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problem with set color to alpha
(02-03-2019, 08:06 PM)rich2005 Wrote: There is a color erase but the same problem as before, in an RGB image you can remove elements of the 'erase' color but liable to leave remnants of the rest behind.
I use color erase quite a bit and found you can use the curves tool to clean up the remnants. 
This tut is a bit old but it still works.
(02-02-2019, 01:35 AM)rickk Wrote: New to all of this, Perhaps I am trying to go about this the wrong way.....not sure.
But, for instance,  suppose I wanted to cut an image of somebody  out of picture "A", and insert it into an already existing image "B"

SO, I painstakingly draw a white area  all around the perimeter of the desired target image, then extend that white area sufficiently large that I can select and crop to a rectangular  image for use.
Then I review the image and find a color not in use in the  target image, say  HTML color h1cf905 for  example sign of that color anywhere in the target image , and then change the white area surrounding the target to that color, and then set that color to alpha.

Hoping to paste that image, complete with the transparent surround, into the "B" image.
Except when I set the surround color to alpha, it's causing the colors in the target image to get all whacked out.  For instance  with the  color mentioned  set to alpha, everything is shifting into violet colors.

Almost seems like the "color to alpha" function isn't discriminating properly, and is perhaps shifting all greens to alpha?
Thanks in advance for pointing me in the right direction

Found your problem thread searching for the same solution as you .  
Tried the suggestions - was rather cumbersome. 
Finally solved the problem by adding an "Alpha Channel" to the image.  {Layer >Transparency >Add Alpha Channel}
(If you are working with multiple layers, you'll need to add the Alpha Channel to the specific layer you wish to edit.)
I made sure I did not have open lines to other areas in the image containing similar colour as my background.  Background happened to be white.  Once that was done, I used the Fuzzy Select Tool on the background, then chose "Colour to Alpha".  {Colours >Colour to Alpha}
Worked like a dream.  The "Select by Colour" tool is a bit different - it will change every piece of the colour you picked to Alpha.  Of course - you can also adjust the Threshold on either colour picker tool to ensure you have less chances of grabbing reds as well, if you only want a pink back ground erased.

Really posting my little story for others looking for a quick solution.  Hope this helps!
(01-04-2022, 10:39 AM)Jeanne Wrote: Really posting my little story for others looking for a quick solution.  Hope this helps!

Hi Jeanne,  thanks for your help.

I've come such a long way in these past 2 years thanks to the patience and expert coaching of Rich, Ofnuts, and the rest of these folks.

Bighty's suggestion to use one of the paint tools in erase mode was an epiphany of sorts for me, never really realized that was possible. I had used gimp sparingly for about 15 years prior, and never managed to discover that.

Now that I'm old and have time on my hands, I've been amazed with what I have learned.

Now, my "go to" solution for the original problem is a targeted use of either fuzzy select or bucket fill in erase mode...but using the "threshold" adjustment at different levels until I get the desired output. I may end up having to undo, adjust, and reattempt 8-10 times, but with the right setting it can be lots easier than the way I've historically done it. 

I've also found that in highly varied background environments, for instance gravel,  (using bucket fill in erase mode) it really helps to try and retry clicking on various parts of the drawing, hoping to find a "sweetspot" in the color spread. Hit the right color, and you get perfect removal, but hit a color to either end of the range, and the results are  often disappointing.   Blush

Hello rickk
That problem  with colour to alpha and the colours in the image fading, depending on the shape etc etc (using the correct brush also helps)  the image can be brought back up to full colour without affecting the the background if you carefully work around inside the image with the erase tool set to anti-erase.

Hope it helps you.

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