On Thu, May 19, 2011 at 8:39 AM, kevin gilbert <kmg...@bigpond.com> wrote:
> In the section "5.8 Practical Uses of Blending Modes" of the book "Grokking
> the GIMP" (see, e.g., http://www.dig.cs.gc.cuny.edu/manuals/Gimp2/Grokking-
> the-GIMP-v1.0/node57.html ) there is an example of how to use blending modes
> to alter the colour of a cat.
> Specifically, I refer to Figure 5.29(a) where the re-coloured cat does not
> appear to sit naturally on the person's shoulder (cf Figure 5.26(a)).
> How can the defect be overcome? Would feathering the selection help? But would
> the bucket fill also be feathered?

The defect arises from:

1. use of the Color layer mode (which, until recent versions of GIMP,
was a fairly inaccurate way of recoloring anything. particularly, it
performed quite poorly at maintaining similar perceptual brightness.).
A better alternative might be the Color layer mode in more recent
(2.7+) GIMP versions, or the LAB color mixer found in G'MIC

2. Failure to consider radiosity (the coloured light which is
reflected from the shirt to the cat, and the coloured light that is
reflected from the cat to the shirt.
If you change the color of one part of a photo, you will need to
adjust the surrounding objects to appear to reflect the right color.

3. Quite frequently also the new colors are poorly chosen, so that it
mismatches the overall lighting conditions. You need to ensure that
the recoloring matches not only in perceptual brightness, but also in
vividness: the apparent vividness of an object is heavily limited by
the lighting conditions

4. Lastly, the actual shape of the mask you are using to mark which
pixels count as 'cat' for purposes of recoloring; merely selecting
those regions which seem to clearly be cat regions results in an area
that is too strongly defined.
You can sometimes get away with using feathering, if the object is not
particularly in focus. The rest of the time, some edges need to be
fuzzy and some need to be crisp (basically in relation to their angle
to the light.)
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