Selection by color range

K1TesseraEna · **Joined:** Sep 22, 2012 **Posts:** 2089 **Location:** US

Two questions

1. When I use Select by Color Tool in gimp is there any way to find out the RGB values of the selected pixel (suppose threshold is 0)
2. Is there a script that would select pixels by color range, the same as Colors>Color to Alpha but for a wider range?
PS has a tool for that, gimp doesn't. I've tried Select by color with increased threshold or Shift-Click to add to selection, but the results are not the same.
When I do color range selection in PS it differs significantly.
You can do some experimenting in gimp to get a better idea of what i mean. Make a solid red layer below greyscale image and try to select pixels with Color to Alpha and then with Select by Color tool in gimp (change threshold value)
I know RobA wrote a scripts for that, but the link is broken and I can't find it anywhere to run a test.
Any workaround that you know of?

saulgoode · **Posted:** Sun Feb 24, 2013 1:36 pm **(#2)**

http://chiselapp.com/user/saulgoode/rep ... lar-colors

molly · **Joined:** Apr 07, 2010 **Posts:** 14182

Thanks saul, cool

Esper · **Joined:** Jul 17, 2012 **Posts:** 2769

interesting post, K1te, i remember needing something like this in the past

and thanks for yet another great script, saulgoode ! :bigthup

ofnuts · **Joined:** Oct 25, 2010 **Posts:** 4812

K1TesseraEna wrote:

[font=Tahoma]Two questions

1. When I use Select by Color Tool in gimp is there any way to find out the RGB values of the selected pixel (suppose threshold is 0)

The Pointer dialog (Windows/Dockable dialog/Pointer) will tell your the RGB (and HSV) values of the pixel under the pointer.

K1TesseraEna · **Joined:** Sep 22, 2012 **Posts:** 2089 **Location:** US

thanks Saulgoode, ofnuts!

pointer dockable dialog is something i've never happened to use before, i'am certainly going to make a use of it now.

saulgoode, thanks for the script, i have yet to run some tests and see if this is what i need. Will report back later.

molly · **Joined:** Apr 07, 2010 **Posts:** 14182

Quote:

The Pointer dialog (Windows/Dockable dialog/Pointer) will tell your the RGB (and HSV) values of the pixel under the pointer

I have never seen that before, that is a really cool tool.

Akros · **Joined:** Jan 10, 2013 **Posts:** 863

K1t maybe http://blog.patdavid.net/2012/07/gettin ... color.html can be utility.

K1TesseraEna · **Joined:** Sep 22, 2012 **Posts:** 2089 **Location:** US

Akros wrote:

K1t maybe http://blog.patdavid.net/2012/07/gettin ... color.html can be utility.

Yes, helpful info on using multiple pointers to mark up canvas, thanks Akros.

Akros · **Joined:** Jan 10, 2013 **Posts:** 863

K1t, rolling menus (most never used before or clicked) found the Rotate Colors (Colors>Map).
It seems to me another tool that could have this functionality "Selection by color range"

At the top you determines the range of colors to be changed, and in the low range of colors you want to get.

It works differently from the PS Color Range, and is not as immediate and efficient, but I think from now on I'll use it enough.
:seeya

ofnuts · **Joined:** Oct 25, 2010 **Posts:** 4812

Akros wrote:

K1t, rolling menus (most never used before or clicked) found the Rotate Colors (Colors>Map).
It seems to me another tool that could have this functionality "Selection by color range"

At the top you determines the range of colors to be changed, and in the low range of colors you want to get.

It works differently from the PS Color Range, and is not as immediate and efficient, but I think from now on I'll use it enough.
:seeya

"Rotate colors" should really be "Rotate hues".

rich2005 · **Joined:** Mar 04, 2011 **Posts:** 2613

It is worth remembering that gmic has a color select filter, flexible and useful because it can output a mask.

K1TesseraEna · **Joined:** Sep 22, 2012 **Posts:** 2089 **Location:** US

@Akros I agree with Ofnuts, we don't want to change the hue.

It seems I've found a workaround but it's a bit tedious. Still investigating, too early to post.

@rich selecting a color in gmic, creating a new layer, adding a greyscale image mask to it, copying that mask to main image does not really sound like an easy solution here, but at least gmic has a preview window. Replace Color filter seems to be more spot on, since it has an option to replace selected color with transparency.

Saulgoode's script replicates the Select by Color tool with the exact same tool options, and by definition (Scheme script) can't have a preview which is available in Color>Color_to_Alpha or PS's Color_Range.

Not much info in gimp documentation about the mechanism behind Color to Alpha function. But that's what I actually need with fuzziness/threshold control to pick analogous colors.

saulgoode · **Posted:** Mon Feb 25, 2013 11:16 am **(#14)**

K1TesseraEna wrote:

Not much info in gimp documentation about the mechanism behind Color to Alpha function. But that's what I actually need with fuzziness/threshold control to pick analogous colors.

If you have GAP installed, try "Video->Bluebox...".

ofnuts · **Joined:** Oct 25, 2010 **Posts:** 4812

K1TesseraEna wrote:

@Akros I agree with Ofnuts, we don't want to change the hue.

It seems I've found a workaround but it's a bit tedious. Still investigating, too early to post.

@rich selecting a color in gmic, creating a new layer, adding a greyscale image mask to it, copying that mask to main image does not really sound like an easy solution here, but at least gmic has a preview window. Replace Color filter seems to be more spot on, since it has an option to replace selected color with transparency.

Saulgoode's script replicates the Select by Color tool with the exact same tool options, and by definition (Scheme script) can't have a preview which is available in Color>Color_to_Alpha or PS's Color_Range.

Not much info in gimp documentation about the mechanism behind Color to Alpha function. But that's what I actually need with fuzziness/threshold control to pick analogous colors.

It is only reverse alpha-compositing. The color-to-alpha "contract" is simple:

Code: Select all

layer with removed color stacked over layer filled with removed color = original layer

This definition leads to an ambiguity, for instance, if you remove white from an image with gray, the gray in the original layer could become either:
- fully opaque original gray
- partially opaque black
- any gray in the original gray->black range, with an opacity somewhere between the two above.

C2A makes it unambiguous and shoots for maximum transparency, so you would get partially transparent black in the case above.

One way to get a color selection using the rotate color:
- duplicate layer
- use Rotate Color to map your wide range of selectable hues into a very narrow one
- add another layer, fill with some color (not gray), and put in saturation mode
- new from visible
- color selection on that layer
- discard all added layers, but keep the selection.

K1TesseraEna · **Joined:** Sep 22, 2012 **Posts:** 2089 **Location:** US

@saulgoode thank you, Bluebox seems more versatile that C2A or Color range!
@ofnuts I've been playing with C2A for a couple of days now, pretty much figured out how it works, for color range to work I run a sequence of C2A filter to get a color range out of the image.
Is it possible to get a gradient (as a color range) out of the image? Not talking about Full saturation spectrum gradients, of course. Simple dual-color or a bit more colored gradients. Just curious, don't see a practical need for that so far but it would be interesting to experiment with that.

Thank you all for contributing, that was a very useful and interesting experience.