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Color adjustment; curving certain colors

Nov 9, 2011, 17:19 (This post was last modified: Nov 9, 2011 17:22 by Marscaleb.)
Post: #1
Color adjustment; curving certain colors
I draw images by hand and then scan them and digitally color them.

The white of the page is not a solid white, but full of variance.
Also, there are many tiny specs of dust and things that show up as grey specs on the image.

Prepping a black-and-white line drawing is easy; I just use color curves to set the mostly-white areas to pure white, doing the same with black, and tweaking the curve how I want to make my black lines sharper or smoother and cutting out those grey specs.

But I've been trying to work out how to add a second level of detail in these drawings. This comes in the form of a light blue. This light blue can be used for so many great things, like points to help line up an image that was drawn on multiple sheets or to represent lines that will becomes shadows or highlights that I don't want to show up in the final image.

Previously I had been scanning my pages as greyscale images to completely wipe out extra color variations that I didn't want. If I do that with the blue lines they turn grey and they get eroded when I adjust the color curves. If I scan it as full color I now have subtle color variants that throw things off, and adjusting the color curves still erodes those blue lines even worse.
If I try adjusting the color curves by color channel I turn the whole image to a particular tint.

How can I adjust the colors of an image to turn anything mostly white into white, mostly black into black, and mostly light blue into light blue; and preferably be able to adjust the scale it does this by so that I might prevent dust from becoming black spots and even maintain the greyscale range at the very edges of my lines?
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Nov 9, 2011, 22:11
Post: #2
RE: Color adjustment; curving certain colors
There are some contradictory requirements here. You cannot really differentiate a light gray pixel that is in "partial" pixel along the border of your lines from the same light gray pixel that is just due to scanner noise... At least not by color alone...

I would try something like this:

1) make all the lines black on the paper even what you want to be light blue
2) scan and load in Gimp
3) select by color the black lines
4) save selection to channel
5) grow the selection by a couple of pixels to make sure you got the borders in (and maybe avoid gaps in some lines). Check for selection patches in the background (dust specks) and remove them (roughly circle them with freehand while depressing Ctrl)
6) use the levels tool to extend the range of values of the selection from black to white.
7) invert the selection
8) paint in white
9) create a new selection that roughly encompasses what should become blue lines
10) intersect selection with the saved selection channel above (you should get only the future light-blue lines selected
11) add a light blue layer below
12) make sure the scan layer has an alpha channel, and delete. You should have changed the black lines into light-blue ones.

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Nov 13, 2011, 06:18
Post: #3
RE: Color adjustment; curving certain colors
It's not about turning the lines blue, it's just about isolating them from the rest of the drawing.
What you describe there,apart from return the lines to blue, is basically what I have been doing so far to isolate special portions of the drawing: copy out the lines and have a version with those extra lines erased.
I was hoping to find a better way to do that.

But what puts a greater challenge to me is to remove the blue lines from drawings where they were used for guides.

Perhaps I wasn't clear about this, but I am talking about modifying a color scan. I have a colored scan of black lines and light-blue lines on a white paper. And of course, each of those elements has a lot of noise. The black lines, because of errors in the scanner, have a slight tint on either side of them, about one pixel thin, one side red and the other side blue-green (I had not seen this particular distortion until now; I think I will switch my guide lines to yellow so I can't confuse them with the left edges of the black lines.) On top of that, those "black lines" now also have minor color fluctuations inside them. (This is hard to identify unless I zoom in about 800%)
On my greyscale scans I just used color curves to set all the black lines to a straight black and the white to a pure white, adjusting the middle ground to give me just as much fade between the two as I want (makes the lines smoother) but still keeping the solid parts solid.
I want to be able to do that with a color image, but the simple value-adjustment I had been making turns the edges into a colorful mulch of hippie dreams (which can be cured with desaturation) and the light-blue lines into a wide scale of value, as its natural value falls in the range I keep on the edges of the black lines.

I want to turn the grainy blue into a solid blue without warping the rest of the drawing.
This is just so that I can isolate them from the rest of the image; whether I delete them or shift them into a new layer for later evil purposes, I just need to isolate lines of a particular color from an image with very fine grain.
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Nov 13, 2011, 10:25 (This post was last modified: Nov 13, 2011 10:42 by rich2005.)
Post: #4
RE: Color adjustment; curving certain colors
This is similar to another post where you can use The G'mic plugin and colors -> lab curves -> B&W recolorize
Play around with it and you might get something acceptable.

Screenshot using the worst piece of paper I had to hand.
http://i.imgur.com/57rov.jpg

this shows cleaning up black and blue but you can remove the blue completely, depends on the settings.

** https://www.gimp-forum.net/ now answering questions**
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Nov 14, 2011, 23:53
Post: #5
RE: Color adjustment; curving certain colors
Wow....
I will take a closer look at that one. Thank you!
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