Interesting, I did try a normal black to white rgb gradient in MV Color Cube but I guess I didn't look close enough to notice the differences. Maybe I'm getting lazy like you.

...I just realized where you guys came from with your 1786 colors B&W and why I missed a serious part of this topic, indeed, I've never noticed that desaturating always output to 256 colors... I thought that only image indexing would do quantization...

That is the weekness of 8-bit jpegs anarkhya. A 16-bit image can give you 16384 true grayscale values. There is a 12-bit b/w mode for jpegs used in medicine which gives you 4096 values. Unfortunately there doesn't appear to be any online support for such files I do not believe.

Question might be: Do our eyes would perceive a range of greys in a 16-bits or wider color space ?

The biggest thing this technique helps with is gradient banding (or postering) which you may get in empty sky areas under certain conditions. Sometimes, you can reduce this effect by adding noise (grain) believe it or not for uber clean selectively. Most of the time, as you alluded to, you won't see a difference between 8-bit grayscale and 12/16 bit grayscale. Pseudogrey is more a perception tool then anything. The biggest eye-opener is if you create a black to white transition gradient in GIMP then count the colors. That's what made a believer out of me.

Ok, I guess an empty sky might indeed be the only place where unwanted posterization would occur because of the vast blue fields hard to interpret (this relates to the saturation vs value issue we discussed earlier, isn't it ?). And I immediately believe you when you said selective noise is the key, because this is how technical beauty in pixel art works, optical illusions

@anarkhya

Yes, I believe we can distinguish more shades of grey than 256, and posterization in skies is not the only place this is useful.

Assume our eyes could not actually distinguish the possible individual change of a larger space of luminance values:

The surrounding pixels will still have an influence on our perception of the result:


Square A and B are exactly the same color...
(love that illusion)

I found an illuminating example of this using the same model.
Where is the white knight?

ahah I once saw this one yes, very frustrating !

I also encountered, while pixelling, these kind of (unwanted) illusions, but with colors
... Oh it seems I can't upload it here because the white background breaks the illusion

I noticed that one of XNview filters renders a more than 256cols grey image (I don't know if it's really pure grey), it's the gammasat filter used at min setting 0.01...

I have tested with a photo, I'm getting 3294 colors and it "looks" grey to me.

I just tried this Anarkhya; though a lot of gray color, I still saw a few over colored objects in a test image when I set the slider to minimum (the firehydrant yellow/green stayed).

With the latest version of Photivo, there's now extra resolution in the settings for B&W conversion, so I got 2364 shades in this:



Kevin

Sure, you win !

Wouldn't consider this a win; just an observation Anarkhya; could be a cool feature used for selective colorization too so potentially a great discovery by you.

haha, that's fair !

Selective colorization, let me think about it.. ah, you would use it to strengthen colours to ease a selection made by the magic wand, something like that, right ?

The link is broken. But I think this would be another option.
Where is the white knight?

In Sample Colorize Dialog there are two check-boxes, Hold Intensity and Original Intensity.

In the image that tested (2168 x 1288)
If neither is checked you will get 1786 colours. (img1)

If both are checked you will get 2519. (img2)


Then reduce the same images to (542 x 322)
The image Img1 now has 1860 colors,

while the image img2 now has 1815.


The reduction in size has decreased the number of colors in the image img2 (2519 to 1815) and increased in other (1786 to 1860)

Thank you I updated the URL.
Squeezing 1786 Grey Levels out of 256 Possiblities
...probably more than you ever wanted to know about pseudoGrey

Just trying out Pat David's .scm.
original

pseudogrey compared with desaturate(lightness)