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RE: Font Metrics concerns

From: Stephen Zilles <szilles@adobe.com>
Date: Sun, 8 Feb 2015 06:46:55 +0000
To: Alan Stearns <stearns@adobe.com>, Chris Lilley <chris@w3.org>, "Tab Atkins Jr." <jackalmage@gmail.com>
CC: "public-houdini@w3.org" <public-houdini@w3.org>, John Daggett <jdaggett@mozilla.com>, Bram Stein <stein@adobe.com>
Message-ID: <BN1PR0201MB08026DE747B9CB5B57FF8519AE260@BN1PR0201MB0802.namprd02.prod.outlook.com>

> -----Original Message-----
> From: Alan Stearns [mailto:stearns@adobe.com]
> Sent: Saturday, February 07, 2015 7:58 PM
> To: Chris Lilley; Tab Atkins Jr.
> Cc: public-houdini@w3.org; John Daggett; Bram Stein
> Subject: Re: Font Metrics concerns
> On 2/8/15, 2:52 PM, "Chris Lilley" <chris@w3.org> wrote:
> >Hello Tab,
> >
> >Sunday, February 8, 2015, 3:55:38 AM, you wrote:
> >
> >> On Sun, Feb 8, 2015 at 1:45 PM, Alan Stearns <stearns@adobe.com> wrote:
> >>> The TextMetrics interface has attributes for some of these metrics
> >>>in CSS  pixel units. This makes sense for the canvas measurements
> >>>being made by  that interface, but I’m thinking the font interface
> >>>we’re designing should  be expressed in more abstract units. One
> >>>possibility is to add a  unitsPerEm attribute and express all the
> >>>other metrics in those  font-specific units. That’s probably the most
> >>>natural thing to do for  typographers. But I think it could also work
> >>>to rationalize on a single  unitsPerEm value or use an em percentage.
> >
> >> An em percentage is fine with me.  No need to indirect through some
> >> variable amount of units.
> >
> >I think that people with a type background are going to want to use the
> >units for their preferred font format (so 1000 or 2048, in
> >practice) and will resent having to mentally convert to em percentage
> >when they normally think in other units.
> >
> >So values in units, together with units-per-em (like the descriptor),
> >is a better method for the intended audience. This can also be
> >expressed as a percentage, for people who are comfortable with that.
> I think this depends on how the values will be used. If every use ends up being
> metric/unitsPerEm * actualEmInPixels
> Then giving the metrics in em percentage makes using the interface easier.
[SZ] I would note that Typographers are very fussy about alignment and that the above expression is likely to have rounding errors when it is computed. That, however, seems to be inevitable, especially when one considers alignment corrections made in device pixels such as subpixel positioning. Therefore, it seems to make sense to use units that are relative to the fontsize, hence percent of the Em.
> Thanks,
> Alan

Received on Sunday, 8 February 2015 06:47:26 UTC

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