Clipping

Clipping is when parts of the character (typically top or bottom; ascenders, descenders and diacritics) are not visible on the screen (or even in print) when a font is used in certain programs.

Clipping happens when glyphs are drawn too large in relation to the font size: some exceed their bounding box. In order to prevent this, the type designer needs to test each font in applications that are known to be sensitive to this problem — in particular Microsoft Office (Word) on Windows. If clipping issues happen, the vertical metrics need to be adjusted.

This resource from Google is very helpful when thinking about how to set up your font files to avoid clipping issues. 

Set these values to be the same across all masters to ensure that output instances have equal vertical metrics:

• TypoAscender and hheaAscender are set to height of tallest uppercase glyph with single accent (probably  or Å)
• typoLineGap and hheaLineGap set to 0
• TypoDescender and hheaDescender set to lowest a-z letter (p, j, q)
• winAscent and winDecent set to yMax and yMin (absolute highest and lowest point in the font)
• fsSelection bit 7 enabled
  • In GlyphsApp, set Use Typo Metrics custom parameter set to true in the Font tab of Font Info.
  • In RoboFont, this is under Font Info > OpenType > OS/2 Table > fsSelection > USE_TYPO_METRICS.

Expected result: vertical metrics should be around 130% of UPM. Anything greater, and the metrics may look too loose.

Helpful Resources

• Karsten Luecke: Font Metrics (pdf)
• Glyphs App tutorial: Vertical Metrics
• Typophile: Vertical Metrics How-To
• Google Fonts: Vertical Metrics (github)