No, I do know what I am talking about. And if I could generate example images, I wouldn't be requesting the feature.
Sorry if that sounded short, but I get the feeling that you don't understand me: In all combine modes, pixels from that layer and the composite of the layers below are subjected to a function - for example, in Multiply mode, the pixel values are multiplied. For overlay mode, the value is obtained via a slightly more complex formula; usually something like:
if a < 128 then
result := (a*
else
result := 255 - ((255-a) * (255-
However, this will return different images based on which layer, a or b, is placed on top. A Symmetric overlay is calculated by generating the image both ways round and averaging them. I could do this in CodeLab, except for the fact it _should_ be a combine mode rather than an effect.