Reilly and Munsell
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Adding Saturation

Shadow colours and Munsell Colour Space

The whole area of colour perception as luminosity changes is an area that has interested me for some time.
Dan Margulis noted that painters used less colour saturation in the darker tones than in the lighter areas of a painting and suggested using a mask, normally the Luminosity channel of LAB, when adding colour saturation during post-processing.

My difficulty in differentiating (especially green) hues in the Colour Clock experiment and the findings from the Learning from Masters project, reinforced the idea that the Munsell colour space is a serious contender when manipulating images.

Sammy Lee brought to my attention the advice by Frank Reilly to painters on how to mix colours as the object became darker. Basically, as an object colour in a scene being painted becomes progressively darker, its Munsell value, not surprisingly, decreases. The rule of thumb says that the chroma also decreases, by about the same amount proportionally.

Although it seems counterintuitive, the hue of a shadow is not nearly as important as its chroma. As the shadow becomes darker, and the chroma decreases, the hue becomes nearly indistinguishable.

Paul Centore has written an article that explains all this, but in a nut shell it says...

Suppose that the colour when lit has Munsell coordinates H V/C

For example 10GY 7/12.  The left axis is a vertical line of neutral greys, numbered from N1 through N9. Continue the neutral axis downward, to where the value would be -1.

Draw a line from the value -1 on the neutral axis, to the center of the square
containing 10GY 7/12.

The shadow colours of 10GY 7/12 fall on this line.

This is obviously approximate as the chances of a colour exactly matching a Munsell swatch is not that high.


First we better see if this theory holds true for Photoshop images.

I have taken this 3 toned box and constructed 2 examples for 4 sets of colours.  One was made by adding a colour layer (in Colour blend in the Lab colour space) over this basic b&w image. 

The other was by painting each face according to Reilly's advice.

I have muddled up the examples, so you can see if you agree with the results.  Lower down the page I say whether the top or bottom image is the Reilly one.


Here I have done exactly the same thing but darken 2 of the surfaces, just to check it out with a wee bit more shadow (and for another reason you will see later!).

The above example had (N6 & N4 - Lab 62&41) under a N7 (Lab 72) surface. 

This image has N5 & N3 (Lab 51&31) under the same top face.

Again the answer (top/bottom) is further down the page.

Now that we agree (?) that the Chroma change looks much more realistic, lets see what happens if we make changes to the luminosity.

This project has been about shadows, but what about the reverse!  What happens when we lighten shadows in an image?  I find this is a fairly common adjustment that I make to photos. 

So I will attempt to alter the second darker faces (N5 & N3 - Lab 51&31) to bring them back to the lighter ones (N6 & N4 - Lab 62&41), keeping the top face (N7 - Lab 72) the same as far as possible.

In other words increasing the shadows by 1 stop (of exposure).  I have masked out the background from these changes to aid comparison with the original lighter box.

This is the original box

Changes made in Lab
The results are (almost) the same if I curve or
place the lighter grey box in a Luminosity layer over it

Jacob's Ladder
The sort of results one would expect as this is processed in Lab space

Changes made in RGB
by placing the lighter grey box in a Luminosity layer over it

Curving in RGB

RGB Lighten Shadow Action

CurveMeister HSB
curving in B only

Attempting to apply a Saturation correction
in CurveMeister HSB space
One must remember that I started from a constructed darker box where the relative values were different from the lighter box

So we must not expect an exact match.

The results remind me that I should always post-process in the Lab colour space.  They say Lab is a bit of a "bull in a china shop", but I cheat using CurveMeister to display curve dialogs over 3 times bigger than the large photoshop ones, on a second hand-me-down old LCD screen, so that I can do quite delicate adjustments.

[Light box answer is bot,bot,top,bot and the darker one top,top,top,bot for the Reilly/Munsell image]

When comparing these image on 3 different screens, only one of which was calibrated, the differences in one case were overly pronounced.  So the effect one creates may not be obvious to others who view your images on their own screens - obvious really, but worth remembering.

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