Re: fontSize and percentages

Yves' question has highlighted that there's ambiguity in the current spec. Before we raise this as an issue is there some wording I've missed that says that EM cell aspect ratio should be preserved when one length specification is provided? My search yielded zero results.

For reference, my understanding of the words as currently written in 8.2.9 is as follows:

  *   When length is specified in percentage, this is (as in the table in 8.2.9) "relative to parent element's font size, or if no parent element, then relative to the Computed Cell Size"
  *   In this example there's no parent element font size so it's relative to the cell size, which is 20px (width) x 32 px (height).
  *   This sets the 100% size of the glyph's EM square, which in this case is not square but rectangular. [I suspect this is the point of divergence in the interpretation]
  *   The wording "If a single <length><http://www.w3.org/TR/ttaf1-dfxp/#style-value-length> value is specified, then this length applies equally to horizontal and vertical scaling of a glyph's EM square" for a percentage means that the same percentage scaling is applied to both dimensions.
  *   The note "The expression 1c means one cell, where 'c' expresses the cell length unit as defined by 8.3.9 <length><http://www.w3.org/TR/ttaf1-dfxp/#style-value-length>. When a single <length> is expressed using cell units, then it refers to the height of the Computed Cell Size." does not apply because the length is expressed in percentages not cell units.
  *   Therefore the width is 640/32 * 75%.

I'd also note that I agree that the desired behaviour should primarily be to select a font with the required height (or as close to it as possible) and if any scaling is to be applied, do so equally in both dimensions if the scaling factor is the same in both dimensions, regardless of whether one or two values are provided.

The wording in 8.3.9 is perhaps a model for this: "When specified relative to a font whose size is expressed as a single length measure or as two length measures of equal length, the unit of measure em is considered to be identical to that defined by [XSL 1.1]<http://www.w3.org/TR/ttaf1-dfxp/#xsl11>, § 5.9.13; however, when specified relative to a font whose size is expressed as two length measures of non-equal lengths, then one em is equal to the inline progression dimension of the anamorphically scaled font when used to specify lengths in the inline progression direction and equal to the block progression dimension of the scaled font when used to specify lengths in the block progression direction."

One could argue that the first paragraph in 8.2.9 sets up this problem:

The tts:fontSize attribute is used to specify a style property that defines the font size for glyphs that are selected for glyph areas generated by content flowed into a region.

Perhaps rewording this to be:

The tts:fontSize attribute is used to specify a style property that defines the desired font size for selecting and scaling glyphs that are selected for glyph areas generated by content flowed into a region. Where tts:fontSize is non-anamorphic, i.e. has the same computed value for height and width, glyphs should be selected primarily to match as closely as possible their height to the computed value, and any scaling should preserve the ratio of height to width for each glyph.

would remove the issue?

Nigel


On 19/08/2013 14:08, "Glenn Adams" <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:




On Mon, Aug 19, 2013 at 5:11 AM, Nigel Megitt <nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>> wrote:
Hi Yves,

The draft text for the next edition of the specification, at http://www.w3.org/TR/ttaf1-dfxp/#style-attribute-fontSize includes a note that hopefully answers your question, as well as making clear that when only one length value is specified it applies equally to horizontal and vertical scaling, and is proportional to the cell size when specifed as a percentage.

The answer to your first question is yes, the width is 640/32 * 75% = 15.

Actually, this is wrong, since it doesn't preserve EM cell aspect ratio. If there is one length specification, it applies uniformly to both dimensions, however, the resolution of a percentage size is in accordance only with the height of a cell (if resolved related to a cell unit).

So, for example, if the intrinsic size of an EM square is 1px (width) x 1px (height), then your example would scale this EM square to 24px (width) x 24px (height).

On the other hand, if you specified fontSize="75% 75%", then this would scale the EM square to 15px (width) x 24px (height).

The relevant spec language is at 8.2.9:

If a single <length><https://dvcs.w3.org/hg/ttml/raw-file/default/ttml10/spec/ttaf1-dfxp.html#style-value-length> value is specified, then this length applies equally to horizontal and vertical scaling of a glyph's EM square; if two <length><https://dvcs.w3.org/hg/ttml/raw-file/default/ttml10/spec/ttaf1-dfxp.html#style-value-length> values are specified, then the first expresses the horizontal scaling and the second expresses vertical scaling.


Non-proportional scaling is the expected behaviour to allow font sizes that are 2 rows in height and 1 column in width to be denoted.

Nigel

________________________________
From: Nigel Megitt
Sent: 19 August 2013 12:02
To: De Muyter, Yves (Contractor); public-tt@w3.org<mailto:public-tt@w3.org>
Subject: RE: fontSize and percentages

________________________________
From: De Muyter, Yves (Contractor) [Yves.DeMuyter@eu.sony.com<mailto:Yves.DeMuyter@eu.sony.com>]
Sent: 19 August 2013 08:30
To: public-tt@w3.org<mailto:public-tt@w3.org>
Subject: fontSize and percentages


Hello,



I have a question regarding percentages on fontSize and having 1 or 2 values on the tts:fontSize attribute.



Given this example (some unneeded attributes are omitted for readability):



<tt tts:extent="640px 480px" ttp:cellResolution="32 15">

<head>

<layout>

                <region xml:id="r1" tts:fontFamily="monospace"  tts:fontSize="75%">

</layout>

</head>

<body>

                <div region="r1">

                                <p>This is text</p>

                </div>

</body>

</tt>



It means the height of the fontsize is 480/15 * 75% = 24



Does it also mean the width of the fontsize is 640/32 * 75% = 15  ?



Or does it mean we need to use the scaling value of the height and apply that equally to the width to have proportional scaling ? The spec doesn¹t clearly say that having a single scaling factor means you need to have proportional scaling, that¹s why I¹m hesitating right now for this detail.



Yves De Muyter

Techsoft Centre

Technology and Software Centre Europe (TSCE)



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