W3C home > Mailing lists > Public > www-style@w3.org > January 2010

Re: Making pt a non-physical unit

From: David Singer <singer@apple.com>
Date: Thu, 7 Jan 2010 10:43:42 -0800
Cc: Ambrose LI <ambrose.li@gmail.com>, www-style@w3.org
Message-Id: <F2D30788-CA03-4F5E-8497-13762D743EBC@apple.com>
To: Boris Zbarsky <bzbarsky@MIT.EDU>
(personal opinion)

It's fairly clear that physical measurements apply to material that is at 'normal reading distance' (and I bet there is an ISO standard for that), or is back-calculated from its actual distance to what size it would be at normal reading distance.  That's why powerpoint works on projectors;  if you are at the distance from the screen such that the screen and a piece of paper held at reading distance subtend about the same view angle, it all works.  Perhaps this should apply to all physical units:  1in means that distance that subtends the same angle at the eye as 1in would at the standard viewing distance, and so on.

This fixes glasses-mounted displays, and so on, as well.

We can deem that computer displays are also at normal viewing distance, I suspect.

If manufacturers want to ship small devices that they deem are necessarily viewed close-up, and thus have a scaling factor also 'built in', that would also be permitted.

Zoom means making things bigger or smaller than nominal, so a zoom of 2x means that if the page asks for 1in, that distance subtends 2in at the nominal viewing distance.  But see below for fonts.

On Jan 7, 2010, at 7:43 , Boris Zbarsky wrote:

> On 1/7/10 10:38 AM, Ambrose LI wrote:
>> I don't understand why we are stressing the importance of physical
>> accuracy in projections. Do people expect units to measure as spec'd
>> when projected?
> I have no idea, but the spec says they should at the moment.  Just like it says they should measure as spec'd on an iPhone.  Or on an eye-glasses display.  Or a contact lens display.  That's what makes the physical units physical.
> That's also what makes them clearly nonsense for anything where you don't control the device; 12pt font on a contact lens display would be ... interesting.
>> When we spec 12pt on PowerPoint does it result in 12pt type on the projection
>> screen which looks like a tiny dot 15 ft away? No.
> That's precisely how Webkit and IE treat pt in general and how Robert is proposing Gecko treat pt.  It's a clear violation of what the CSS spec says to do with pt at the moment.

What you write below does not sound right at all, to me.  A zoomed version of a page should be just that, zoomed;  not re-styled as if the designer had asked for fonts at twice the design size.

> On Jan 7, 2010, at 9:23 , Boris Zbarsky wrote:
>> I would hope not.  A UA should be choosing the design size most closely corresponding to actual rendered size, as much as possible.  For example, if you have a page that says it wants 12px fonts and it's zoomed to 2x in Gecko, Gecko will use the 24px font instead of scaling the 12px font.

David Singer
Multimedia and Software Standards, Apple Inc.
Received on Thursday, 7 January 2010 18:44:16 UTC

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