Color/Luminosity separation

Started 3 months ago | Discussions thread
Mark Hollister Regular Member • Posts: 482
Re: yes

Tom Axford wrote:

Mark Hollister wrote:

Tom Axford wrote:

Mark Hollister wrote:

Tom, in GIMP is it possible to make selections based on HSV values? I still haven't delved into GIMP because (as far as I know) it still doesn't have adjustment layers. Thanks.

I am not quite sure what you are asking, but I'll try to give an answer.

GIMP has an operation to decompose an image into three separate layers, one for H, one for S and one for V.

Alternatively, there is an operation to select just one of the three. So no new layers are created, but the image is changed into its Hue or its Saturation or its Value.

Normally, I don't use either of those because any changes I want to hue and saturation I normally do with the usual tools operating directly on the RGB image.

I want to use HSV for operations like Curves and sharpening so that they only operate on the V channel. The easiest way I have found to do this is simply duplicate the layer and set the mode of the top layer to HSV Value. Then changes to the top layer are interpreted as only applying to V when the top layer is blended with the bottom layer. The advantage of this method is that you can see the results as you apply the operations, no need to recombine the layers later.

Note that I could have extracted the V component in the top layer before using Curves, etc., but it is not necessary as the results are exactly the same for operations like Curves which are applied equally to R, G and B (and hence to V as it is just the maximum of R, G and B).

I hope this is helpful.

Well, I learned something about GIMP I didn't know. Thanks Tom. The more I find out about GIMP the more intrigued I am. I'd like to eventually switch to it but I want to be able to do everything I can do in Photoshop before I do.

The making of selections I was asking about are found in Photoshop's menu Select > Color Range, which, when you do that, highlights a color you sample wherever it's found in the picture, with "marching ants." This selection can then be used for any purpose - for example, to saturate those selected locations, or desaturate them, to denoise them or sharpen them, or whatever else you'd like.

So what I'm describing is selecting particular areas of a picture based on color. But now you've got me wondering if GIMP can do likewise, ie make selections based on particular Hues, degrees of Saturation or Value. Hmm...

By the way, does "Value" have the same meaning as "luminosity" in Photoshop?

Thanks again.

Value is just equal to the largest of the R, G and B values for each pixel. It is a measure of lightness/brightness/luminosity, but not quite the same. Luminosity is a weighted average of R, G and B for each pixel.

I am sorry but I do not use selections, so I am not familiar with the various ways of defining a selection. My processing is nearly always done globally on the whole image.

Thanks for this info Tom. If you ever decide to try out selections, this is a good video by Sean Bagshaw.

https://youtu.be/BaTgZvJNn6A

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