What does "normalizing" an image or histogram usually mean?
Asked 12/8/2019
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A colleague remembered a digital signal processing class where they talked about "normalizing the curve" or "normalizing the image" to improve accuracy or reveal more detail. He described a photo histogram with three narrow peaks, then a "normalized" version where the peaks were more spread out. He was working in MATLAB.
Is this the same as gamma correction or HDR, or is it a different image-processing step?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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What your colleague meant, based on your description, was: Setting the white and black point of the image.
On an 8-bit grayscale image, you have 256 different values ranging from total black (0) to total white (255) and various shades of gray (values 1-254).
An image with content only in the ranges between e.g. 0 and 120 (under exposure) might feature "three sharp peaks". Setting the white point to 120 will make the peaks in the histogram have 'larger spread'. However, the absolute location of the peaks must change in this process (unlike what your colleague has drawn).
Widening the peaks without change of peak location is definitely not what is meant by 'normalization' - still considering, that this term is used in different places with different meanings.
Originally by user47612. Source · Licensed CC BY-SA 4.0
user47612
6y ago
0
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Based on the description, this most likely refers to setting the black and white points, or stretching the tonal range.
In an 8-bit image, tones run from 0 (black) to 255 (white). If an image only uses a limited part of that range—say 0 to 120—it can look flat or underexposed. "Normalizing" in this context often means remapping those values so the darkest used tone becomes black and the brightest used tone becomes white. That expands contrast and can make details easier to see.
So this is closer to histogram stretching / level adjustment than to HDR or gamma correction.
One caveat: if the histogram peaks become wider, their absolute positions would normally also shift during this remapping. So the sketch your colleague remembered is probably simplified or slightly inaccurate.
In short: likely tonal normalization by adjusting black and white points, not HDR.
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