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The Levels dialog shows you the color distribution of an image and lets you modify brightnesses in order to bring those colors into an acceptable range. This dialog can be accessed from the Levels node.
The main portion of the dialog contains a graph, known has a histogram, that shows how frequently certain brightnesses appear. By default all three color channels are shown simultaneously, but you can filter to just a single channel using the Channel selector at the upper left of the dialog.
The x-axis of the graph indicates intensities, while the y-axis signifies how frequently that intensity appears. Thus, the point on the graph indicated in the above illustration means that 0.7% of the pixels in the image have an intensity of 50%.
The left part of the histogram contains a red region, while the right portion contains a green region. These regions signify pixels that are either darker than 0% (black) or brighter than 100% (white), respectively. This information is typically lost when the image is exported, with pixels brighter than 100% indicating the presence of a blown-out highlight. This problem can be fixed by setting new black and white points. The easiest way of achieving this is by clicking the Auto button at the top right of the dialog, but custom parameters can also be set, either by dragging the handles at the bottom of the histogram, or with the numeric controls along the bottom of the dialog.
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