For the last 5 years a huge focus of my work has been on fonts and text. You know you went to far when you can tell the difference between a Helvetica and an Arial by just looking at the printed ‘a’.
Its unsurprising that people end up asking me what the difference is between leading and linespacing, why the customer claims we don’t do kerning for that specific font, why WYSIWYG actually fails for most people. I naturally can’t be because I’m the only one stupid enough to claim to know this, right?
I considered claiming ignorance, but then my weird behavior may no longer have any reason, so that would just make me a worse freak. Instead I just wrote an application that shows everyone how text works. And I’ll write a blog or two about what you can see on screen and how that relates to your questions even before you have them. Genius or what?
So, here is an example text;

let me quickly go over the different parts. In red we have the outline, this is the total amount of space that the text takes. This is what is reserved in your user interface for the text. The sizes you see on the right hand side are the major anatomical dividers of a font face. Much like your body has a head and legs (I’m making assumptions about that, work with me). The baseline is the only one that is really interesting to point out. it’s the zero-point for a font. All measurements start from there. So you have a part that’s above and a part that’s below the baseline. I’ll leave it to your imagination to mirror that to your own anatomy.
In blue we have the size taken by the individual characters. But when we are talking about fonts we actually should be talking about glyphs. There are subtle differences, but I won’t bore you with that. Each glyph has its own rectangle as you can see in the blue. This is useful to see since the m is wider than the j, which is useful to know since you position the characters next to each other. The blue little gradients are helpers (called bearings) to position the glyphs better so they visually look more pleasing.

Ok, with the basics behind you here are a bit more interesting things; consider the two following screenshots.

The only difference is that kerning is turned on for the first and turned off for the second. Notice how the blue boxes overlap in the first image and how they are simply placed side by side in the second.
In general you want kerning to be turned on, its on by default in Qt because it increases readability.

Last example; this one is tricky.

Fonts are designed in a way that they can be scaled and reused for any size. Which is a pretty neat idea since it avoids using a crystal ball to figure out which sizes to ship your font in. There is a little problem, though. A font that is printed on paper at 10cm per character needs a lot of detail but if you use the same character at on screen at just a couple of millimeters height, you have problems to make that one look good.
So, font makers ship something called ‘hints’. Which make their fonts look better at smaller sizes then the computer could do automatically. This is enabled per default and is practically speaking exactly what you want.
Except for one problem; if you add all those little adjustments they can add up. So much that if you have a sentence you can have a word that fits just fine when you show the font on screen, but if you then take the same width and same text but on paper, those little adjustments may just move a word to the next line.
In other words; hinting gets in the way of what you see is what you get text-layout. So, in Qt you can turn this off. Allowing you to get the exact same line-breakings on screen as you get on paper.
Look at the following two screenshots. You will see a little spacing between the little blue squares in the one where the hinting is turned off. We call this mode ‘designer metrics’.
if you want to play around with this stuff yourself, here you can find the sources;

svn checkout svn://labs.trolltech.com/svn/graphics/fontAnatomy

For the people that are still here, thanks for sharing the pain! And I’ll answer the question of “why should I care!”. The concepts shown in this blog have good support in Qt4. The point to take home is that the font is the one that specifies all the information. If the font doesn’t have kerning, game over. If the font has horrible sizing information, you are out of luck. With this little tool at least you can see the differences that different fonts make.