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CSS px is an Angular Measurement

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Re: CSS px is an Angular Measurement

#31

Post by voyou »

JoelSutherland wrote: post #7
nthitz wrote: post #2

Ok, but does it matter?

Increasingly. To my knowledge, all desktop browsers ignore this spec and treat each pixel as a pixel. (This will likely change with the upcoming Retina Macbook Pros) For a while all mobile devices treated all pixels as a pixel. But then iOS and Android devices began to dramatically increase their DPI. In the case of iOS, the math is easy, everything gets multiplied by 2 (though chasing pixel precision in a browser do…

"all desktop browsers ignore this spec and treat each pixel as a pixel"

That's not ignoring the spec, though, it's following it. Where the device pixel is close to the reference pixel (as it is, on desktop browsers), the px measurement is supposed to represent one device pixel. See the CSS 2.1 spec: http://www.w3.org/TR/CSS2/syndata.html#length-units

Re: CSS px is an Angular Measurement

#32

Post by kickingvegas »

JoshTriplett wrote: post #29
kickingvegas wrote: post #23

I've mentioned this before and I'm not the first to say it, but it's time to stop using px and start using in its place absolute measurements like inches or millimeters. It's the only sane approach to supporting different resolution displays.

Unfortunately, devices stopped actually making "mm" a millimeter long a while ago; for instance, various mobile devices scale "mm" by a factor of 2 because they assume that sites don't actually want a physical unit of measure.

If you don't mind my asking: which devices?

Re: CSS px is an Angular Measurement

#33

Post by voyou »

gwillen wrote: post #5

This is dumb. The fact that one pixel subtends a particular visual angle does not imply that N pixels subtend N times that angle. It should be very plain to anybody implementing or using a rendering engine that pixels are intended to be linearly additive. This implies that N pixels will subtend less than N times the angle one pixel subtends. On a flat screen, this is normal and expected, because a pixel further from…

You're right, and the article linked here is wrong. The CSS spec doesn't say that px is an angular measurement, and the formula given that is supposed to convert between px and radians has no basis in the spec. The spec specifically defines px as a length; angular measurements only become relevant in calculating the reference pixel, which is a length used to define the size of 1px in circumstances where physical pixels are significantly different in size from the physical pixels of a computer monitor.

Re: CSS px is an Angular Measurement

#34

Post by JoshTriplett »

kickingvegas wrote: post #32
JoshTriplett wrote: post #29

Earlier quoted context omitted.

Unfortunately, devices stopped actually making "mm" a millimeter long a while ago; for instance, various mobile devices scale "mm" by a factor of 2 because they assume that sites don't actually want a physical unit of measure.

If you don't mind my asking: which devices?

Take a look at http://robert.ocallahan.org/2010/08/css-units-changes-landed... for some of the details.

Re: CSS px is an Angular Measurement

#35

Post by gwillen »

gwillen wrote: post #5

This is dumb. The fact that one pixel subtends a particular visual angle does not imply that N pixels subtend N times that angle. It should be very plain to anybody implementing or using a rendering engine that pixels are intended to be linearly additive. This implies that N pixels will subtend less than N times the angle one pixel subtends. On a flat screen, this is normal and expected, because a pixel further from…

Is anybody else having problems with notifo-based comment notifications? I am receiving them, but the links stopped working sometime between Jan. 22 and March 8.

Re: CSS px is an Angular Measurement

#36

Post by Stratoscope »

JoelSutherland wrote: post #7
nthitz wrote: post #2

Ok, but does it matter?

Increasingly. To my knowledge, all desktop browsers ignore this spec and treat each pixel as a pixel. (This will likely change with the upcoming Retina Macbook Pros) For a while all mobile devices treated all pixels as a pixel. But then iOS and Android devices began to dramatically increase their DPI. In the case of iOS, the math is easy, everything gets multiplied by 2 (though chasing pixel precision in a browser do…

> To my knowledge, all desktop browsers ignore this spec and treat each pixel as a pixel.

Not really. Most desktop browsers support pixel scaling with Ctrl+Plus and Ctrl+Minus or user preferences, and they may even remember this setting for each domain.

So not only is a CSS pixel not always a device pixel, but it may be a different (fractional) number of device pixels on different websites.

Re: CSS px is an Angular Measurement

#37

Post by bigiain »

gwillen wrote: post #27
crazygringo wrote: post #16

Earlier quoted context omitted.

That's actually a very good point, don't know why you're being downvoted.

