VTT zone shift

ewalthert

Dear @Mike Duggan or whom I may concern,
Is there a way to shift a zone — for example the x-hight — up in a specific ppm as one is able to do in FontLab or Glyphs?
I’m sure there is, but I was not able to find a reference or figure it out with help of looking at the sample font.

Bonus questions, does this also work in variable fonts?

Jens Kutilek

The vttdemo.ttf font does include a sample of "zone deltas" in the Control program. Basically you write a delta instruction after the CVT definition:

LowerCase
  SquareHeight
    6: 974 /* replace the 0 with the measured square x-height */

ASM("SVTCA[Y]")
ASM("CALL[], 6, 89")

/*The following delta, increases the lowercase x-height @9,22 & 28ppem*/
Delta (1 @9;22;28)

It will be compiled to the following code in the Pre-program:

SVTCA[Y]
CALL[], 6, 89
DLTC1[(6 @0 8)(6 @13 8)]
DLTC2[(6 @3 8)]

And yes, this works in variable fonts too. Which may prove tricky, because if for example your x-height is variable, the deltas should not apply at the same ppm along the axis because at some point the x-height zone will round to the grid differently, thus the delta may not be needed or needs to have a different shift.

There is the GETVARIATION[] instruction which will give you the axis positions of the current variable font instance. You could use it to make delta instructions dependent on the axis location. The hard thing is finding the "breakpoint" along the axis where a delta needs to be de-/activated.

But if the x-height varies along two axes, you get literally hundreds of breakpoints dependent on the combination of axis locations. I've given up on that ...

ewalthert

Thank you @Jens Kutilek again and again!

Luckily the x-height is not variable but evidently too low in 19ppm.

I will give this a try.
No doubt the information was to find, but above my head and therefore not recognised by me.