Topic: Lightness, Black and Brightness

[derekfountain]

The 3 colourspaces that separate out lightness from colour provide between them three differently named lightness channels: Lightness (from LAB), Black (from CMYK) and Brightness (from HSB). Whenever I need to alter lightness I do the work in LAB mode, but only because I'm pretty familiar with that colourspace.

Can anyone offer any guidance on what the differences are between these apparently very similar channels, and when I might want to try K or B over L?

[Greg Groess]

K really relates to Black ink.  It is very useful in masking and tonal control.  You can construct black out of the other three CMY channels but it is lacking in depth.  K adds the "pop" to the image and makes well for lack of a better description makes the blacks black.  There are multiple levels of Black that can be set in the CM module.

The L channel separates all brightness information from Color.  You can make changes withour affecting color.

http://www.curvemeister.com/support/curvemeister2/help/Wizard/CSMatrix.htm

Greg

[curvemeister]

Good summaries from Greg.  I generally don't use the B channel in HSB very often, but it has some interesting characteristics.  For example, adding contrast to a very dark image in HSB will bump the color saturation, similar to the composite RGB curve.  But bumping the contrast of a light image in HSB mode will reduce the amount of color.  Also, the brightness of RGB(100,0,0) and RGB(255,255,255) are the same in HSB, but different in Lab.

The K channel of wgCMYK or CMYK is a different animal.  It provides wonderful control of shadows, and the heaviness of the K channel can be controlled by the GCR dropdown.  As you increase the GCR, you get more control over the image using the K channel. 

BTW, as you increase GCR, the amount of control provided by the C, M, and Y channels is reduced, which turns them into the most subtle color adjustments available in Photoshop.  IOW, you can make very large moves in the CMY curves and have very subtle control over your colors.