from Hacker News

CSS px is an Angular Measurement

by ned on 3/12/12, 6:46 PM with 60 comments

  • by gwillen on 3/12/12, 8:09 PM

    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 eye will subtend less angle than a pixel closer to the eye.

  • by javajosh on 3/12/12, 9:20 PM

    This is one of the more important Hacker News submissions I've seen in a long time. It seems that there is, potentially, a fundamental disconnect between what "px" is supposed to mean and what it means in practice. Given the incredible importance of the web, particularly the front-end of the web, and the extraordinary increases in screen resolution (today the iPhone 4 and iPad 3, tomorrow most computers), it's very important to resolve this discrepancy.

    Personally, I'm disgusted at the W3C standard. It's a great idea to have an angular measure (really great) but to call it a "pixel" is horrible. A pixel is the smallest controllable dot on a physical display, and nothing else. Call it an "aixle" abbreviated "ax" and short for "angular pixel" but don't overload the term "pixel".

  • by colanderman on 3/12/12, 8:40 PM

    Interesting. The angular resolution of the human eye is about 0.02° according to Wikipedia (http://en.wikipedia.org/wiki/Naked_eye#Basic_accuracies), which, using the provided calculator on this page, corresponds very nearly to one pixel (0.938 to be exact). Pretty sweet.

  • by yonran on 3/12/12, 8:57 PM

    Not only is the definition of pixel unexpected, but 1in is now defined to be 96px in CSS 2.1, because this was the default in Windows for so many years. In retrospect, given whole page zoom from Opera and high-resolution displays from Apple, probably CSS shouldn't have had units named "in" and "px" at all but instead should have had a single unit like SVG.

  • by nthitz on 3/12/12, 7:58 PM

    Ok, but does it matter?

  • by kickingvegas on 3/12/12, 11:50 PM

    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.

  • by tantalor on 3/12/12, 10:25 PM

  • by evincarofautumn on 3/12/12, 8:52 PM

    This matters more still for a display with head-tracking. As I read this, the apparent angular size of 1px within a human’s FOV ought to remain constant, regardless of viewer position or physical pixels.