Side bearing deltas / intermediate sidebearings

Var Test.glyphs (230.8 KB)

VariableSpacingTestVF.ttf (2.6 KB)

In this example, I want spacing at width 50-70, adding
pos @All <0 0 0 0 (wdth:50) 250 0 500 0 (wdth:70) 0 0 0 0>;
but it adds spacing at width 0, weight 500, which is where the second master is: