Hinting by grid-fitting, which applies to all sizes of a glyph, is more efficient than manually editing bitmaps for every size of a glyph. However, grid-fitting does have limits because it neither relates to a particular point (or ppem) size nor specifies a particular number of pixels to be used. As a result, at some point sizes, the computer might choose the wrong pixels. Often you need to make an exception to the global hinting to improve the pixel pattern around the round parts of the glyph.
With Visual TrueType, you can apply two kinds of exceptions to any control point:
· A “delta” moves the control point a set number (whole or fraction) of pixel(s), based on the ppem size. Each delta applies only to a particular size.
· A “move” moves the control point a set number (whole or fraction) of pixel(s), too, but regardless of the ppem size. Each move applies to all sizes of the glyph, so typographers find moves useful for improving the step pattern of pixels on diagonals.
After you finish hinting one size of a glyph, you review the glyph size by size. Your review should include all ppem sizes likely to be displayed on screen. If a particular size doesn’t look good, you may need to fix its pixel pattern by creating a delta or move. Either exception tells the computer to do an additional operation to correct the problem and ensure a high-quality font.
Note Each delta or move increases the disk space and amount of memory required for a font. Applying an exception to the same point at many different ppem sizes not only increases the font size for each ppem size, but also indicates a need to change how you’re grid-fitting. Don’t create an exception unless you’re sure it’s the best solution to a hinting problem.
You can tell Visual TrueType to apply the exception before or after it applies the outgoing links and other hinting:
An “inline” exception moves the point after the incoming hints have been applied, but before any outgoing hints have been applied. An inline exception affects all subsequent links, interpolations, and other hints. This approach works well for adjusting the position of entire features, such as the position of a crossbar.
A “pre” exception moves the point after any incoming or outgoing hints have been applied, but before the computer automatically smoothes the outline. Links and interpolations significantly adjust the outline, introducing many irregularities. Smoothing instructs the unhinted control points to follow these dents and bumps “accordingly.” This approach works well for adjusting the weight of features without affecting subsequent hints.
A “post” exception (the default) moves the point only after Visual TrueType has applied all other hints and smoothed the outline. This approach works well for adding or deleting individual pixels on the bitmap at the specified ppem size.
Each time you create a delta or move, Visual TrueType automatically updates the size run and on-screen glyph because you need to see the results of this change immediately.
After finishing the exception hinting on a font, consider reviewing the glyph at all sizes again, but with a different rasterizer.