23. A system comprising: one or more computers configured to perform operations including: receiving a 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 glyph outline using the identified darkening amount to modify the 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 glyph outline.
23. A system comprising: one or more computers configured to perform operations including: receiving a 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 glyph outline using the identified darkening amount to modify the 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 glyph outline. 28. The system of claim 23 , further configured to perform operations including modifying hints of one or more vertical stems of the hinted glyph outline using an identified darkening amount.