Extrema Issues (FontForge)

Thomas WeakleyThomas Weakley Posts: 59
edited March 2020 in Type Design Software
Hey all,

I'm looking for some clarity on solving a rendering issue in InDesign which I suspect has to do with missing extrema points.

The Issue:
A few (not all) of my rounded characters are rendering in InDesign with a small chip missing from the apex of the character. The missing chip does not show up in any other adobe software and it goes away in InDesign when switching the view to Overprint Preview. The issue is also not present upon export to any format. 

See screenshot of the missing chip:

Maybe I should not worry about this but I believe it is a result of the only validation error for these characters: Missing points at extrema. I'd like to correct these errors but I have not found a way to do so without causing other issues.

When I use FontForge's "Add Extrema" feature it adds seemingly unnecessary points to my form and slightly distorts it so that doesn't seem to be a fix for this.

Apparently the hot pink crosshair points on the screenshot below are what FontForge identifies as the extrema of the form. I don't understand why the yellow points I have plotted are not at the extrema of the form. Does it have to do with the control handles? Why the heck are there four points of extrema on the insides of the bowl? I can't make sense of this. 

Here's a screen shot of the points FontForge adds to the top when I use the "Add Extrema" Feature:

Any clues or direction would be super helpful.

Many thanks!


  • Thomas PhinneyThomas Phinney Posts: 2,746
    Your "real" extrema are above where you think they are, because your off-curve control points go too far—too high at the top and too low at the bottom. They should never cross over the line created by extending the neighboring control point.

    That's in turn causing bad rendering under some circumstances. Basically, the curve does indeed go a fraction of a unit above the overshoot area.

    Actually, the same problem happens in the X direction with the other nearby off-curve points. I didn’t notice that when I made the graphic, but it's there. Just a little bit in all cases, pretty subtle. I’m impressed that you noticed the rendering glitch! Good eye.  ;)

  • @Thomas Phinney Thank you!!

    This extrema thing has been a mystery to me throughout this process. I'm so glad to finally understand what is going on. 

    Here's my new /O

Sign In or Register to comment.