Luminous Landscape Forum

Raw & Post Processing, Printing => Digital Image Processing => Topic started by: paulbk on June 07, 2006, 06:34:35 am

Title: Histogram: RGB v Luminosity
Post by: paulbk on June 07, 2006, 06:34:35 am

Why is the RGB histogram so much different than the Luminosity histogram?

RGB Histogram -- Is it true that a point on the RGB histogram represents the sum of pixels (Y axis) at a given RGB value (X axis)?
For example, if I have a file with:
1000 pure Red pixels at R=200, and
1000 pure Green pixels at G=200, and
1000 pure Blue pixels at B=200, then,
the crest of the histogram represents 3000 pixels (Y axis) at 200 (X axis).

While the Luminosity histogram is similar to the RGB histogram, it is not similar enough and therefore the question.

Title: Histogram: RGB v Luminosity
Post by: Hermie on June 07, 2006, 06:46:23 am

Paul,

See http://www.cambridgeincolour.com/tutorials/histograms2.htm (http://www.cambridgeincolour.com/tutorials/histograms2.htm)

Title: Histogram: RGB v Luminosity
Post by: paulbk on June 07, 2006, 03:50:42 pm

Thanks! Excellent link. Lots of good information.

Bottom line: “Luminosity” accounts for the bias in human vision to perceive Green as brighter (more luminous) than equally intense Red or Blue. Which is odd since the cone distribution is: red cones 64%, green cones 32%, and blue cones 2%. (Reference Cone Details (http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html))