USM in LAB

[Alex]

Hi,

I read somewhere that doing USM in LAB colour space under "Lightless" channel is a better way of sharpening your photos? Is this to reduce noise.

Related to this, I just bought "Photoshop LAB Color The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace" from Amazon. Seems like a good read.

Thanks
Alex

[Heath Bennett]

So many ways of sharpening for different applications. High pass sharpening is another of the lesser known ones. Problem being knowing when to use!

[Aussie Dave]

Hi Alex
there are a few methods when talking about sharpening, all of them are good when used in their right context. Sometimes a certain method will work better than another for one particular image, but not on another.

I usually use the LAB method you have described (and have setup a Photoshop Action to do so, but sometimes the High-Pass method, or even Smart-Sharpen in PSCS2 suits the intended purpose better.

I think you'll find THIS interesting reading....and a good start !

Hope this helps...

[Alex]

Thanks Heath and Dave. Dave, looks like an interesting read thanks.

Alex

[DaveB]

Good pointer Dave. I use Pixel Genius's tools for all my sharpening these days (referred to towards the bottom of that article).

For the technical background of why sharpening the L channel in Lab is better than sharpening in RGB, I'll have a go at explaining it:

Sharpening is done by enhancing the contrast at edges. Thus at edges we make the dark side darker and the light side lighter. Do it too much and you notice "halos", but the issues go deeper than that.
In an RGB image, the sharpening IS APPLIED TO EACH CHANNEL INDEPENDENTLY. Thus at an edge in red we'll make the lighter side a bit redder and the darker side a bit less red (and so on for green and blue). Unfortunately this can cause colour shifts at edges. Consider this (agressively sharpened, and then enlarged 500%) edge between yellow and grey:

The yellow side of the edge hasn't got any lighter (it can't) but the grey side has become blue! That's because the reds and greens have been darkened.

In the L*ab colour space, instead of red/green/blue coordinates we use lightness, and "a" and "b" which together define the colour. By sharpening just the L channel we get this:


However, note that the Lab colour space is huge: it describes lots of colours that simply don't exist (if that makes sense). At least, it describes a colour space that's MUCH larger than your RGB working space (e.g. sRGB, AdobeRGB, etc). And in each of these channels, in 8-bit mode there're only 256 steps available (and many of those usually impossible colours). This means that in 8-bit mode Lab is often less precise than RGB! The same thing happens with use RGB spaces such as ProPhoto RGB. Not only can posterisation be introduced, but the precision in some calculations is reduced.

So I never use Lab unless I'm in 16-bit mode. The above L-sharpening was done by converting to 16-bit before transforming to Lab, and converting back once the image was in RGB again. Have a look at what happened when I left it in 8-bit mode all the way through:

Where did that texture in the grey come from? From rounding errors in the colourspace transformation on the way to Lab and then on the way back again! The 16-bit version is perfectly smooth.

So there's an example of why sharpening the lightness channel is theoretically better. This is only a small part of the picture (pun not intended) and is often very subtle, but should provide some insight into why there are so many different ways of sharpening (and the comprehensive ones often use L sharpening, even if they don't make it obvious to you the user).

[Alex]

Thanks, Dave. Very useful explanation. Can I have that reference to Pixel Genius tools? Also I normally work in tiff (16-bit), convert to LAB, do my USM on "L" channel then return back to RGB and only when I save as jpg do I go to 8 bit mode. Is this OK to go to 8-bit after you converted back to RGB while still in 16 bit?

Alex

[DaveB]

Can I have that reference to Pixel Genius tools?

Pixel Genius's PhotoKit Sharpener is referenced in the article that Aussie Dave linked to earlier.

I normally work in tiff (16-bit), convert to LAB, do my USM on "L" channel then return back to RGB and only when I save as jpg do I go to 8 bit mode. Is this OK to go to 8-bit after you converted back to RGB while still in 16 bit?

What you're doing sounds OK. By staying in 16-bit mode until the end you avoid losses. I work on a variety of 8-bit and 16-bit originals, so in my Actions I explicitly switch to 16-bit where required (which if the file's already 16-bit will do nothing). To do this you need to record the action on an 8-bit file.

[jethro]

Dave you must teach PS surely. Very well explained

jethro

[Alex]

Dave,

Thanks for your help.
Alex