Curves are hard to use.

Started Apr 25, 2014 | Discussions thread
Tom Axford Veteran Member • Posts: 4,923
Re: Curves are hard to use - a couple of basic points

gordonpritchard wrote:

Curves are designed to be used to alter tonal contrast rather than color. If you use curves on an RGB image both color and cotrast will be affected. To adjust contrast only, which is the point of the tool, either convert the image to Lab mode and select only the L channel to apply a change with the curves tool, or, duplicate your RGB image into a new layer and set that layer to "Luminosity" then apply a change in contrast with th curves tool. Either of those methods will adjust contrast without altering the saturation of the image.

This is not totally correct. If you change L in Lab mode then the hue and saturation (i.e. the 'colour') will not be changed while you stay in Lab mode. However, when the image is converted back to RGB (which is necessary to view the image), some Lab mode pixels may be too bright to be represented in RGB and one or more of the R, G and B channels may be clipped (as happens when you overexpose the image in the camera) and the colour saturation will be reduced (and the hue may be slightly affected also).

The only common colour model which does not risk clipping on conversion back to RGB is the HSV model. Any curve can be applied to the V channel without causing any changes to the hue or saturation of the visible image. This doesn't mean that HSV is the ideal colour model in other respects, but it is the only one that guarantees no changes to saturation and hue when you change the contrast using the V channel.

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