Adding information regarding UPM values other than 2048

en-US/Design_Metrics.md

@@ -7,7 +7,7 @@ category: Font Metadata
 title: Design Metrics
 ---
 
-The curve shapes, letter widths, diacritic positions, and contextual adjustments in a font are defined in relation to a grid of either 1000 x 1000 units (Postscript and CFF-based OpenType fonts) or 2048 x 2048 units (TrueType and TrueType-based OpenType). Other units-per-em (UPM) values are allowed, but there is very little real justification for doing so, despite what some experts argue. Although CFF-based fonts are supported by most systems, TrueType-based fonts have much better support, especially on older systems. **We recommend that you develop TrueType-based OpenType fonts, and set your UPM value to 2048.** The rest of this discussion on metrics assumes a 2048 UPM grid.
+The curve shapes, letter widths, diacritic positions, and contextual adjustments in a font are defined in relation to a grid of either 1000 x 1000 units (Postscript and CFF-based OpenType fonts) or 2048 x 2048 units (TrueType and TrueType-based OpenType). **Other units-per-em (UPM) values are allowed, but such situations are rare and should be  avoided unless there is a very specific reason to do so (see [Special UPM values](#special-upm-values))**. Although CFF-based fonts are supported by most systems, TrueType-based fonts have much better support, especially on older systems. **We recommend that you develop TrueType-based OpenType fonts, and set your UPM value to 2048.** The rest of this discussion on metrics assumes a 2048 UPM grid.
 
 The 'em' is a unit that has roots in traditional type founding, but is now well removed from that heritage. The idea was that the letters of a font would normally be sized so that every letter of the design could fit within that 'em-square'. That is no longer the case, and the em/UPM is now used to mathematically determine the physical size of a letter when rendered at a particular point size. Adam Twardoch wrote (in a [recovered Typophile post][Twardoch]):
 
@@ -37,5 +37,18 @@ If you have set your x-height high and also have long ascenders, then your norma
 
 For example, say you decide that you want your font to appear as large as Georgia (about 960 units in a 2048 UPM font), and you set the height of the ‘x’ to 960. But then you also decide to give it long ascenders, much longer than Georgia’s, between 1600-1800, and deep descenders as well. Then it shouldn’t be a surprise that you get lines colliding. You can fix this technically. However, it may be that you’ve simply chosen an oversized x-height for a font that also has long ascenders. You may not be able to have both the perceived size of Georgia and tall, elegant ascenders. **This is a design issue, not a technical one.**
 
+## Special UPM values
+
+It can be useful in certain cases to set the UPM value for a TrueType font to something other than 2048 - specifically 1000. Some of the technical reasons to avoid this are less relevant than in the past, and it's very likely that a TrueType font with a UPM of 1000 will work without problems. **It's still a good idea to use 2048 by default, however, a UPM of 1000 may be appropriate in certain situations:**
+
+- The font is based on some other source font with a UPM size of 1000, and you want to avoid rounding errors in scaling to 2048.
+- You want to import data or metrics from a font with 1000 UPM, such as kerning or hinting.
+- You are working with a team of designers that are all used to working in 1000 UPM and you want to make it easier for everyone to use consistent measurements.
+- When the fonts need to be delivered in multiple technology formats, such as both TrueType and OpenType/CFF.
+
+Some designers have chosen a UPM greater than 2048 (such as 4096) to gain greater design precision, but that is quite rare.
+
+
+
 
 [Twardoch]: http://typophile.com/node/77906