Simon Wood wrote:
> 
> Hi all,
> I'm new to Gimp, yes I have read the manual....but I'm having difficulty
> getting my head around the following problem.
> 
> I would like to create a RGB  image, as a layered collage of monochrome
> patches (cut out from other images) each of  which will have a different hue
> (i.e. one patch is black/green, another is black/mild-pink, etc.).
> 
> Q). What is the best way to change the hue of a patch of 'grey-scale' to a
> specific setting?
> 
> I can select 'image-color-hue/saturation', but this doesn't seem to work
> with a 'grey-scale' patch in a RGB image.
> 
> The only way I've found if to use two layers for each 'patch', the top with
> the grey-scale image set to 'multiply', and another of sold colour (of the
> desired hue) with the same bounds (shape) as the top.
> 
> The main problems with this are:
>         * it requires twice as many layers to compose the image.
>         * it is difficult to keep track of the bounds of each patch (when
> scaling etc.).
>         * the 'layer-pairs' interact with each other until their elements
> are merged together.
> 
> Thanks in advance for any help or suggestions,
> Simon Wood

Hi Simon,

I've thought about this and I have a suggestion for how you might be
able to pull it off with only two layers.

Here is how you would do it.

Arrange the 'patches' using layers and set guides to border each one. 

Once you have placed all the images where you want them and you have all
the guides in place flatten the image.

Right mouse click and select "Image > Colors > Desaturate". This will
make the image look like it is greyscale.

Now duplicate the single layer and change the new layer's mode to
"Color".

Using the retangle marquee select each 'patch'.  This will be easy
because the marquee will "snap to" the guides.

Open up a Pallete of colors that you like and selece a color then use
the paint bucket fill tool to fill each 'patch' with color. (and do any
other modifications you need to do to the hue/saturation, etc.)  

I have made a screengrab that will help illustrate how this works:
http://obscura.obscurasite.com/images/screengrabs/01-02-00_235039_screenshot.jpg

I just sliced up a single image with two guides but I could have, just
as easily, pasted four different 'patches' into a blank canvas.  

Now, I know you're probably thinking... "What if the patches are not
rectangles with perfectly vertical and horizontal sides?"  

Easy enough.  Allow me to demonstrate with another quickie tutorial:

Start with a blank image and then paste a 'patch' that has been
de-saturated into a new layer.  

Then use the transform tool to warp and distort the patch however you
like.  

Duplicate the layer with the patch and change the duplicate to "Color"
mode.

Use "alpha to selection" to select the patch in the "Color" layer and
then use the paint bucket fill to color it.  

Right mouse click on the "Color" layer and select "Merge Down" to merge
it with its de-saturated mate.

You will end up with a layer for each patch.

This screengrab shows three 'patches'.  I stopped short of the "Merge
Down" step on the green patch so you could see how it works:

http://obscura.obscurasite.com/images/screengrabs/02-02-00_002443_screenshot.jpg

I hope this helps.  If this is indeed the technique you were looking for
let me know.  I might consider writing up a proper tutorial.  If this is
not what you were looking for let me know and I'll make another attempt.

Happy Gimping!
--
Jon Winters
visit the Obscura Lounge in OpenVerse
http://www.openverse.org/

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