Cool 8-bit curves trick!

Not too sure if this link has been posted yet...pretty cool.

RGB 8bit, Grayscale 16bit, and beyond.

Hi Uncle,

Is the site down ?, because i cannot seet it.

me neither

No joy here either.

El mismo aqui.

Here's a link to the Google cache of the referenced URL. I'd post the text here, but since the site is commercial, I'd rather not offend the copyright gods.

http://www.google.com …lnk&cd=1&client​=firefox-a

Weird?
Was up yesterday?

Anyway it describes a less-destructive way of editing curves in 8-bit images.
1. Create your curves adjustment layer and adjust as needed.
2. Duplicate the curves adjustment layer.
3. Set the layer blending mode of the first, lowest curves layer, to COLOR.
4. Set the layer blending mode of the secont, top curves layer, to LUMINOCITY.

Compare the histogram of an image adjusted this way vs. one with just one traditional curves layer.

The idea here is to separate brightness (luminosity) from color (hue + saturation) adjustments by creating separate curve adjustment layers for each. Standard curves adjustments can cause tonal changes and shifts in hue and saturation along with the brightness changes. Anything but very gentle curve adjustments can lead to unpleasant color shifts and posterization.

Generally, when making curve adjustment your actual goal is to increase or decrease contrast which is a function of both color and brightness differences. I tried working with separate layers for luminosity and color when using curves but gave it up fairly quickly. The standard way seems best, and is only improved in my experience by resorting to the LAB color space, which gives you independent control of its channels for color and luminosity.

LAB space is best to do this sort of thing.

Intresting....
When I was observing the histogram of several RGB images given the traditional curves treatment vs. the 2-curve-layers the 2-curve-layer imaqge was definitely less "chopped up", more smooth, in all cases.

For those not pursuing a LAB work-flow, this seems to be a good option.

If you are really committed to this approach while working in RGB color space, then this article will interest you: http://www.xs4all.nl/~​tindeman/raw/curve_too​ls.html

The author developed a theoretically sound approach to uncoupling luminance (or lightness) from color while editing in RGB color space. It takes some work to understand and apply his "CurveTools" but you can gain a lot of insight into how and why the tools we use everyday in PS work the way they do. Also, his tools work as advertised!