Severe contrast enhancements: on 45% rule — and cats

Started Jun 28, 2013 | Discussions thread
Tom Axford
Senior MemberPosts: 1,539
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Re: HSV works better than either RGB or LAB
In reply to knickerhawk, Jul 2, 2013

knickerhawk wrote:

Tom Axford wrote:

For adjusting the tone curve, HSV works better than either RGB or LAB.

The V channel may be adjusted (even to extremes) without causing colour shifts and without the risk of channel clipping on conversion back to RGB.

Well, I use Photoshop, so I don't have the option of converting to HSV as you do in GIMP. However, it's my understanding that applying a luminosity blend in RGB (which is what I normally do if I'm not otherwise working in LAB), also achieves the goal without affecting color. There are times (for instance with already reasonably contrasty, colorful images) that the relative linear nature of the L channel in LAB can allow you to do more targeted curve adjustments. Since I've never worked with HSV I can't say, but isn't the V channel relatively non-linear? If so, it might make it somewhat more difficult to control the affects of the curve adjustment.

Different strokes for different folks (using different tools)...

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That's a very interesting question you raise about whether a curve adjustment in LAB produces a more pleasing result than a similar curve adjustment in HSV. I do not have enough experience of using both to even give an opinion (I almost always use HSV because it is convenient for me and it does avoid clipping).

The mathematical definition of V is simply the maximum of R, G and B; whereas L is a much more complex formula chosen to approximate to the luminosity as seen by the human eye. I would agree with you that, on the face of it, using L feels to be the more sensible thing to do (provided you are careful to avoid clipping). In practice, however, ... I don't know.

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