Tuesday, April 21, 2020

Digital Black and White Conversion ~ Correct Human Tonal/Value Perception

Mike Johnson's The Online Photographer blog is a continued source of inspiration, education, and insight.

I read a post about how humans correctly perceive colors as they are converted to black and white.  In the article Mike references a Twitter post by French photographer Tim Soret.

Following the recipe M.Soret lays out, I tried two things.  Using a color wheel collection that I use to understand digital black and white conversion:

First, in the Gimp, I added a black layer over the color wheels and set the blend mode to LCh (Luminosity Channel - which in RGB terms translates to 71%Green 21%Red and 8%Blue).  Here is the result.

To my admittedly aging eyes this looks perfect.

Second, in RawTherapee, using the "Black and White" module I set the "Method" to "Luminosity."  Taking the above image and letting Rawtherapee convert the color side, we can note any differences between this software package and the Gimp.

Between the two images it looks like a perfect match.  The conversions are visually the same.

Revisiting my earlier comparisons and taking another look at the "Luminosity" conversion my mind's eye begins to appreciate the subtle differences in perception. 

In this earlier study I concluded that the "Green-Yellow" filter more closely matched what I had in mind than any other conversion process.  Now, after reading M.Soret's Twitter thread I'm not so sure.

As a reminder, here is the "GreenYellow" filter conversion.

Considering the color blue in particular, the "Luminosity" conversion approach seems to be more accurate for how the human eye perceives relative color values expressed in Black and White after-all.  The math doesn't lie. 

I think I'll have to sit with this a bit, make more digital color to B&W conversions and see how things proceed.

No comments: