Adjusting Color Levels
*note* This tutorial is almost useless if you haven’t calibrated your monitor properly.
A Practical guide to adjusting color levels in Gimp or Photoshop for beginners.
You don’t really need to understand the technical aspects of this adjustment to appreciate the results. This tool can be your best friend for bringing life back to a dead image, or hiding small mistakes. This is like the brightness and contrasts tools rolled into one on steroids. I will provide a link at the end of this post for further reading on the subject, but this tutorial is aimed at the beginner and how to get good results fast.
Explanation of Features:
The first thing you will notice is the histogram graph. This graph represents the amount of brightness in the color regions from absolute black on the left side, to absolute white on the right. With that in mind, if you had an image that was almost all black, the graph would be large on the left side with almost nothing on the right.
Gimp offers two Histogram Types. The default being linear, and the alternative being logarithmic. Here’s a quote from the Gimp manual of the difference.
These buttons determine whether the histogram will be displayed using a linear or logarithmic Y axis. For images taken from photographs, the linear mode is most commonly useful. For images that contain substantial areas of constant color, though, a linear histogram will often be dominated by a single bar, and a logarithmic histogram will often be more useful.
If you hover your mouse over the image above, you can see the difference of graphs for the same image. The Photoshop graph is also showing the same image, so you can see that there are many ways to represent the same data.
Under this histogram you will find three Input Sliders. From left to right are black level, gamma, and white level. This is where you will make most of your adjustments. By moving the black slider to the right, you will be making the image darker, and inversely by moving the white slider left, you will be making the image brighter. Moving the gamma slider left or right will effectively make the image lighter or darker without changing where the maximum white or black levels are. This is a very oversimplified explanation, but this should get you started.
Below the input sliders are the Output Sliders. These set the range of values you want in your output. So if you moved the black slider right a bit, your image will not show full black by the amount you move it. You will almost always want the full range so you will rarely touch these.
The Auto button will attempt to set the proper levels for you and sometimes gives great results, but not always. You will probably just ignore this as you get more comfortable adjusting the level manually.
The Eyedropper Tools are there to set the levels by selecting points from your image that should be pure black, middle grey, and pure white. When you do this, it will adjust the levels for you based on your pick points. This method gets a little more involved, especially determining the middle grey point, but still can be useful. It’s worth trying these out to experiment. Further reading is required to harness the full power of these tools.
At first glance, the image on the left seems a little washed out like it has a whitish haze over it. The blacks in the image don’t really look black either. By moving the black slider right to where it starts to spike should turn those dark areas black. Moving the white slider left a bit to meet up with the right spike should bring out the whites more as well. I didn’t need to fiddle with the gamma slider on this one, but sometimes it’s worth a try.
For reference, I also tried the auto button on this image and it almost matched my manual adjustments perfectly. The only bad thing about the auto adjustment is that it applies the fix without actually moving the sliders or changing the numbers, so it’s hard to tell what exactly it adjusted.
You might also notice that no detail was actually lost in this image. It actually just corrected it to appear as it was supposed to. It is very easy to overuse these controls, so try to not overdo it to the point that you lose details. If you watch the video at the bottom of the page, I show more examples of how this can be used to actually fix blending problems in images as well as some other fixes.
To wrap up, this is a very powerful tool that can really improve your images if used responsively. It can repair images that were either scanned or converted to digital improperly, or even help with blending after removing unwanted objects from things like posters and backgrounds. With a little practice, you will wonder how you lived without it.
All videos will be available in HD, so make sure you select HD for playback on youtube. (click the gear icon and select 1080p)
Resources: Detailed information on adjusting Levels
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