I find it interesting that I was downvoted to the negatives, then you posted this reply, and then I got upvoted to the top. I wonder what the causation is there. I suspect I know why I was downvoted; I used unnecessary emotive language ("dumb") and didn't explain my point clearly. Most of the rest of the commenters were focused on one part of the article's point, which is very relevant -- the idea that a pixel is no…

I worked out a while ago that the comments I expended least effort on were the ones that were most likely to get significantly upvoted (and also downvoted) - longer comments get way fewer votes. My theory is that a single, almost throwaway sentence is easier to agree (or disagree) with, and hence earn a reflexive vote-click. When I write a few paragraphs (or more) as a comment, particularly with researched links and/or data, and thoughts/commentary on those links/data, I get way fewer votes, either up or down.

My initial reaction to this "discovery" was to decide to post shorter and more concise comments. But a few moments of reflection revealed that for me that's a pointless change of behavior for two reasons: 1) I don't comment with the aim of getting voted up, and shouldn't change my comment behavior just because there's a metric to be gamed, and 2) because at least for me, the bulk of my karma has come way more from fortuitously being first to submit a popular link (mostly assisted by my non West Coast GMT+10:00 timezone) - since the karma to be gained from a several hundred upvoted submission is _way_ less effort than writing thoughtful comments - and it's clear people are gaming that to pump karma (who was it that posted a while back about seeing bots stalking their rss feeds to autosubmit new posts? patio11 maybe?)

As you can see, I'm rambling all over the place with this comment - almost certainly in a way that makes it more difficult for readers to chose whether to up or down vote, and I'll guess resulting in neither.

Possibly stupid idea floating around my head right now - what if the voting system allowed you to not just up or down vote a comment, but to selectively up or down vote paragraphs or sentences or sentence fragments? Maybe I could choose to upvote your "the article author's claim that the function mapping real pixels to CSS pixels is nonlinear (which I think is just a misreading of what the spec intended.)" and possibly downvote another bit (there's not actually any of your post I'd choose to downvote, but maybe for example the "I suspect I know why I was downvoted"), then choose how to split my 1 unit of vote between the bits I want to vote up and the ones I want to vote down, so I could say 2/3rds for the up vote and 1/3rd for the down vote giving you a total of +1/3rd of a unit of karma - and more importantly, giving you feedback on why you're seeing the voting numbers you are…

Re: CSS px is an Angular Measurement

#38

Post by ars »

gwillen wrote: post #35
gwillen wrote: post #5

This is dumb. The fact that one pixel subtends a particular visual angle does not imply that N pixels subtend N times that angle. It should be very plain to anybody implementing or using a rendering engine that pixels are intended to be linearly additive. This implies that N pixels will subtend less than N times the angle one pixel subtends. On a flat screen, this is normal and expected, because a pixel further from…

Is anybody else having problems with notifo-based comment notifications? I am receiving them, but the links stopped working sometime between Jan. 22 and March 8.

http://blog.notifo.com/notifo

Re: CSS px is an Angular Measurement

#39

Post by gauravk92 »

JoshTriplett wrote: post #28
gauravk92 wrote: post #21

Earlier quoted context omitted.

So basically use em, like we should have all along?

Use em, yes, but meanwhile "px" should have continued to mean "pixel", not "96th of an inch except on mobile where ...". Occasionally you really do need to talk about pixels in CSS, and redefining "px" makes that impossible.

I was at the same time telling myself that we really need to start using em more often. I have used px so many times at this point because it's more convenient for precise aligning. This is actually going to be a problem considering retina display macbooks might be coming soon.

Re: CSS px is an Angular Measurement

#40

Post by nwatson »

modeless wrote: post #22
gwillen wrote: post #5

This is dumb. The fact that one pixel subtends a particular visual angle does not imply that N pixels subtend N times that angle. It should be very plain to anybody implementing or using a rendering engine that pixels are intended to be linearly additive. This implies that N pixels will subtend less than N times the angle one pixel subtends. On a flat screen, this is normal and expected, because a pixel further from…

The fact that this measurement is an approximation (and a very good one in fact) doesn't make it "dumb". In fact, it's flat displays that are "dumb". An optimal display would be curved so that each pixel subtends a roughly equal visual angle, and it's only the fact that most displays subtend a relatively small visual angle that allows us to approximate this with flat displays.

A curved display (let's say, spherical) centered approximately at the "primary" observer's eyeballs whose pixel elements all subtend equal solid angles would (a) lead to weird unintuitive and hard-to-program-for locations in their most natural expressions the further you wandered from the center-horizontal or center-vertical row/column of pixels; (b) make it very hard to those whose eyeballs aren't smack-dab in the center of the sphere to make an intuitive mapping from their distorted view of the screen to something that makes sense.

A flat screen is best for general use.

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