Why is this color mapping not working?

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Why is this color mapping not working?

BrentBrotine
HI,

I'm using Gimp 2.8.18 on Mac High Sierra.  I want to do what I hope is very
simple -- switch out one color with another.  I wanted to change the white to a
dark gray and teh black to a lighter gray.  I first changed the white color,
which went ok.  Then, I want to change all the black to a gray color -- #D3D3D3.
I open my image, go to the Color menu, and select "Map" and then select "Color
Exchange".   Then I select the relevant colors and click "OK".  However the
result (attached) is disappointing.  It is very difficult to read the numbers in
the scale.  Am I missing something?  Or there others colors at play here?

Thanks for any advice, - Dave

Attachments:
* http://www.gimpusers.com/system/attachments/815/original/scale.jpg
* http://www.gimpusers.com/system/attachments/816/original/scale_gray.jpg

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laredotornado (via www.gimpusers.com/forums)
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Why is this color mapping not working?

BrentBrotine
>HI,
>
>I'm using Gimp 2.8.18 on Mac High Sierra.  I want to do what I hope is
>very simple -- switch out one color with another.  I wanted to change
>the white to a dark gray and teh black to a lighter gray.  I first
>changed the white color, which went ok.  Then, I want to change all
>the black to a gray color -- #D3D3D3.  I open my image, go to the
>Color menu, and select "Map" and then select "Color Exchange".   Then
>I select the relevant colors and click "OK".  However the result
>(attached) is disappointing.  It is very difficult to read the numbers
>in the scale.  Am I missing something?  Or there others colors at play
>here?
>
>Thanks for any advice, - Dave

Not the best way, guessing some sort of selection to choose the background
leaving semi-transparent pixels around the text.  A jpeg especially tends to
promote artefacts around text.

A better way.

Remove the background using Colour -> Colour-to-Alpha White is default

Lock the transparent pixels on the layer using the alpha lock in the layers
dialogue.

Bucket fill the layer with the d3d3d3 gray.

Add a new layer under and fill with the darker gray.

Something like this is best exported as a png which because of the 2 colours is
smaller than a jpeg.

screenshot and xcf is attached. xcf should still have alpha lock in place as
example.

rich: www.gimp-forum.net

Attachments:
* http://www.gimpusers.com/system/attachments/818/original/no-selection.jpg
* http://www.gimpusers.com/system/attachments/819/original/scale.xcf

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rich404 (via www.gimpusers.com/forums)
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Re: Why is this color mapping not working?

Ofnuts-2
In reply to this post by BrentBrotine
On 02/13/18 23:55, laredotornado wrote:

> HI,
>
> I'm using Gimp 2.8.18 on Mac High Sierra.  I want to do what I hope is very
> simple -- switch out one color with another.  I wanted to change the white to a
> dark gray and teh black to a lighter gray.  I first changed the white color,
> which went ok.  Then, I want to change all the black to a gray color -- #D3D3D3.
> I open my image, go to the Color menu, and select "Map" and then select "Color
> Exchange".   Then I select the relevant colors and click "OK".  However the
> result (attached) is disappointing.  It is very difficult to read the numbers in
> the scale.  Am I missing something?  Or there others colors at play here?
>
> Thanks for any advice, - Dave
>
> Attachments:
> * http://www.gimpusers.com/system/attachments/815/original/scale.jpg
> * http://www.gimpusers.com/system/attachments/816/original/scale_gray.jpg
>
Color>Invert and then use Contrast/Brightness to adjust the result.

The catch in your method is that you have more that two colors in the
picture, besides the two obvious ones (background and character bodies).
There are also:

- the JPEG compression artifacts (recognizable by the 8x8 blocks pattern)

- The pixels on the edges of the characters, that can be any blend of
the background & character colors to  create  a smooth-looking edge.

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