The Adobe technology involved in interpolation is called ‘Multiple Masters’. That’s because we need multiple, i.e. at least two masters, in order to calculate steps between them. To be precise, Multiple Master used to be a font format that would allow the user to interpolate his own fonts. Probably due to its complexity, it never really gained much traction, and was discontinued in the late nineties.
However, for making large font families, the Multiple Master interpolation technique still plays a significant role in modern typeface design. Needless to say, it is built into Glyphs. But before dealing with Multiple Masters, you must understand the difference between masters and instances.
Masters vs. Instances
Masters are what you draw. They are the input for the ensuing interpolation. Masters are organized in the Masters tab of the Font Info. You draw them on different layers of each glyph. When you are working on a family, you constantly jump back and forth between masters to make sure they will interpolate nicely.
Instances are what the computer calculates. They are the output, the result of the interpolation. Instances are organized in the Instances tab of the Font Info. In the case of Glyphs, they are spit out as prêt-à-porter OpenType fonts right away. If everything works the way it should, you should never get to fiddle with the points or paths of an interpolated instance.
Setting up Masters
Alright, let’s get the party started. Once you have created a new Glyphs file, you pick File > Font Info and navigate to the Masters tab. One Master is already there:
It is defined as a regular weight and regular width master. Attention: this does not imply anything about the design of the master. As a matter of fact, the regular master may look very thin or even very fat. Rather, it says something about the relative position towards other masters on the same axis.
‘What is an axis?’, I hear you say. Easy: Axes are your interpolation dimensions. Glyphs lets you interpolate along the Weight axis and the Width axis, and, should you need it, on a third 'custom' axis. For instance, if you have a very thin and a very fat master, they will constitute a Weight axis. If you add a semibold master between them, you still only have one axis, since you will only be interpolating one-dimensionally. Actually, you can have any number of Masters on a single axis:
If you then add a narrow master, you will open up a second axis, Width, and be able to interpolate two-dimensionally:
Most people will choose to draw four Masters on two axes, though:
But even in 2D you can have an intermediate master:
Want to add a third dimension to your font, or do the terms Width/Weight not apply to your design? In Font Info > Masters, you will find a text field called
Custom. You can enter a custom name for your Master, for example,
Display for a display master.
Whoa, wait a minute, let’s keep it simple for the moment, and add just one fat master. To do that, we simply click on the plus button in the lower left corner of the Masters tab in File > Font Info. Glyphs will add a second master called Regular.
Now, you will agree that you cannot interpolate between the middle of an axis and the middle of the same axis. So, we need to move the second master to one end of the axis. To do that, you activate it by clicking it in the sidebar and change its Weight entry to Bold:
Congratulations, now you have two masters on a weight axis!
Switching Between Masters
Now, drawing is easy. Actually it is just like drawing anything else in Glyphs. The only difference is that you have to draw on two layers, corresponding to the two masters you have set up. To switch between the master layers, simply press Cmd-1 and Cmd-2, i.e. the command key and the number of the master. This button appears as soon as you have two or more masters in your font, which you can alternatively use to switch between the master layers:
Drawing the Masters
Okay, switch to the Regular master and draw a letter. Personally, I like to start with the n because you can recycle its shape for a lot of other letters. So here is our n:
Remember: the term ‘Regular’ only says something about its position on the axis, and nothing about the design of the letters. So, don't worry if your n looks a bit too light for what people would call Regular in real life.
Once you are finished with your n, select all and copy it to your clipboard. Then, press Cmd-2 to switch to the Bold master, and paste your n there. Then, start moving the nodes and make the n super-fat. Usually, you want to reduce white space, so the letter can appear darker. After all, a bolder weight is supposed to achieve a darker color on the page. Thus, it is as important to make the counter smaller as it is to reduce the sidebearings a bit.
Pro tip: When moving a node or a segment, simultaneously holding down Ctrl and Opt may prove to be helpful, because it will proportionally adjust the surrounding handles while you are dragging or arrowing. This is referred to as 'nudging'.
You can always switch between them with Cmd-1 and Cmd-2. But once you are finished with the bold n, you deserve to take a look at them both side-by-side. You can do that with Edit > Show all Masters. Here is what our two n masters look like, including a subtle reminder about the shortcuts for accessing the masters:
Setting the Master Values
Now is a good moment to find out how thick our stems are. The width of the stem is a useful key value for determining the weight of a type design. That is why we recommend using the stem width as master values on the weight axis.
To quickly determine the width of a stem, simultaneously hold down Ctrl, Cmd and Opt. This will temporarily bring up the Measurement Tool (L). Add the Shift key, if you still have a free finger, and you can drag a horizontal line across your stems to see what their widths are. Do that for both masters. In our case, they happen to be 90 and 300:
For each master in File > Font Info > Masters, you can now replace the default Weight value of 100 with the number you have just determined:
Entering Other Values
Well, I suppose I couldn’t keep this a secret for long, because you have seen all the other entry fields in the Masters tab of the Font Info. Yes, as you would expect, all the number values there will be interpolated, as long as all entries are reciprocated in all masters. In other words, it will, e.g., not suffice to enter alignment zones in only one master. You need to enter the corresponding zones in the same order in all other masters as well. This way, Glyphs knows between which numbers it is supposed to interpolate. This also includes stem values, vertical metrics and all numbers entered in custom parameters.
Update 2014-06-20: Fixed typo in Interpolated Nudge pro tip. (Thx Jeff Kellem.)
Update 2015-07-21: Now with 3D and updated for Glyphs 2.
SAMPLE FONTS: Graublau, Jordana