Color Range Selection using Lab values

ED
Posted By
Ellen_Day
Aug 4, 2008
Views
465
Replies
6
Status
Closed
Is there a way to select pixels in Photoshop based on given Lab values? I am using Select –> Color Range with a specified "fuzziness" threshold, but I’d rather make the selection based on specific Lab values instead of random pixels that I choose with my eye dropper. Is there a way to do this in Photoshop, or does anyone know of a plug-in that might do this?

Also, if I end up having to use the basic Color Range tool, I’d like to know what "fuzziness" means. I understand what it does (selects more or less colors based on the threshold around the point you choose with your eyedropper), but what is that based on? A color difference (delta E) value around that point maybe?

I am using Photoshop CS3 v10.0 on a Windows XP machine.

Any help would be greatly appreciated.

Thanks, Ellen

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JM
J_Maloney
Aug 4, 2008
If you go to select color range, your current foreground color is the first selection. No idea on the fuzziness factor.

J
F
Freeagent
Aug 4, 2008
You can target specific Lab values in the color picker, which then, as J Maloney says, becomes your first selection.

I’ve always thought of fuzziness as applying a gaussian curve either side of the specific values, which grows or shrinks with the set percentage. Works for me.

But I agree it would be nice to know precisely what it means.
C
Curvemeister
Aug 5, 2008
Select Color range actually does use Lab values for its selection, with the fuzziness denoting the range of lab values.

If you want to experiment with this, the Labmeter image is a text image with a grid labeled in a and b values – each range of selected color shows as a square with shaded edges.
http://curvemeister.com/tutorials/LabMeter/index.htm

For a more quantitative control, convert the image to Lab, dupe the image to a new layer, and use the "Blend If" sliders in the layer properties to select specific ranges of a and b.
GA
George_Austin
Aug 5, 2008
It may be helpful to compare the Color Range command with the Magic Wand Tool. A Magic Wand "selection" is 100% opaque and all pixels outside its tolerannce have zero opacity. So the selected pixels are either totally selected or totally unselected. There is no partial selection.

Not so with the Color Range command. The pixel clicked establishes the "base" color. Whereever that exact color occurs in the image, it is totally selected (100% opaque). The degree to which other pixels are selected (their opacity) depends on the Fuzziness setting.

At zero Fuzziness, all pixels other than those with the base color are totally unselected (zero opacity). As the Fuzziness setting increases, pixels with values other than the base value begin to be selected, but not 100%. There is a bell-shaped (Gaussian) curve of opacity centered on the base value and dinminishing on either side of it.

The breadth of the Gaussian curve (let’s define that as the color value difference between 50% opacity on the left and 50% opacity on the right is directly proportional (not equal to) the Fuzziness. Double the Fuzziness and you double the breadth. Marching ants appear at the 50% opacity point. But be careful interpreting this because with the Magic Wand, for example, the 50% opacity point occurs at the same point as zero and 100%, there being no gradation of opacity at the edge of a Magic Wand selection.

You need to be aware also that the Color Range command operates in each color channel and the deviation from base value is determined by the most restrictive of the three channels. If a pixel is within Fuzziness limits for one channel but not another, it doesn’t pass muster. And, yes, CR uses Lab colors, but you need not be in Lab mode.
ED
Ellen_Day
Aug 5, 2008
Thank you all so much for your very quick replies. I think that it definitely helps me a lot to know that the foreground color is the first selection in color range.

Thanks!
F
Freeagent
Aug 5, 2008
And let me add that George Austins detailed explanation was very valuable for me.

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