As soon as you start adding OpenType features to your fonts, it is very important to keep an oversight of how they all work together. Fortunately, FontCreator has several powerful tools that allow you to test your font, so you can detect errors easily.

The first quick test to perform is checking characters with diacritics in the preview toolbar. In this case we have taken a closer look at Cambria, testing several Latin lowercase letters with diacritics.

Within the font character grid select a group of characters to inspect. Press the “p” key on the keyboard to use them within the preview window. The font has advanced, anchor-based, positioning features for diacritics, which we want to inspect. Therefore we enable these features: ccmp, mark, and mkmk.

As you can see, there are two glyphs that seem to misbehave, so we want to track down those issues within the OpenType features. First we copy the line of text (containing the characters that we are inspecting: ṙṛṝṟṡṣṥṧṩṫṭṯṱ) from the upper right of the preview toolbar to the clipboard.

Then we open the OpenType Designer. At the bottom of the dialog, do ensure that you have checked the box on the left for “_shaper.” This will ensure that the lookups are processed in the same way that other shaping engines would process them. The shaper is not perfect, but it works great for Latin, as you can see below.

Now copy the line of text from the clipboard. To jump quickly to the possibly invalid item, we first click the macron, but that doesn’t trigger anything, which looks suspicious. Since clicking the macron failed, we now click the letter r itself, which then causes the upper part of the dialog to jump to the glyph composition / decomposition (ccmp) feature.

Bingo! You can see the multiple substitution lookup contains rdotbelowmacron, which uses a modifier macron while it should have been the combining mark named macroncomb. The glyph named uni1E69 has a similar issue, as it uses dotaccent instead of dotaccentcomb.

We can easily fix the issues by changing the output field for these items. “r dotbelowcomb macroncomb” and “s dotbelowcomb dotaccentcomb” do the trick.