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According to the manual, the "histogram" graph plot has the X axis represents values of pixel, from 0 to 1 (or 0 to 255), while the Y axis represents the number of pixels in that tonal range.

When I draw a "sample line" on my image, I got the graph plot that looks like it has the X axis represents the pixel point/position, while Y axis represents the brightness (luma) for each points.

There is no further explanation on the graph plot of sample line in the manual.Can someone clarify whether the XY axis for "sample line" graph plot mentioned above is correct or otherwise?

enter image description here

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    $\begingroup$ The units of measurement for the sample plot are Luma, which is the weighted sum of gamma-compressed R′G′B′ components of a color video—the prime symbols ′ denote gamma compression., explained in detail here: en.wikipedia.org/wiki/Luma_(video) $\endgroup$
    – TLousky
    Nov 5 '19 at 9:20
  • $\begingroup$ @TLousky Thank you! I have corrected it in my questions. Do you happened to know the answer? $\endgroup$
    – NabilaM
    Nov 6 '19 at 6:40
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That plot you get is not an histogram. It is used for video analysis, it is called Waveform Monitor. And it is, as you correctly deduce, a plot of a spatial horizontal axis (pixel along the line), and a luminance vertical axis (brigthness for that pixel.

See the page as a reference: https://www.bhphotovideo.com/explora/video/tips-and-solutions/introduction-waveforms-scopes-and-exposure

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I checked tha Blender Manual. Indeed there's options for different kinds of scopes, just as any regular video edition software:

[https://docs.blender.org/manual/en/latest/editors/image/scopes.html][1]

You can display: histogram, waveform, vector scope, and sample line, as follows (image from the Blender Manual page):

Blender scopes

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