Re: fontSize and percentages

On Mon, Aug 19, 2013 at 9:45 AM, Glenn Adams <glenn@skynav.com> wrote:

>
> On Mon, Aug 19, 2013 at 8:19 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:
>
>>  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?
>>
>
> This is equivalent to saying "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".
>
>
>
>> 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).
>>
>> When you say "it's relative", it refers only to the process of resolving
> percentage specifications. It does not imply that the EM cell is the same
> as the TTML cell size, and also doesn't imply anything about the aspect
> ratio of the EM cell.
>
> See the note in 6.2.1:
>
> "The use of a uniform grid is employed only for the purpose of measuring
> lengths and expressing coordinates. In particular, it is not assumed that
> the presentation of text or the alignment of individual glyph areas is
> coordinated with this grid. Such alignment is possible, but requires the
> use of a monospaced font and a font size whose EM square exactly matches
> the cell size."
>
>>
>>    - 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]
>>
>> I'm not exactly sure what you mean by "this" in "This sets ...".
>
> In the case of outline fonts, the EM square is always square. In font
> files, e.g., OpenType, the EM square is associated with a coordinates space
> by specifying a resolution on this square, for OTF, this is unitsPerEm,
> defined in the 'head' table [1].
>
> This resolution is always uniform in both dimensions. In many Postscript
> based fonts, this is specified as 1000. In many OpenType and TrueType
> fonts, this is specified as 2048; however, the number can vary from 16 to
> 16384, as determined by font designer. Each glyph's outline is specified in
> coordinates within this coordinate space (but notes that outline points are
> not constrained to be interior to the EM square).
>
> [1] http://www.microsoft.com/typography/otspec/head.htm
>
> In the case of bitmap fonts, one may have a non-square "EM square", though
> here, the EM square is generally associated with a font bounding box, whose
> width and height can be construed as the EM square. For example, one might
> have for a BDF font [2] the following:
>
> FONTBOUNDINGBOX 16 16 0 -2
>
> [2] http://en.wikipedia.org/wiki/Glyph_Bitmap_Distribution_Format
>
> Here, one could create a double height font using a bounding box of, say:
>
> FONTBOUNDINGBOX 16 32 0 0
>
> or one that corresponds to the example "cell size" in this thread:
>
> FONTBOUNDINGBOX 15 24 0 0
>

This should have read

FONTBOUNDINGBOX 15 32 0 0


>
> So, let's say we are using a bitmap font whose bounding box is 16px by
> 16px (square) with this example file whose cell size is 15px by 32px
> (non-square). Then a fontSize="75%" is the same as specifying font size of
> (32px * 75%) 24px. So, we need to scale this font by (24/16) 150% in both
> width and height.
>
> Let's say that instead we had specified fontSize="75% 75%", then this is
> the same as specifying of (15px * 75%) 11px (rounding to nearest integer)
> width and (32px * 75%) 24px height, i.e., the same as fontSize="11px 24px".
> So, we need to scale this font by (11/16) 68.75% in width and (24/16) 150%
> in height.
>
> So, we have:
>
> fontSize="75%"     : scale(24/16,24/16) : scale(150%,150%)
> fontSize="75% 75%" : scale(11/16,24/16) : scale(69%,150%)
>
> If instead, we used a bitmap font whose bounding box is 7px by 16px
> (non-square), then we would arrive at:
>
> fontSize="75%"     : scale(24/16,24/16) : scale(150%,150%)
> fontSize="75% 75%" : scale(11/7,24/16)  : scale(157%,150%)
>
>
>>    - 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.
>>
>
> No. If two values are specified, then scaling is independent in width and
> height.
>
>
>>
>>  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."
>>
>
> This text is correct, and defines how to interpret the meaning of the 'em'
> unit in accordance to whether it applies in width or height.
>
>
>>
>>  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.
>>
>
> I'm not seeing a problem here. Perhaps your misunderstanding came from an
> incorrect association with computed cell size and em square, since I notice
> you didn't quote the language I noted from 6.2.1 above.
>
>
>>
>>  would remove the issue?
>>
>>  Nigel
>>
>>
>>   On 19/08/2013 14:08, "Glenn Adams" <glenn@skynav.com> wrote:
>>
>>
>>
>>
>> On Mon, Aug 19, 2013 at 5:11 AM, Nigel Megitt <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
>>> *Subject:* RE: fontSize and percentages
>>>
>>>   ------------------------------
>>> *From:* De Muyter, Yves (Contractor) [Yves.DeMuyter@eu.sony.com]
>>> *Sent:* 19 August 2013 08:30
>>> *To:* 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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>>
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