How To Use The Histogram In GIMP To Assist In Editing
One of the most overlooked tools in GIMP that can vastly improve your editing is the histogram! It can tell you a lot about your photo and where to start your editing. The histogram can also alert you to when you’ve over-edited an image.
In this article, we’ll explore the inner workings of the GIMP Histogram and how to use it when editing.
If you’re ready, let’s do it!
Table of Contents
What Is the GIMP Histogram?
The histogram is a graphical representation of the tonal ranges in your image. The peaks and valleys are the details within your image.
The histogram displays all of this detail in up to 256 different color channels. Each channel contains both RGB color values and the brightness of your image. These channels span the tonal range of your image; Black points, Shadows, Exposure, Highlights, and White points.
Knowing how to read the histogram by deciphering this visual data can guide you in editing.
Histogram Tool Deep Dive
Before we jump into using the histogram for editing, let’s first explore the tool itself. To display the Histogram panel, go to Windows > Dockable Dialogs > Histogram.
Peaks & Valley’s
There are two main Histograms and three options for altering how the peaks and valleys are displayed.
A linear histogram is a representation of how your camera sees the data. When using this histogram type in GIMP, it will closely match your camera.
A logarithmic histogram represents how your eyes see the data in your photo.
Using the “Values” menu options can help you pinpoint specific colors and or single out the brightness levels.
Take a closer look at the histogram, and you’ll notice 4 vertical lines. This divides the histogram into five sections. The first section represents the Black points (pure black). The second section consists of details in your Shadows. This is followed by Exposure, Highlights, and the White points (pure white).
How To Read the Histogram For Editing
Let’s explore how to read the histogram to assist you with your editing. As mentioned previously, the histogram represents data for different tones in your image.
Ideally, you’ll want detail spanning from left to right and the peaks within the histogram. If not, this means you lost detail at the time of capture!
Over Or Underexposed?
There are two different ways to know if your image is over or underexposed. The first visual cue is if a gap exists on the left or right side of the histogram.
The second is the height of the peaks. If they create a “flat-top,” you’ve clipped detail in your image. Both of those are your visual cues that the image is over or underexposed. Let’s look at some histograms and their corresponding images to learn how to read them.
Due to the bright sun, the background is very overexposed. The histogram reveals what part of the tonal range has lost data… the White points. Notice how high the peaks in that tone range are compared to other sections.
For this particular image, I could have balanced the tonal range by increasing the foreground’s amount of light. However, the museum we shot in wouldn’t allow studio strobes to be setup. So, I exposed the couple to the available light.
I’m ok with the background being void of detail since the main subjects are the couple and the statues.
Straight out of camera (SOOC), this image had details in every part of the tonal range. However, if you look at the histogram you can clearly see that it’s underexposed.
The data is skewed to the left with large peaks in the Black points. This is telling us that detail has been lost in this part of the tones. So, I overedited the image and, in the process, clipped data.
How To Fix an Overexposed Photo
Due to situations out of my control, there wasn’t much I could do to capture a balanced exposure in-camera. There are two ways I could have fixed this in GIMP, though.
The first would have required taking two images (HDR). The first image would be exposed as I did initially. The second image would have been exposed for capturing the details in the background.
Then, in GIMP, I could have merged the two together by separating the background and masking with a Layer Mask.
Fixing this image would require filling in the gap. This can be done with one of several tools; Levels, Curves, or the Shadow/Highlight tool, to name a few.
For this example, I used the Levels tool and adjusted the mid-tones (exposure) to the right. Compare the histogram to the original, and you can now see the gap has been filled. Oh, and it’s no longer overexposed!
There’s one problem, though. Notice the gaps in the histogram. As you stretch it from left to right or vice versa, GIMP has to push the details to fill in the gap.
This results in a loss of detail throughout the different tones. The end result is a lower quality image.
To avoid this, try to nail your exposure in-camera! Use your camera’s built-in histogram to reveal if you should retake the image… for better exposure.
How To Fix an Underexposed Photo
Fixing an underexposed image is not much different from overexposed. Use your favorite editing tool to fill the gap!
This time, I used the Curves tool to increase the image’s overall brightness until the gap was (almost) filled.
Like before, gaps in the histogram are visible, and the result is increased digital noise and artifacts. These can be reduced with GIMP’s built-in noise reduction filter.
To avoid over-editing, read your histogram as you edit! Compare this histogram to the over-edited version. Notice how I retained detail for the entire tone range by nailing the exposure in-camera.
This results in a higher quality image. Plus, you’ll spend less time fixing what you could have done at the time of capture.
Editing isn’t something you can master in a day or two, weeks or even months. It takes time and lots of practice. To help you along with your editing journey, check out additional free tutorials on this site.
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