[PLUG-TALK] Changing color of an image with the GIMP

[Rogan Creswick]

On Sun, Nov 16, 2008 at 3:07 PM, Richard C. Steffens <rsteff at comcast.net> wrote:
> I have a clip-art image I want to change from being black on white to
> being orange on white. I've found all kinds of interesting color tools
> in the GIMP, but I haven't found one that is connected to the DWIM (Do
> What I Mean) button.  I've found some instructions on the web that say I
> should play with the hue-saturation controls. But, when I go to that
> panel, I get a message that tells me that hue-saturation only works on
> RGB color layers. I get that message with a number of the color

You've probably figured out a bunch of this by now, but just incase
it's of use to someone...  I'm also not an expert, but I think I can
help clarify some of this.

The amount of "stuff" you can do to an image is generally restricted
by the way the colors are stored for the image.  For instance, a
"true" monochrome image may only be able to store one of two colors
per pixel (This would be a black / white image, for example, with no
grays.)  An indexed image has a pallet, as you've found by now, and
each pixel is designated a color from that pallet.  (I do not know how
alpha channels affect things, if indeed, they even come into play.)

The gimp won't let you add colors to a black / white image (keep in
mind, there are many *color* images that only use two colors...), or
add colors that aren't in the pallet to an indexed image.

You can change the _Mode_ (Indexed, RGB, etc.) of an image in the gimp
with the Image -> Mode menu.  Once you've switched your indexed image
to RGB, you will be able to do more things with in.  (of course, you
may not be able to save it as indexed again without regenerating a
pallet, and that may or may not cause issues down the line -- probably
not though.)

> Does anyone know how to identify a color in an image and then change
> every instance of that color to another color?

I've often wondered how to do this in the general case.  Say, to
change all the blues in an image to greens with corresponding
saturation / value.  I have a hunch the process is similar to that of
making a sepia-tone image, but I have never figured it out.

> The "Foreground Selection Tool" tells me to draw a rough circle around.
> When I do, everything outside the circle turns blue -- I think to
> indicate it's selected?

I think your logic is reversed -- the blue portion looks to be the
_unselected_ area.  Gimp won't let you do things outside a selection,
so that could be why no operations were working on that area.

--Rogan


>
> --
> Regards,
>
> Dick Steffens
>
>
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