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Re: [CSS21] Issue 149 - px vs. pt

From: Robert O'Callahan <robert@ocallahan.org>
Date: Wed, 30 Jun 2010 07:27:55 +1200
Message-ID: <AANLkTilcgWCYuYhZyj1A_pxA43yncOUZ26kBsur4LkxC@mail.gmail.com>
To: David Singer <singer@apple.com>
Cc: Brad Kemper <brad.kemper@gmail.com>, fantasai <fantasai.lists@inkedblade.net>, "www-style@w3.org" <www-style@w3.org>
On Wed, Jun 30, 2010 at 4:56 AM, David Singer <singer@apple.com> wrote:

> The problem comes when you have a device with say 300dpi.  If you want 96
> px/in, and CSS pixels are an integer number of device pixels, then you have
> to pick 3:1 for that ratio, which results in a CSS inch being 288 (3*96)
> device pixels, which is about 4% short of a true inch.  Basically, something
> has to give:
> * 96 CSS px/in
> * integer relationship
> * true measurements
>
> If you anchor in true measurements, and the 96 px/in is inviolable, then it
> has to be the integer relationship that gives.  I think in print media in
> particular, people expect inches to be inches, not 4% wrong.
>

The best approach would depend on the details of the device.

For genuine printing, even at 300dpi you don't get sharp-edged pixels
because of effects like ink bleeding, and you don't care about the
performance impact of non-pixel-aligned drawing, so there is no reason to
make 1px is an integer number of ink dots; you would prefer to make 1in be a
physical inch.

On the other hand, on a 300dpi LCD screen you would probably want to make
1px 3 screen pixels.

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]
Received on Tuesday, 29 June 2010 19:28:30 GMT

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