1. A method performed by data processing apparatus, the method comprising: receiving a hinted glyph outline for a glyph to be rendered, the glyph to be rendered being associated with a character of a particular font and at a specified scaled size, the glyph to be rendered having one or more horizontal stems; identifying a darkening amount to be applied to the one or more horizontal stems of the glyph; modifying hints of the one or more horizontal stems of the hinted glyph outline using the identified darkening amount to modify the hinted glyph outline, wherein the modifying includes increasing a spacing between edges of each horizontal stem by twice the darkening amount identified for the respective horizontal stem; and rasterizing the glyph using the modified hinted glyph outline.