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Why is Gnome fractional scaling 1.7518248558044434 instead of 1.75? (unix.stackexchange.com)

submitted 10 months ago by [email protected] to c/[email protected]

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[–] [email protected] 93 points 10 months ago* (last edited 10 months ago)

The actual answer in on Stack exchange in their comments.

https://unix.stackexchange.com/questions/740319/why-is-gnome-fractional-scaling-1-7518248558044434-instead-of-1-75

It is related to a mix of actual display resolution vs conversions to virtual resolutions (the scaled resolution), and use of single precision floating point calculations.

Essentially my understanding is what it is doing is storing the value needed to convert your actual resolutions number of pixels (2160p) to a virtual resolution number of pixels (2160/1.75 horizontally) but that gets you fractions of a virtual pixel. So instead of 1.75 it scaled by 1.75182... to get to a whole number of virtual pixels to work with. Then on top of that the figure is slightly altered from what we'd expect by floating point errors.

If you take the actual horizontal resolution 2190 and divide it by the virtual resolution it's trying to use 1233 pixels, you need a conversion value of 1.75182.... to convert to it so you don't get fractions of a pixel. If you used 1.75 you'd get 1234.2857... pixels. So gnome is storing the fraction that gets you a clean conversion in pixels to about 4 decimal places of a pixel.

Full credit to rakslice at Stack Exchange who also goes into the detail.

[+] [email protected] -33 points 10 months ago (4 children)

For the same reason a lot of programming languages can't calculate 0.1+0.2 properly.

There's a website explaining it: https://0.30000000000000004.com/

[–] [email protected] 57 points 10 months ago (3 children)

Floating point error? Yeaahhh no. No. Just... no. That is NEVER as big as 0.01 unless the number is also insanely massive.

The error is relative in scale. It's not magically significant fractions off.

[–] [email protected] 4 points 10 months ago (1 children)

TBF the error can become that big if you do a bunch of unstable operations (i.e. operations that continue to increase the relative error), though that's probably not what is happening here.

[–] [email protected] 3 points 10 months ago (1 children)

To get to 0.01 error, you'd need to add up trillions of trillions of floating point errors. It will not happen solely because of floating point unless you're doing such crazy math that you shouldn't be using primitives in the first place.

[–] [email protected] 2 points 10 months ago

That's why I said unstable operations. Addition is considered a stable operation (for values with the same sign)

[–] [email protected] 1 points 10 months ago

0.001, but still

[–] [email protected] 1 points 10 months ago* (last edited 10 months ago)

As the answer in the link explains, it's adjustment of your scaling factor to the nearest whole pixel, plus a loss of precision rounding to/from single/double floating point values.

So I'm not really sure of the point of this post. It's not a question, as the link quite effectively answers it. It's more just "here's why your scaling factor looks weird in your gnome config file", and it's primarily the first reason - rounding to whole pixels.

[–] [email protected] 30 points 10 months ago

If I'm not mistaken 1.75 can be correctly stored as a float, as it consists of 1/2 and 1/4 only

[–] [email protected] 1 points 10 months ago

True, but it is not that difficult to trucante (or round) the value at the second decimal value.

[+] [email protected] -22 points 10 months ago* (last edited 10 months ago) (3 children)

Gnome is coded with JavaScript (lmao 🤣) ~~so yeah, I Think you are right.~~

EDIT: Actually, even if JavaScript and other languages have this issue, the value 1.7518248558044434 has not this issue. There is another reply that explains it and makes totally sense. But still pretty lame to know the desktop runs with JavaScript. (Yeah, I hate Gnome)

[–] [email protected] 15 points 10 months ago (1 children)

It's not a "language" issue it's a "computer" issue. This math is being done on the CPU.

IEEE 754

Some languages do provide for "arbitrary precision math" (Java's BigDecimal for example) but it's slower to do that. Not what you want if you're multiplying a 4k matrix every millisecond.

[–] [email protected] 4 points 10 months ago

I see, thanks for the explanation.

[–] [email protected] 15 points 10 months ago (2 children)

GNOME is primarily written in C

[–] [email protected] 4 points 10 months ago (2 children)

the desktop shell is mostly javascript though

[–] [email protected] 15 points 10 months ago (2 children)

Closer to 50/50, and other parts of the GNOME desktop like mutter, are largely C. Saying the entire GNOME desktop is mostly JS is silly.

[–] [email protected] 1 points 10 months ago (1 children)

No one here said GNOME desktop is mostly JS.

[–] [email protected] 1 points 10 months ago (1 children)

You're right, they said the desktop shell, which is still incorrect, but I guess a little less incorrect. My bad.

[–] [email protected] 1 points 10 months ago

Well, I started this thread saying it runs on JavaScript, and I mean that they need JS for most of the interactions with the desktop, like gesture or mouse events. 😞 Even if most of the code is C, we all know we need to write much many lines of code of C to do the same with JS, so most of the logics on GNOME is computed by JS. We need some rust here. 🦀 🦀 🦀 🦀

[–] [email protected] -2 points 10 months ago (1 children)

Yeah, on their git says 46% of the code is JavaScript: https://gitlab.gnome.org/GNOME/gnome-shell

That's pretty much, almost half of the code.

[–] [email protected] 11 points 10 months ago (1 children)

That page also shows that there is more C. That page is also specifically the shell, not all of the desktop.

[–] [email protected] -3 points 10 months ago

There is less than 4% more code in C than JavaScript. That's pretty much, many features on the gnome-desktop is using JavaScript too, like gestures and mouse events.

[+] [email protected] -14 points 10 months ago (1 children)

Okay, but still needs JavaScript, they are slowly trying to remove or improve it. But it is a fact that it also runs on JavaScript. 🤣

[–] [email protected] 11 points 10 months ago (2 children)

Using JavaScript isn't inherently a bad thing. JavaScript can be very useful when used for scripting. Obviously anything with a new for performance will be done in C.

[–] [email protected] 0 points 10 months ago

JavaScript isn't the best language to make a desktop interface in my opinion, it can be very efficient, but you can see in bugs (at least in the past) how bad performance it had, and they needed to re-factor it to replace to C or improve the JavaScript. I'm just laughing and making fun of it using JavaScript, not saying it is slow, Gnome is pretty fast nowadays.

[–] [email protected] 1 points 10 months ago* (last edited 10 months ago) (1 children)

It's mostly C.

And Gnome is far from the only desktop that uses JS, KDE Plasma, for example, also uses a lot of JavaScript.

It's weird when people bash Gnome for using JS, when practically everybody else uses it a lot too. Shows that they're just regurgitating "Gnome = bad!!!" nonsense.

We get it, you think disliking Gnome is a quirky, edgy personality trait.

[–] [email protected] 1 points 10 months ago (1 children)

Mostly C because you need to type more C code to do the same with JavaScript, so I suppose most of the logics are using JavaScript. Plasma desktop has 2% JavaScript (https://invent.kde.org/plasma/plasma-desktop), it's not comparable. 🙂

[–] [email protected] 1 points 10 months ago* (last edited 10 months ago) (1 children)

There's a lot more to your UX than just the Plasma desktop. And you're also trying to pass off Gnome's shell as being Gnome desktop. Pretty disingenuous.

[–] [email protected] 1 points 10 months ago

But at least the desktop itself isn't using JavaScript that much like Gnome do. Show me the repo with the % to see what are you referring.