On 8/16/19 10:31 AM, Ofnuts wrote: > Specifically, I'm wondering about the "Split", "Merge", and the group > "Passthrough" modes. > > The only relevant Google answer is a paying video tutorial. > > Of course I have checked the online doc... There's indeed no organized documentation for all the new layer modes yet, unfortunately. For Pass Through mode, see: https://docs.gimp.org/2.10/en/glossary.html#glossary-pass-through Merge and Split are complementary modes, as their names suggest. They are similar to Normal and Erase modes, respectively, but they handle alpha values differently. The short version is that (when using their default composite modes) Merge adds the alpha values of the input layers, while Split subtracts them. In other words, if the alpha values of the bottom and top layers are A and B, respectively, the resulting alpha values for Merge and Split are `min (A + B, 100%)` and `max (A - B, 0%)`, respectively. --- Wall-of-text warning. Skip to the end for practical examples. --- The slightly more advanced version has to do with how alpha values are interpreted. Alpha values can be seen as a property of the sub-pixel geometry of a pixel; they specify how much of a pixel's area is covered by the layer's content. For example, when you rasterize an ideal shape, such as a circle, pixels along the circumference will only be partially covered by it; their alpha values specify how much of their area is covered by the circle. It's even more instructive to think of alpha values in terms of probability: when picking a random point within a pixel, the alpha value specifies the *probability* of the point belonging to the layer. The information conveyed by the alpha value is, of course, only partial: while it tells you *how much* of a pixel's area is covered by its content, it doesn't tell you exactly *which* portion of the pixel is covered. The latter becomes relevant when you combine multiple layers, though: two layers having a pixel with 50% alpha each, when overlaid on top of each other, can result in a pixel having anywhere between 50% alpha (if the areas covered by both layers coincides exactly) to 100% alpha (if the areas covered by both layers don't coincide at all). There is no correct answer here: we have to make a certain assumption about the relationship between the two layers. The "ordinary" assumption, used by most layer modes, is that the two layers are independent (in the probabilistic sense): whether a given point within a pixel belongs to one layer doesn't affect the probability of it belonging to the other layer as well; this leads to the familiar `A + B - AB` formula for combining alpha values. I call this the "diffuse" assumption. While it's a reasonable assumption in the general case, in can lead to artifacts when applied inappropriately. Most notably, it can lead to semitransparent "seams" between opaque objects that should, in theory, intermesh perfectly. That's a common problem with Inkscape, for example; it happens because Inkscape renders each object separately, throwing away the actual sub-pixel geometry in favor of alpha values, and then combining the alpha values with the (wrong) assumption that the objects are independent of each other. There are at least two other simple assumptions we can make about the layers: the "complement" assumption, where the two layers are thought to be mutually exclusive (i.e., to *complement* each other), and the "coincide" assumption, where the two layers are thought to occupy the same area (i.e., to *coincide* with each other). Note, however, that this formalism places some restrictions on the layers' alpha values (namely, that `A + B <= 100%` for "complement", and that `A = B` for "coincide"). A more precise formalism is that with "complement" the intersection of the two layers is as small as possible, and with "coincide" the intersection is as big as possible. One day, GIMP might allow this assumption to be controlled separately from the layer mode, through an additional layer attribute. For now, Merge and Split are singled out as two useful cases of non-diffuse modes: Merge is simply Normal mode with the "complement" assumption, and Split is Erase mode with the "coincide" assumption. --- Merge mode should be used when combining layers that are, logically, separate parts of a whole. The archetypical example is that of cutting an antialiased selection, and pasting as a new layer. When using Normal mode for the pasted layer, there's usually a semitransparent "seam" around the selection's boundary (as mentioned above). Using Merge mode instead fixes the issue, as the two layers are two parts of one whole, reproducing the original image. Split mode is complementary to Merge: it allows you to erase one of the parts out of the whole, producing its complement; the two parts can then be combined using Merge mode to reproduce the original. For example, you can split an image into two layers by first isolating one of the parts into a separate layer, combining the isolated layer with the original image using Split mode and creating a new layer from the result, and then combining the two layers using Merge mode: https://streamable.com/r7yie Note that the original image in the example deliberately has semitransparency to begin with, otherwise the same result could have been achieved with Erase mode instead of Split (but not with Normal instead of Merge). -- Ell _______________________________________________ gimp-developer-list mailing list List address: gimp-developer-list@xxxxxxxxx List membership: https://mail.gnome.org/mailman/listinfo/gimp-developer-list List archives: https://mail.gnome.org/archives/gimp-developer-list
