GIMP gives us a number of options for influencing the tone and colour of our photos, however “Levels” and “Curves” are the most versatile and hence the ones which you will use almost exclusively. Most of these other tools are pretty self explanatory and can be found in the Colors dropdown menu, there are also some fully automatic tools in Colors > Auto. There are also some new options in GIMP 2.6 in the form of GEGL Operations, these can be accessed by selecting Tools > GEGL Operation… and selecting from the dropdown, I’m particularly pleased with the color-temperature tool.
I had intended to write seperate tutorials for Tone and Colour and indeed the “Levels” and “Curves” tools, however I have chosen to go a different route. I shall instead improve both the Tone and Colour together, working through Levels and on to Curves as I do so. Hopefully this approach will be educational, as it will allow you to see how changes made in Levels are implemented in Curves.
So here is a shot from an underground passageway in an old ruin near where I live, you can see that the image is washed out and lacks contrast, there is also a blue colour cast on the stone work and rubble in the foreground. So lets begin, I am assuming that you understand the basics of the image Histogram, if not please read Histograms: A Levels and Curves Primer first.
In order to make this followable I am going to split the process into three parts, Tonal Range, Whitebalance (colour cast) and Tone (brightness/contrast).
1. Launch the Levels tool by selecting Colors > Levels… and try our automatic options. Auto Levels will work very well on the majority of photos (this is one example where it fails, Auto Levels results in far too much contrast and does little to remedy the colour cast). If Auto fails click on Cancel and relaunch the Levels tool again.
2. If you are happy with the colour in your image you may wish to skip ahead to step 3. This is certainyl not the case here so we’ll try the three Colour Picker buttons, selecting a Black point from inside the chimney, a White point from the brightest point on the wall outside and a Gray point from the stonework gives quite good results on this image, however it failed to fully get rid of the colour cast. However it does give us a good basis for further work.
Hint: You do not need to use all three colour pickers, good results can sometimes come from using two or even just one. Also I usually use the Gray picker last, the reason for this is that it can be difficult to tell what is neutral grey, hence the grey step may have to be repeated.
3. Launch the Levels tools by selecting Colors > Levels… We can see that the Value channel histogram does not extent to either end of the scale, this compressed tonal range is a reflection of the washed out, low contrast nature of the image, so we need to increase the tonal range.
4. Grab the left triangle pointer and drag it out to the start of the histogram graph, this will remap the tones in our image so that the darkest tones will be rendered as pure black.
5. Most images will contain some white areas so I would normally drag the right pointer to the right edge of the graph, however in this case I know that this image contains little or no white, nor do I want it to, so grab the right triangle pointer and drag it slightly to the right. The aim it so increase the tonal range without remapping the brightest tones to pure white.
Hint: The center triangle pointer represents the midtones, moving this to the left or right will darken or lighten the image respectively.
6. Take a look at the histograms for the Red, Green and Blue channels. Note the changes made by the use of the three color picker buttons, the tonal range has been increased and the center triangle pointer has been moved away from the default 1.00 position, thus increasing Red and Green in the midtones and reducing Blue.
Hint: If your histogram is very flat and shows no discernible peaks or detail try switching the mode from Linear to Logarithmic, I have circled the buttons on the above graphic for the Red channel.
7. Time to get rid of that colour cast, select the Blue channel from the dropdown in the Levels dialog box.
8. Unfortunately in this case moving the center pointer further to the right (reducing blue) simply makes the image much too green, it’s time for Curves to come to our rescue. Launch the Curves tool by selecting the Edit these setting as Curves button in the Levels dialog box.
9. Take the time to look at the Curve for the Value channel. Observe that the graph is no longer the 45 degree angle we expected. The top and bottom of the graph have been moved inward, this is the curves representation of the increase to tonal range we made earlier using Levels.
10. Now take a look at the curves for the Red, Green and Blue channels, again we can see the tonal range increase but this time we can also see the center of the curves has been raised for Red and Green and lowered for Blue. This is the curves representation of the changes made using the three Color Picker buttons back at the beginning.
11. We wish to remove further blue from the highlight (brighter) areas so select the Blue channel. Grab the top right of the curves and drag it down until the line becomes diagonal.
It is probobly good practice to repeat this step for the Red and Green channels also, however I am not going to do it in this case for one very simple reason. Histograms are neither right nor wrong, I am happy enough with how the blue has been removed while keeping the red of the brickwork in the ceiling and the green of the mossy walls. My attitude would be that it is better to make any edits based on how the image looks rather than upon what may be suggested by the histogram.
12. As a final step we will alter the tone slightly so as to better match my memory of what is basically a damp undergroung passageway. Select the Value channel and click of the center of the curve to create a control point (circled below), next click on an area in the lower (dark) portion of the curve and drag it downwards slightly. Next click of an area in the upper (bright) oprtion of the curve and rise it slighly. This is the classic “S” curve you will frequently hear mention of, we have used it here to darken the dark areas while simutaneouly brightening the brighter areas. This is the real power of curves, it gives us very fine control by allowing us to affect as many control points as we see fit, 5 in this example.
Hint: Soft gentle curves tend to work best. One way to make very small adjustments easier is to use the arrow keys (on PC, I’ll check this in Ubuntu), the up and down keys move the selected control point up and down, while the left and right arrow keys allow us to scroll through control points.
13. When you are finished click OK.
Levels and Curves can be a tricky at first, however any time you put into learning them will be time well spent. Unfortunately these tools give us so much control that it now becomes difficult to know when to stop, I’m thinking that I overdone the contrast a little in this example and am considering starting again, but it should serve for demonstrative purposes.