One of the most important tools in my abstract-generating kit is Layer Mode, also called Blend Mode and Mixing Mode. In an image editing program like GIMP or Photoshop, Layer Mode is a setting that controls how a top layer interacts (blends) with the layer below it. There are many different Modes and their effects vary dramatically, although the difference between some of them can be fairly subtle. I've been wanting for a while to make a chart of the Layer Modes using the same images for each Mode, so that their effects can more easily be compared and contrasted.
This post provides such a chart of almost all the Layer Modes, using a flipped version of the same image as the top layer. Then, a truncated version of the chart is made available, illustrating the major Layer Modes with two different images for each layer. Finally, a couple of practical applications of using Layer Modes will be discussed. It's modal madness!
(Btw, the photo I have chosen is one I took in Barcelona's Gothic Quarter. One of my favourites, especially with my signature initial filter passes; the Mean Curvature Blend and Cartoon filters work really well with dressed stone. I made an applied design out of it - see this blog post.)
Bottom layer on the left - Top layer on the right
For no particular reason, I rotated the image 90. In this example, the horizontally flipped, top layer completely overlaps the bottom layer, covering it entirely. Also, the opacity of the top layer is set to 100%. Almost forgot - these images were generated with GIMP, although (I believe) the effects are the same in Photoshop. Hm, now I have to test that; it would be good to know. I chose GIMP for this because it has few more Modes than PS.
Lighten - Addition - Screen
The two Modes I use the most by far are Lighten and Darken. When it comes to GIMP, these are actually four Modes, because there are two subtly different versions of each. We'll just think of them as Lighten and Darken. These Modes (especially Lighten) do the most to transform the results by removing some of either light or dark values and filling in detail from the opposite value. So, Lighten will remove some dark values and replace them with light values from the other layer, as you can see in the example above - e.g., the diamond shaped dark outlines are missing at the top centre. This is a very productive way to erode and fragment an image into abstract unrecognizability.
Darken - Multiply - Burn
Dodge - Divide - Grain Extract
Soft Light - Hard Light - Hard Mix
Pin Light - Overlay - Luminance
Difference - Subtraction - Exclusion
One might be tempted to wonder, "Just how, exactly, do these Modes work?" The answer is "Algebra." Please do refer to the previously mentioned Wikipedia article about this for all the math one could wish for.
Having spent some time now producing and evaluating the above examples, I think using a flipped version of the same image as the top layer probably isn't the very best way to illustrate these Mode interactions. It's something I do all the time, if not invariably, when I'm creating abstract patterns, but having both layers be two versions of the same image makes it less obvious which layer is contributing to the result.
Here's an abbreviated chart of the Modes I find to be the most productive. The bottom layer photo was taken in Amsterdam, the top layer in Sintra, Portugal.
Bottom Layer - Top Layer
Overlay - Multiply
Pinlight - Burn
Difference - Subtract
A Couple of Practical Applications
Using Darken Mode is a fast and easy way to superimpose an object over a background with out any selecting, copying and pasting (or erasing from a duplicate layer). You don't have to define the perimeter of the object because Darken does it for you. In order for this to work, the object must me fairly dark, while the area outside the object is as light and uniform as possible - the closer to white, the better. The background that you are superimposing the object on to should also be quite light, or at least not too dark.
Darken is also useful for filling in light areas against a very dark background. For example:
Here are the same three images using Lighten, which fills in dark areas against a white background:
That's more than enough on this topic for now. I have more Portfolio work to do!