Why Microsoft rescaled their HSL ranges to [0-240]?

Question

I'm studying digital image processing in university, and now we're doing a work where we have to convert RGB values to HSL, and it has to be equal to the Microsoft Paint. When I finished the implementation I saw that it was not equal to Paint, and searching I found that they don't use [0-360] range, as the usual way to implement it, but [0-240] range.

Source

But even reading it I couldn't got why to do this.

Answer 1

Short Answer

The reason for a hue of 240 is only about using one single int (8 bit integer) for the value, a remnant from a bygone era.

Longer Answer

Regarding the accepted answer: it has nothing at all to do with digital video code values, nothing to do with AtoD converters, nor the other comments on the purpose of video range, etc.

The use of a hue with 240 values instead of 360 dates back to when computers did not have math coprocessors (i.e. possibly before the OP was born, considering he's at university now, but I date myself LOL). Without a math coprocessor, doing anything in floating point was a significant performance bottleneck. Moreover, low CPU speed, limited RAM, etc... back then much was done to be efficient in ways that might not make sense today.

Remember we are talking about decades before it was possible to have a hand-held computer connected to all the world's knowledge that also happened to double as a phone, camera, map with your current location, music player, speech to text interpreter, video production system, and luminance adjustable flashlight...

LOL 😂

Back in the late 80s/early 90s, 8bit integer math was how almost everything was done, and going out of that 8bit integer zone was... was uphill... both ways!! ....

Reducing 360 to 240 keeps that value in 8bit, and has the advantage that it is exactly 2/3rds of 360.

Since there are three primaries in an RGB monitor, HSL breaks things down accordingly. If you think of sRGB as a "cube" with one corner as black, one corner as white, then you have 6 corners as the max values of full saturation, such as #f00 for red and #0ff for aqua.

HSL is not a perceptually uniform color model — it is just a low-tech way to make color adjustments "intuitive" for the user.

Side Note

Color (hue) does not exist as an a "angle". Color is not actually real, there is simply wavelengths of visible light, from longer than 700nm (red) to shorter than 400nm (blue). The sensation of color is just a perception; a function of your neurological system.

Nevertheless encoding those to a circle makes it fairly intuitive. So in any application where color values are defined as an angle, it is purely a construct of convenience.

Speaking of convenience, my hand held computer that is connected to all the world's knowledge is beeping at me to remind me it's time to feed my cat. And it's not even a beep like we had in the 80s. It's a stereo harp sound with some tubular bells in the background...

Edit to add:

The "uphill both ways" is an old person joke that is common here in the US, and that I find myself making more and more often, at my age, LOL... I don't know how well it tranlates to EU be honest...

Answer 2

There are several reasons.

We really prefer ranges which have just 256 values. In such case we can store data in a byte. We may use two bytes, but so we will double the memory usage. Or just using more then 8-bits, but so we have to do bit operation on every pixel before doing anything interesting, so very inefficient. Note: with two bytes we can use half-float, which are very useful in some context, but float points are also inefficient on various calculation (but it you do filters and layers composition).

The second reason it is stated in the link you included in the question, but it may need some more information: in many format (especially for video or televisions) we have a limited range (not 0 to 255, but 16 to 235 or 16 to 240). There are few reasons: the analog signal could have more black than black (which may be impossible) and more white then white (realistic). We just calibrated TV so to define white and black points (and possibly broadcaster will filter out the point that are below black. This was useful for some reasons (converting analog data, and more white then white is a colour we experience a lot). With digitalization, TV needed analog to digital converters (and the inverse). These converters costs (and more bit: more costs), and we already used data outside "visible colours" range (in analog signal) for other reasons, outside colours. So instead to have different converted depending on the kind of data (or just more bits on converter), we just keep limited range).

For every colour operation, one should check the range (and the definition of colour space). Quantization may be also a problem (many professional software use floating points, so we are less limited on range (but there are also other reason for floating point, e.g. precision and efficiency when working in linear space [without gamma correction]). Note: you should care only on the ratio value to maximal value. Angles have various measurements: 360 degree, 400 degree, or 2 Pi (the most common, sometime, especially on past you may write then as decimal degree (or other factors, so e.g. 3600, 36000, or 21600 [minutes of degree]). User interfaces may just show a different convention (more convenient for users).

And to make all more complex, the HSL conversion is not so exact, if you want exact H, S, and L (in stricter definition), just a quick shortcut.