Green channel values

[Nicolas Hanson]

I have noticed that when clipping the lowest values of the green channel (green values below 0 IRE), the image loose more details than when clipping the same amount of green or blue. Is this is a correct observation, and why?

[Timothy Whiting]

In digital cameras the green channel contains more detail than the red and blue channels. Most digital cameras have twice as many green filters on the sensor than blue or red, because the human eye is more sensitive to green. 

Edited December 18, 2016 by Timothy Whiting

[Bruno Mansi]

An image showing what Timothy was describing...

[Andy Minuth]

Spatial resolution is mostly perceived via Luminance. In Rec 709 Luminance is calculated from RGB with this formula:

Y' = 0.2126 R + 0.7152 G + 0.0722 B

The Green channel contributes more than 70% to the Luminance signal. 

[Nicolas Hanson]

Great information guys, thank you very much!

[cameronrad]

On 12/18/2016 at 2:20 AM, Andy Minuth said:

Spatial resolution is mostly perceived via Luminance. In Rec 709 Luminance is calculated from RGB with this formula:

Y' = 0.2126 R + 0.7152 G + 0.0722 B

The Green channel contributes more than 70% to the Luminance signal. 

Coming from stills, I've read slightly different. I've read that's how "Luminosity" is calculated rather than "Luminance". However there seems to be confusion in terminology between stills/photoshop world and rest of the world. I think photoshop/stills world is incorrect and a bit behind/backwards to be honest.

 

Quote

How is a luminance histogram produced? First, each pixel is converted so that it represents a luminosity based on a weighted average of the three colors at that pixel. This weighting assumes that green represents 59% of the perceived luminosity, while the red and blue channels account for just 30% and 11%, respectively.

 

Quote

*Technical Note: Strictly speaking, these should really be called "luminosity histograms." Unfortunately, the terms "luminance" and "luminosity" are often used interchangeably, including by Photoshop, even though each describes a different aspect of light intensity. Luminance refers to the absolute amount of light emitted by an object per unit area, whereas luminosity refers to the perceived brightness of that object by a human observer.

via http://www.cambridgeincolour.com/tutorials/histograms2.htm

Edited December 21, 2016 by cameronrad

[Andy Minuth]

Yes, I was not precise enough about terminology in my last post:

Luminance should not be used in this context, because it is an absolute value that describes the amount of light in physics. The light emitted by a display for example is described by luminance and measured in cd/m2.

Here the term relative luminance Y or luma Y' should be used. Relative luminance Y is a linear value and normalised to 1 or 100. Luma Y' is the same achromatic part of an image but nonlinear, it uses gamma compression. In video systems usually luma is used, but the formula is also true for linear components. 

Y = 0.2126 R + 0.7152 G + 0.0722 B

Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'

Both formulas are valid. The ' indicates nonlinear values due to the gamma compression. These coefficients are intended for Rec.709 HD systems.

For standard definition Rec.601 other coefficients are used:

Y′ = 0.299 R′ + 0.587 G′ + 0.114 B′

These coefficients are also intended for JPEG images, which might explain the use in Photoshop. A quote about the term luminosity from Wikipedia:

Quote

In Adobe Photoshop's imaging operations, luminosity is the term used incorrectly to refer to the luma component of a color image signal

 

For further reading I recommend Charles Poynton and Wikipedia:

http://www.poynton.com/PDFs/YUV_and_luminance_harmful.pdf

http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html

https://en.wikipedia.org/wiki/Luma_(video)

[Thomas Singh]

Great insight, thank you!