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 GMT

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