LAB question

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bruce1951
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LAB question

Post by bruce1951 » Tue 02 Oct 2018 12:20

I've been experimenting with lab colour space/mode.

I open a simple image that is RGB. Layer/Convert layer type and make it lab. Save the file with a new name. Open the file and everything is fine. Image is lab. Layer is lab. So I then try to use a plugin on the layer and the plugins are grayed out. Nik and Topaz plugins. (I tried to address some noise issues with Topaz DeNoise).

So simple question - why won't those plugins work on lab layers? Am I missing something basic with lab?

bruce

pixel8tor
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Re: LAB question

Post by pixel8tor » Tue 02 Oct 2018 13:39

It all depends on the plugin author what color spaces they support - RGB, CMYK, Lab, 8bit, 16bit, or others - they are all handled differently because of the different channel configurations. So if you find plugins that support more than one color mode, the developer had to explicitly write it to handle them.

HTH

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Hoogo
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Re: LAB question

Post by Hoogo » Tue 02 Oct 2018 18:22

Once I tried to discuss that with the developer of NeatImage, but I wasn't too successful, see here. At that time he asked to disable all channels except L, then NI would filter the L channel like a gray scale layer.

If I remember correctly, Ximagic filters could handle Lab layers, but the denoiser was very technical, and it seems there's not much development going on anymore.
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bruce1951
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Re: LAB question

Post by bruce1951 » Tue 02 Oct 2018 23:47

Thanks.
At one point while trying my layer went to grey scale!!! No idea what I did for that to happen.

So it's the plugin rather than lab being the problem?

One more question. (No I have a zillion questions). If I work with lab layers is it ok to convert the layer to RGB just to use a plugin that doesn't support lab and then convert it back again. I realize this is counter productive with some tasks but wouldn't be such an issue with other tasks.

I'm hooked with curves in lab mod. Colours are much easier to control for what I'm looking for.

Why didn't I 'discover' lab years ago? And why isn't it more widely used?

bruce

bkh
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Re: LAB question

Post by bkh » Wed 03 Oct 2018 14:54

bruce1951 wrote:
Tue 02 Oct 2018 23:47
One more question. (No I have a zillion questions). If I work with lab layers is it ok to convert the layer to RGB just to use a plugin that doesn't support lab and then convert it back again. I realize this is counter productive with some tasks but wouldn't be such an issue with other tasks.
Why don't you work the other way round? Most PL tools (including curves) can be used in Lab mode even if the source layer is RGB. Unless you do a lot of processing steps in Lab, this should work equally well.

Converting back and forth between RGB and Lab shouldn't be a problem if you work with a colour depth of 16 or 32 bits.

Cheers

Burkhard.

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Hoogo
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Re: LAB question

Post by Hoogo » Wed 03 Oct 2018 21:42

bruce1951 wrote:
Tue 02 Oct 2018 23:47
Why didn't I 'discover' lab years ago? And why isn't it more widely used?
As far as I understand, Lab is related to a color model named XYZ, which again is based on a measurement of human perception. That makes it great for converting colors between different hardware or color spaces, and it's also beautiful for changing colors by taste.

On the other hand, RGB is much more "physical", and so it's much more useful when you want to simulate natural effects.
Quick example, it's easy to change the color temperature using curves in RGB. Decrease the output of blue, decrease green a little less, and you have a warm light. I'm not sure how I could do that in Lab.
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bruce1951
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Re: LAB question

Post by bruce1951 » Wed 03 Oct 2018 23:29

Thanks. I've done a bit of playing around. On photo images there doesn't seem to be much affect switching back and forth between rgb and lab. But with the work I do there are huge colour shifts between rgb and lab. ( You may get a better idea if you look at my web site https://500px.com/brucetreloar ). On my artistic work the colours change too much for me to swap back and forth. So I will stick to rgb and leave lab just for photo work.

bruce

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russellcottrell
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Re: LAB question

Post by russellcottrell » Thu 04 Oct 2018 03:39

Why didn't I 'discover' lab years ago? And why isn't it more widely used?
I use it a lot. :)

Without knowing exactly what you are trying to do, I would point out as Burkhard did that you can use Lab curves in an RGB image, with all the benefits of using them in an Lab image.

You can also use the Change Channels tool to copy the individual channels of an Lab image to grayscale images that can then be manipulated by plugins and pasted back into the Lab original. Contrast changes, noise reduction, etc. in the a and b channels, for example. Cumbersome, but can partly be done with actions.

Interestingly, Change Channels can use L, a, and b as source channels in an RGB image which can then be pasted as grayscale layers in the same image and manipulated. Unfortunately, L, a, and b are not in the destination channels; if they were, this would be a fairly easy way to use plugins on the Lab channels; it would be very action-friendly.

If your goal is noise reduction, Neat Image uses the YCbCr space, which is an opponent-color space similar to Lab except that the conversion is linear not exponential. For noise reduction, in the Cb channel for example which tends to be the noisiest, it is probably just as good.
Quick example, it's easy to change the color temperature using curves in RGB. Decrease the output of blue, decrease green a little less, and you have a warm light. I'm not sure how I could do that in Lab.
Raise the midpoints of both the a and b channels a little.

bruce1951
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Re: LAB question

Post by bruce1951 » Thu 04 Oct 2018 03:51

Thanks Russell. It's your fault!!! :D It was only after reading your material that I started to play with lab. In reality I'm only using it with Curves. But being curious I pushed other fun bits to see what happened. Some work fine. Other times it's just a learning tool. But for curves in regular images I like lab a lot. Flip flopping back and forth between rgb and lab can work sometimes but other time I get 'strange' results. It's just a matter of knowing when to use lab.

bruce

bkh
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Re: LAB question

Post by bkh » Thu 04 Oct 2018 10:13

bruce1951 wrote:
Thu 04 Oct 2018 03:51
Flip flopping back and forth between rgb and lab can work sometimes but other time I get 'strange' results. It's just a matter of knowing when to use lab.
Probably, you are using Lab colours which are outside the gamut of your RGB colour space. Most RGB profiles are terrible at converting out-of-gamut colours from Lab (and I'm not even sure if the behaviour is consistent between different CMS). Might be a good idea to switch on the single channel warning in PL (Preferences -> Display -> Extreme Values) – this gives you an idea when channels are overflowing when converting back from Lab to RGB (or when playing with Lab curves in RGB).

Apart from such overflow problems, converting between RGB and Lab is very reliable and happens in colour management all the time (after all, your display is an RGB device, even if your image is Lab).

Cheers

Burkhard.

bruce1951
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Re: LAB question

Post by bruce1951 » Fri 05 Oct 2018 01:45

One of the big issues is also the fact that many plugins are also only 8bit. So changing colour mode and also from 16bit to 8bit is counter productive.

It's not a big issue for me. just my curious mind trying to learn!!

As an example. The same image the same editing. One in lab and one in rgb.


bruce
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bruce1951
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Re: LAB question

Post by bruce1951 » Fri 05 Oct 2018 07:50

I posted 2 images but one vanished!
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