Dump contour values relative to each other instead of (0, 0)

[RoelN]

In the spec I read about coordinates:

First coordinates relative to (0,0); others are relative to previous point.

Yet the coordinates dumped with ttx/FontTools all seem to be relative to (0, 0). Is there any way to dump/compile them relative to the previous point?

[behdad]

There isn't. The compiler concerts them to absolute. We consider that a bye encoding detail. Why do you need that?

[RoelN]

When coordinates are absolute you can't move a point farther than 32,767. Currently <pt x="-16383" y="16383" on="1"/> works, but <pt x="-16384" y="16384" on="1"/> crashes ttx/FontTools with the message OverflowError: signed short integer is greater than maximum. This is half the value of an unsigned int, so I didn't expect that.

With relative values I can work around this making steps of the maximum value so I end up at my ridiculous large value.

I am experimenting so I understand you probably won't need this for production stuff.

Edit: this is the full contour, which gives the overflow error. A single point with x="-16384" y="16384" works, but when this gets converted to absolute you'll overflow the SHORT:

    <pt x="-16384" y="16384" on="1"/>     <!-- Top left -->
    <pt x="16384" y="16384" on="1"/>  <!-- Top right -->
    <pt x="16384" y="-16384" on="1"/>     <!-- Bottom right -->
    <pt x="-16384" y="-16384" on="1"/>        <!-- Bottom left -->

[behdad]

You are misinterpreting the error. What we expose to user doesn't limit anything. You get the error because the relative step from -16384 to +16384 does not fit into a SHORT. You are free to take as many large steps as you want as long as the difference between consecutive ones does not exceed the SHORT range (-32768..+32767)

[RoelN]

Ah yes, check. Thanks for the explanation!