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

RE: Making pt a non-physical unit

From: Linss, Peter <peter.linss@hp.com>
Date: Mon, 11 Jan 2010 23:04:38 +0000
To: www-style W3C Group <www-style@w3.org>
Message-ID: <A56057A4896BEC4BA13EE4C45CEB4FED2B85DACBEE@GVW0538EXC.americas.hpqcorp.net>
I will vigorously oppose any change that makes points anything other than 1/72 of an inch (unless we really want to get pedantic and correct it to the Truchet definition from the 1600s of 15625/833118mm, ok, not really).

I accept that the px unit was doomed from the start and I'm OK with having px defined to be 1/96th of an inch just to provide some sanity for the installed base. This will by definition set the ratio between px and all other physical units as we're not going to allow well established ratios of units to change.

Now, how CSS units map to real-world physical units is something that simply has to be handled on a per-media, per-device basis by the UA. For interoperability sake, it makes sense to have a defined algorithm to use when 1:1 physical mapping is unavailable (or unwise). For certain media, like print, there's no good reason that it should be anything other than 1:1. For screen devices, if the environment has a reliable mechanism that allows the UA to determine how many device pixels map to real world units, then sure, go for it, otherwise use the algorithm. (For the record, I don't consider Windows to be in this category, I'm thinking at this point this may be limited to devices like e-readers.)


-----Original Message-----
From: www-style-request@w3.org [mailto:www-style-request@w3.org] On Behalf Of David Singer
Sent: Monday, January 11, 2010 11:19 AM
To: www-style W3C Group
Subject: Re: Making pt a non-physical unit

On Jan 10, 2010, at 15:24 , Jonathan Kew wrote:
> As I've said before, if we're going to change the basis of "pt", we need to change ALL the current "physical" units in the same way. Anything else is a recipe for even more confusion -- besides the risk of making CSS look somewhat ridiculous.

Yes, what happens if we change the spec. to say that once you have determined what a px is (using the procedure described, including rounding to whole numbers of actual pixels), then an inch is 96 of those, a point is 1/72 of one of those inches, mm conversion is the usual ratio, etc.?

If this 'works' then we are left with the question "what do I say if I *want* 2 feet on the actual display surface, not the logical viewing surface?" which gets the immediate response "and how do we define 'actual display surface' for displays that do not have one (projection glasses) or where we have no way of measuring it (we don't know that the video output device hooked to the computer is a projector, and we don't know how large the image is that it is projecting onto a screen)?"

David Singer
Multimedia and Software Standards, Apple Inc.
Received on Monday, 11 January 2010 23:06:05 UTC

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