[AP] An odd saturation information

[ch22]

For (RGB) = (243,249,247), through the Color panel or the Information panel, Affinity Photo claims the saturation is  33%. How such a high value can be obtained ?

 

In my opinion, for this special case, the saturation value should be (249-243) divided by a number between 256 and 243 (depending on the colorimetric model), i.e. near 2%.

[R C-R]

The value is what it is because saturation is a relative value & in the HSL cylindrical model, near the top of the cylinder where lightness is at its maximum, saturation becomes almost meaningless. There is a long Wikipedia article that goes into this in great detail, in part in the Saturation section.

[ch22]

OK.  The AP value for S (33%) is consistent with the formula given in this Wiki paper, in Fig.14-d.  But what does it mean ? Saturation should not become almost meaningless.

 

This Wiki paper does not seem internally consistent for me. In the beginning, the saturation is explicitely defined as

   Saturation =  "colorfulness of a stimulus relative to its own brightness" = Chroma/Lightness" 

In the case of my near-white color, this leads to Saturation =  (Max-Min)/255  (i.e. 6/255 = 2%) and this does not agree with the formula mentioned above.

 

At the end of the paper, I read 

Using the same name for [various] definitions of saturation leads to some confusion [...]  Even worse, the word saturation is also often used for one of the measurements we call chroma above...   Oh yes! I must confess I make the confusion myself ; even worse,  I consider the chroma the most interesting quantity for image processing. 

 

Anyway, thanks for the answer!

[R C-R]

2 hours ago, ch22 said:

OK.  The AP value for S (33%) is consistent with the formula given in this Wiki paper, in Fig.14-d.  But what does it mean ? Saturation should not become almost meaningless.

 

Well, like it or not, that is a consequence of how RGB values are mapped to the cylindrical HSL model. As the Wikipedia article says:

Quote

Both of these representations are used widely in computer graphics, and one or the other of them is often more convenient than RGB, but both are also criticized for not adequately separating color-making attributes, or for their lack of perceptual uniformity. Other more computationally intensive models, such as CIELAB or CIECAM02, are said to better achieve these goals.