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

Re: Making pt a non-physical unit

From: Boris Zbarsky <bzbarsky@MIT.EDU>
Date: Tue, 05 Jan 2010 12:36:42 -0500
Message-ID: <4B4378AA.4050203@mit.edu>
To: Ambrose LI <ambrose.li@gmail.com>
CC: www-style <www-style@w3.org>
On 1/5/10 11:58 AM, Ambrose LI wrote:
> As a user, I don't believe the OS knows what the true size and/or
> resolution of the display. IIRC, when Apple's X server still gave us
> two screens (:0.0 and :0.1) for my dual head setup (:0.0 being Apple's
> own notebook LCD so theoretically its size must be known, and :0.1
> being an external LCD monitor), the numbers didn't look quite right.

Apple's X server is reporting what the underlying OS reports, and the 
underlying OS in this case is clamping the reported resolution to 96dpi.

> But now that it gives me just one merged display, the numbers clearly
> must be wrong

Yep.  I'm not saying the OS doesn't lie about DPI at will!  That doesn't 
mean it doesn't have the real information, though.

> The existence of xinerama alone shows that querying the X server does not guarantee a correct resolution and/or
> size.

It merely guarantees a size consistent with other programs that use 
physical units, sure.

Here's the relevant part of /var/log/Xorg.0.log on my Linux box (lots of 
stuff skipped, obviously):

(II) NOUVEAU(0): EDID for output DVI-D-0
(II) NOUVEAU(0): Manufacturer: DEL  Model: a008  Serial#: 808735308
(II) NOUVEAU(0): Year: 2005  Week: 32
(II) NOUVEAU(0): EDID Version: 1.3
(II) NOUVEAU(0): Digital Display Input
(II) NOUVEAU(0): Max Image Size [cm]: horiz.: 41  vert.: 31
...
(**) NOUVEAU(0): Display dimensions: (410, 310) mm
(**) NOUVEAU(0): DPI set to (99, 98)

The monitor in question is a "20 inch" monitor at 4:3 aspect ratio, 
nominally.  The 410mm by 310mm numbers claimed by X here are in fact 
very close to correct as measured by ruler.

Given bytes 21, 22, 66 and 67 of 
http://en.wikipedia.org/wiki/Extended_display_identification_data#EDID_1.3_data_format 
(and the note in that same article that EDID 1.3 dates back to 10 years 
ago or so), as well as the mentions in the same article that XFree86 
(now Xorg) uses EDID, I suspect that this is in fact just the monitor 
self-reporting.

That same article mentions some software packages for MS Windows that 
can read EDID information.

-Boris
Received on Tuesday, 5 January 2010 17:37:16 GMT

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