3 Opinions

1. 

   saulgoode 05 September 2011 at 12:38 pm

   “Since the top layer will by default completely cover the rest of the layer stack, the final image will be exactly what you saw when you disabled some of the channels.”

   Such is not the case if there are any semi-transparent regions in the image. In these situations, the final image is the projection of the the original projection composited with itself (i.e., the top layer is blended with the original layers). The opacity of a pixel in this final image is 2α/(α^2). For alphas of 0 (fully transparent) and 1 (fully opaque), the final image would be identical; however, anything in between results in a more transparent final image.

   There are a couple of remedies available for this, should it arise. First, after creating your Layer New From Visible, Save As to a PNG file and when the export dialog appears, choose the “Ignore” option (this will save just the active layer and not the merged projection).

   A second approach would be to, instead of using Layer New From Visible, perform an Edit->Copy Visible followed by an Edit->Paste As->New Image. The new image can then be saved to an appropriate file format (you can even save to XCF).

   Thanks for sharing your tutorials. They are greatly appreciated.

2. 

   saulgoode 05 September 2011 at 12:47 pm

   In addition to HTML mucking up my formula, I posted it wrong.

   The alpha of two composited pixels is the product of the two alpha divided by their sum. For two identical alphas, the result should (alpha^2) / (2 * alpha). For example, two pixels each with an alpha of 0.5 would composite to an alpha of 0.25

3. 

   prokoudine 05 September 2011 at 1:38 pm

   Oh, this is an interesting remark! Obviously I’ve got to play with such cases! :)

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