Re: [Gimp-user] Layer Masking

[Steve Kinney <admin pilobilus net>]

On 03/10/2012 12:18 PM, Madeleine Fisher wrote:
> Hello! I am an artist trying to create a certain effect that I
> can't seem to figure out on my own:
> In a picture, I have areas in shadow and areas in light. I want
> the light parts to be the "real colors" (brown barrels, red
> apples, etc.), but in the shaded areas, I want it to be
> purple--perhaps a purple version of the "real" colors.
> Any thoughts? (Especially ones that aren't too technical?)

Hi Madeline!

There is a more or less "standard" way to do this.  Sometimes it
works "the easy way, first try", other times it takes some tweaking,
but it's a good starting point.

Open your original image and duplicate of the base layer twice. 
(The "two rectangles" button in your layers dock.)  You should end
up with three layers, all alike.

Select the top layer, and apply the Threshold tool (Colors >
Threshold), tweaking it as necessary to produce a black and white
layer where your "shadow" is black and the rest is white (or as
close as you can come).

Select the second layer, right click, and "Add Layer Mask."  Accept
the default properties for this.

Select the top layer (the one you reduced to black and white above),
and do Control-C to copy it.

Select the mask on the second layer (just click on it), and do
Control-V to paste the copied layer.  Anchor the pasted layer
(anchor icon at the bottom of your layers dock), and the pasted
layer will replace the all white mask.

Delete the top layer, that you reduced to black and white above.  It
is no longer needed.

The layer mask is, in effect, a stencil.  Where the mask is white,
its parent layer is visible and its contents will replace the first
layer in the finished image.  Where the mask is black, the parent
layer is transparent and its contents will not be present in the
finished image.  Remember to select the "image" part of the masked
layer before applying filters, color adjustments, etc.; otherwise
they will be applied to the mask itself.

Do whatever you want to the base layer, to change the appearance of
the image where it is in shadow.  Do likewise to the layer copy, to
change the appearance of the image where it is more brightly lit.

Note that you can paint on the layer with black or white to subtract
or add visibility; you can also fill selections with black or white
to make parts of the 2nd layer vanish or reappear.  You can apply a
blur filter to soften the edges of the transition, or etc. - any
effect you can apply to a black and white layer can be applied to a
mask, to alter its effect on the parent layer's visibility.

There are a couple of "before and after" examples where I used this
method, on this page, it is most visible in the "Real Estate Photos"
one:

http://pilobilus.net/photo_rework.html

:o)

Steve