General Font Metrics

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Bhikkhu Pesala
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General Font Metrics

Post by Bhikkhu Pesala » Thu Dec 28, 2006 12:27 am

Superscripts and Subscripts

Relevant Page in the ManualSpecifications at Microsoft Typography

The font metrics for superscripts and subscripts are found from the Format menu, Settings, on the General tab. These values can be used by applications to calculate the size and position of superscripts and subscripts. Adding these metrics to your fonts will ensure that superscripts and subscripts are the right size and position in applications that use these metrics.

To calculate the metrics for your fonts, you need to take some measurements using the transform tool or the status line. I have measured the superscript 2, which is found in most fonts. The superscript Y position, which is the desired height of superscripts above the baseline in funits, I call the SupS Position.

Image

To calculate the Superscript y-offset from this value we need to know the size of superscripts relative to the figures in the font. To get this scale factor I measure the zero and the superscript zero. If your fonts don’t have the superscript zero, you could use the figure 2 and superscript 2.

Select the glyph and note the width and height on the Size tab of the Transform toolbar. I call these values SupS Width and SupS Height respectively.

Do the same for the figure 2 to get the values for Figure Width and Figure Height. Note the funits/em value for your font on the Header tab of the Format, Settings dialogue. This value is often 2048, but may be different for your font.

Image

The value for Superscript Vertical is calculated using the formula:

SupS Vertical = (funits/em) x (SupS Height) / (Figure Height)
For example: SupS Vertical = 2048 x 852/1520 = 1148

And the value for Superscript Horizontal is calculated using the formula:
SupS Horizontal = (funits/em) x (SupS Width) / (Figure Width)
For example: SupS Vertical = 2048 x 602/963 = 1280

The values Superscript Vertical and Superscript Horizontal can be used by applications to scale superscripts from the figures. Applications may scale in both directions by the same proportion using only the value for Superscript Vertical, but it is better to use a slightly larger scale factor for width to help to compensate for the fact that scaled figures are too light compared to other glyphs in the font. Ideally, a font should contain properly designed superscripts.

The metrics Subscript Vertical and Subscript Horizontal are calculated in exactly the same way, but since the sizes of subscripts and superscripts are usually the same, there is no need to do the calculations again — just use the values for Superscript Vertical and Horizontal. If you want subscripts to be smaller than superscripts, then you will need to use different values for these metrics.

The position recommended by Microsoft Typography for superscripts is aligned to the tops of figures, but you can define the position that you recommend for your fonts by using these metrics. Too high, and they will clash with descenders in the line above; too low and they may be hard to spot in a line of text. The position of superscripts and subscripts in your fonts is calculated from these formulae:

Superscript y offset = (SupS Position) x (Figure Height) / (SupS Height)
For example: Superscript y offset = 668 x 1520 / 852 = 1192

Subscript y offset = (SubS Position) x (Figure Height) / (SubS Height)
For example: Subscript y offset = 420 x 1520 / 852 = 749

Image

The recommended position for subscripts is bisecting the base­line. The calculation for Sub­script y-offset is similar to that for Superscript y-offset, but the value is expressed as a positive offset below the baseline. Negative values will position the subscript above the baseline.

The Superscript and Subscript x offsets are calculated from the corresponding y offsets using the caret run. For italic fonts, they are used to offset superscripts to the right and subscripts to the left. Negative values will offset subscripts to the left. The caret run / caret rise = tangent of the italic angle. For an italic angle of 11° the caret run will be 194/1000 which is the tangent of 11°. For upright fonts these values are zero.
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General Font Metrics 5.5.pdf
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Last edited by Bhikkhu Pesala on Wed Feb 21, 2007 10:03 am, edited 5 times in total.
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Post by Bhikkhu Pesala » Thu Dec 28, 2006 12:03 pm

Strikeout Metrics

The purpose of strikeout is to indicate that text should be removed from a draft, it is not intended to make the text unreadable. Its weight and vertical position should be designed so that it does not make the struck out text hard to read.

Image

The Font Test dialogue in FontCreator has check-boxes for strikeout and under­lining, so that you can see how your font will look in applications when these attributes are applied. Notice how in this screenshot of Bitstream Vera, the strikeout line is just below the horizontal strokes of the “e” so that there is no confusion between “e” and “o” when strikeout is applied. If it doesn’t look right in the Test Font window, it won’t look right in applications either.

Image

The strikeout position is the offset in funits from the baseline to the top of the strikeout line.
The strikeout size is the thickness or weight of the strikeout line in funits.

Underline Metrics

The underlining size and position should be designed not to obscure des­cen­ders making text hard to read when under­lining is applied.

Image

The underline pos­i­tion is the offset of the top of the under­lining stroke relative to the baseline in funits. It is nearly always a negative value, which is below the baseline. The underline size is the thickness or weight of the underline stroke in funits.

Image
Last edited by Bhikkhu Pesala on Sun Sep 21, 2008 11:45 am, edited 3 times in total.
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Good Stuff!

Post by Dave Crosby » Thu Dec 28, 2006 6:35 pm

More excellent information. Thank you Bhikkhu Pesala!
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Re: General Font Metrics

Post by Bhikkhu Pesala » Fri May 10, 2013 4:46 pm

Update for FontCreator 7.0

The font metrics for superscripts and subscripts are found from the Font menu, Properties, on the General tab of the Font Properties dialogue.

I have attached the PDF of this tutorial for FontCreator 6.5 or earlier to the first post.

The PDF version of the updated tutorial (there are only a few changes) is attached to this post. The caret run, which is used to calculate the Superscript and Subscript x-offsets for italic fonts from the Superscript and Subscript y-offsets, is not found anywhere on the Font Properties dialogue, but it can be measured using the measuring tool or calculated from the Italic angle (it is the tangent of the angle), which is now found on the Identification tab of the Font Properties dialogue.
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General Font Metrics.pdf
Updated with correct formulae
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Last edited by Bhikkhu Pesala on Wed May 15, 2013 4:36 am, edited 1 time in total.
Reason: Corrected formulae for superscript y offset and subscript y offset
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Re: FontCreator 7 Update

Post by Bhikkhu Pesala » Tue May 21, 2013 6:02 am

The latest release of FontCreator 7 now has a calculate button on the General tab of the Font Properties dialogue, which will calculate the superscript/subscript metrics with one click.

Erwin designed this for Word. My calculations were based on the behaviour of Serif™ PagePlus, so the calculate button won't give the right results for Serif™ users. Now that we can add OpenType features for Subscripts, Superscripts, and Ordinals to our fonts, these metrics have become less important. You might like to use the Calculate button on your fonts anyway, so that they will work better in applications that don't support OpenType features.
Calculate General Metrics.png
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Re: General Font Metrics

Post by Erwin Denissen » Tue May 21, 2013 9:54 am

Also be aware WordPad (running Windows 7) seems to use its own defaults, and completely ignores these superscript/subscript metrics values.
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Bhikkhu Pesala
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Re: General Font Metrics

Post by Bhikkhu Pesala » Tue May 21, 2013 11:51 am

As far as I can tell from my tests, Wordpad just scales the glyph by 66.7% and raises/lowers it by 50% of the digit height.
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