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I started using GIMP recently, and came across an option called Curves under the Colors menu. What does this tool do, and how do I use it correctly?

I get some good colors if I make a graph in 'S' shape. But what exactly is the meaning of it? How do I use this tool to enhance my photographs?

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2 Answers 2

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The GIMP's curves tool is virtually identical to PhotoShop's Curves tool. Almost all photo manipulation tools have a way of doing that action. There are a number of good explanations of it out on the internet if you look for photoshop: http://www.google.com/search?hl=en&q=curves+adjustment+in+photoshop

this one however is fairly thorough in it's explanation of what it's doing: http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm

(disclaimer, it's been a good 3 years since I've used the GIMP, it's possible they've changed it since then, but I highly doubt it since the curves tool is one of the more fundamental advanced editing tools.)

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I haven't used Photoshop ! , so comparing it to photoshop doesn't help , but will have a look at tutorial. Tutorial looks comprehensive Thank you. –  sat Nov 3 '10 at 7:11
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The cambridgeincolour tutorial you cite works equally well for Gimp . That is a very good reference you provided. –  labnut Nov 3 '10 at 7:23
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@sat, the point wasn't to compare it to photoshop, but to say that they're the same. You can use the photoshop tutes as a guide, and get the same functionality from The Gimp. :) This is applicable to lots of features that the two have in common, so if you're ever stuck trying to figure something out on the Gimp, google for the same feature name on photoshop and see if the answers make sense with the tools. –  cabbey Nov 4 '10 at 3:01
    
Yeah as in this case. Thank u :) –  sat Nov 4 '10 at 5:46

Curves is a powerful and very flexible tool, which allows to control brightness, contrast and color balance very preciesly.

The way I approach the curves tool, either in Gimp or in any other editor is two-fold:

(1) The curve defines how to change the intensity. Its left side is for the darkest parts of the image, its right side is for the brightest part of the image. Where the curve is above the diagonal, the intensity is increased. Where the curve is below the diagonal, the intensity is decreased.

(2) The curve is the tool to redistribute contrast. Ranges with steeper curve will receive more contrast, and ranges with flatter curve will look more dull.

General tips

  1. Learn to read image histograms.

  2. Keep the curve monotonically increasing. If you make part of the curve flat, you will loose all tone details in that range, and if you make some part of the curve with negative (inverse) slope, that tonal range will be effectively inverted. Usually you don't want that.

  3. Move the beginning and the end of the curve inside of the range to stretch contrast (similar to changing black and white levels in the Levels tool).

  4. Pull the curve up to lighten the image (similar to increased gamma), and down to darken the image (similar to decreased gamma).

  5. Make the beginning (left end) of the curve slightly flatter to compress shadows (and make noise less visible). Pull the curve in the beginning (left end) up, if the image is underexposed.

  6. Prefer slight changes if possible.

Gimp-specific tips

  1. When the curves tool is open, click on the image to see the position of this point on the histogram (a vertical line on the histogram will appear). This helps a lot to find which part of the curve to modify.

  2. Don't stretch contrast too much. Current Gimp (version 2.6) represents images with 8 bit per color channel. Curves tool may lead to posterization (how to notice: the histogram will change from smooth hills into many spikes). Future versions of Gimp should support also 16-bit per channel. Now you better do only slight modifications, or use the curves tool of the RAW processing software (UFRaw, RawStudio, RawTherapee...), or use an editor with 16-bit color depth support (e.g. digiKam's built-in editor).

  3. Increase window size of the curves tool. The larger the curves tool windows is, the easier it is to adjust them precisely.

  4. Enable and disable preview to quickly view the image with the current curve applied and without.

Examples

For example, let's consider a curve like this

A curve to make shadows darker and highlights brighter

This curve will make the shadows darker and the highlights brighter; mid-tones will receive more contrast.

On opposite, the next curve will make the highlights darker and the shadows brighter. Also, it will “pull” more details out of shadows and highlights (they will receive more contrast, at the cost of lost contrast in mid-tones):

A curve to make shadows brighter and highlights darker

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Is there any way to force the curve to be monotonically increasing? That'd be handy. –  mattdm Jan 11 '11 at 17:57
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As far as I know, there is no such an option, but use the preview and trust your eyes. Usually the picture starts looking wrong long before the flat ranges (or ranges with negative slope) appear on the curve. –  sastanin Jan 13 '11 at 10:21
    
Excellent answer. Can you add the resulting histogram of (at least one of) the applied curves? It will make your points clearer. Adding the sample pictures will be a great addition too! –  ysap Sep 4 '11 at 14:13

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