layer without affecting the hue or value (in general), keeping in mind that if a gray pixel has larger red, green, and/or blue values than the corresponding color pixel, then the resulting pixel will tend toward being gray.

Macaw + Grayscale Gradient;

Mode Lighten Only; Opacity 50%

Screen mode, I noticed that each resulting color had higher red, green, and blue values than either of its initial colors, thereby making the final image lighter. However, the formula involved in computing the result was entirely non-obvious, so I was forced to resort to the In examining the changes in RGB values when usingmode, I noticed that each resulting color had higher red, green, and blue values than either of its initial colors, thereby making the final image lighter. However, the formula involved in computing the result was entirely non-obvious, so I was forced to resort to the user's manual . Here's the equation in English:

Invert color 1. Invert color 2.

Multiply the results.

Divide that result by 255.

Invert that result.

Color Pattern; Mode Screen Photo Overlay; Mode Screen

Observations:

It does not matter which layer is on top.

Black has no effect on the final result.

White results in white.

Shades of gray serve to lighten the other color. Darker shades of gray lighten less. Lighter shades of gray lighten more.

If one layer is grayscale and the other is color, the effect of Screen is to lighten the color layer without affecting the hue. That is, if you look at the HSV numbers, hue does not change.

When two non-grayscale colors are involved, the result is a lighter version of a combination of the two. This means the resulting hue can be quite different than either of the initial hues.

The lightening power of Screen is more intense than Lighten only, but not as intense as either Addition or Dodge.

Thoughts:

Use Screen with a grayscale layer and a color layer to lighten the color layer without affecting the hue.

Use Screen on the upper layer of a duplicated image and vary the opacity to adjust brightening of the lower image without affecting the hue.

Macaw + Grayscale Gradient;

Mode Screen; Opacity 50% Macaw Duplicate;

Mode Screen; Opacity 50%

Dodge increases the values of red, green, and blue above both of the initial values. According to the Dodging is a photographic printing technique that decreases the exposure for areas of the print that the photographer wishes to be lighter. In examining my result images, I noticed thatincreases the values of red, green, and blue above both of the initial values. According to the user's manual , the value of each pixel on the lower layer is multiplied by 256. The result is then divided by 256 minus the value of the corresponding pixel on the upper layer.

Color Pattern; Small Blocks on Top; Mode Dodge Photo Overlay; Mode Dodge Color Pattern; Small Blocks on Bottom; Mode Dodge

(Gray background represents transparency.)

Observations:

Because of the way the equation works, it is important which layer is on top.

Black on the upper layer has no effect on the final result.

Black on the lower layer results in black.

White on either layer results in white.

If the upper layer is grayscale and the lower layer is color, the effect of Dodge is to lighten the color layer without affecting the hue until it gets to white. That is, if you look at the HSV numbers, hue does not change until it pegs at white.

Color on both layers can result in either a lightening or a hue change, or both.

The lightening power of Dodge is more intense than Lighten only, Screen, or Addition.

Thoughts:

Use Dodge with a grayscale upper layer and a color lower layer to lighten the color layer without affecting the hue (in general).

Use Dodge on the upper layer of a duplicated image and vary the opacity to adjust brightening of the lower image without affecting the hue (in general).

Macaw + Grayscale Gradient;

Mode Dodge; Opacity 50% Macaw Duplicate;

Mode Dodge; Opacity 50%

See the following articles for examples of using Dodge mode.

The Addition layer mode does what its name implies. The red, green, and blue values of the two layers are added together, pegging at white. This results in an overall lightening of the combined image.

Color Pattern; Mode Addition Photo Overlay; Mode Addition

Observations:

It does not matter which layer is on top.

Black has no effect on the final result.

White results in white.

If one layer is grayscale and the other is color, the effect of Addition is to lighten the color layer without affecting the hue until it gets to white. That is, if you look at the HSV numbers, hue does not change until it pegs at white.

Color on both layers results in a lightening, plus a possible change in hue.

The lightening power of Addition is more intense than Lighten only or Screen, but less intense than Dodge.

Thoughts:

Use Addition with a grayscale layer and a color layer to lighten the color layer without affecting the hue (in general).

Use Addition on the upper layer of a duplicated image and vary the opacity to adjust brightening of the lower image without affecting the hue (in general).

Macaw + Grayscale Gradient;

Mode Addition; Opacity 50% Macaw Duplicate;

Mode Addition; Opacity 50%

If you have specific uses not mentioned here for any of these layer modes, please leave a comment. I'd love to hear about it.

For another review of layer modes in GIMP along with interesting visuals, see the four-part series at emptyeasel.com

For information on blend modes in general, independent of the software that implements them, see Wikipedia: Blend modes

See also:

GIMP's Layer Modes (Somewhat) Demystified – Part 4: A Pictorial Comparison