Question

I know it’s a simple concept but I’m struggling with the font metrics. Centering horizontally isn’t too hard but vertically seems a bit difficult.

I’ve tried using the FontMetrics getAscent, getLeading, getXXXX methods in various combinations but no matter what I’ve tried the text is always off by a few pixels. Is there a way to measure the exact height of the text so that it is exactly centered.

Answer 1

Note, you do need to consider precisely what you mean by vertical centering.

Fonts are rendered on a baseline, running along the bottom of the text. The vertical space is allocated as follows:

---
 ^
 |  leading
 |
 -- 
 ^              Y     Y
 |               Y   Y
 |                Y Y
 |  ascent         Y     y     y 
 |                 Y      y   y
 |                 Y       y y
 -- baseline ______Y________y_________
 |                         y                
 v  descent              yy
 --

The leading is simply the font’s recommended space between lines. For the sake of centering vertically between two points, you should ignore leading (it’s ledding, BTW, not leeding; in general typography it is/was the lead spacing inserted between lines in a printing plate).

So for centering the text ascenders and descenders, you want the

baseline=(top+((bottom+1-top)/2) - ((ascent + descent)/2) + ascent;

Without the final “+ ascent”, you have the position for the top of the font; therefore adding the ascent goes from the top to the baseline.

Also, note that the font height should include leading, but some fonts don’t include it, and due to rounding differences, the font height may not exactly equal (leading + ascent + descent).