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How Long is a Frame?

I’ve recently stumbled upon a topic that differs greatly depending on the background of each game creator. It’s a fundamental question of time; namely, should time be measured in terms of seconds, or in terms of frames?

I am a stalwart defender of frames… but I still think seconds have their use. As for you, let’s dive into the merits of both, and then see how you’re feeling!

The Usual Defense of Seconds

I understand the appeal of seconds, I really do. They are a natural, universal way to measure a relatively short interval of time. A second is 1/60th of a minute, which is 1/60th of an hour. The arguments I hear in favor of seconds generally focus on this point: that seconds are common and natural to understand.

There are other benefits to seconds too… for example, it’s easy to hop online and find physics equations that are in terms of some value per second. But these benefits still tend to reinforce a bias of familiarity with the second.

Just because seconds are intuitive to most people does not mean a second is always the best way to express intervals of time in game design. That familiarity bias can even impact our ability to perceive time in more granular intervals.

Where Seconds Break Down

(For these examples, I am hacking around with some JavaScript code by Richard Davey I found here)

Let’s say we are making a shoot-em-up game, and we have a spaceship that needs to shoot rapid fire projectiles. To start off, let’s make it so where if you hold the fire button, it shoots 1 bullet per second:

When you see this animation, you’ll immediately think “Wow, those bullets need to shoot a lot faster!” So let’s try them faster… instead of a bullet every 1 second, let’s do a bullet every 0.5 seconds:

Still a bit slow! It really only starts feeling naturally like a shoot-em-up game once we get the shot interval down to 0.125 seconds:

When we’re dealing with gameplay actions, especially ones that need to respond to a button press, game feel is determined by changes that transpire over the course of fractions of a second.

Insisting on measuring with seconds means that the programmers and designers are going to be juggling a lot of fractional numbers! Fine tuning these very quick gameplay actions often means deciding over the difference between numbers like 0.2 and 0.25 seconds.

That’s where the familiarity benefit starts to crumble. While one may be able to easily discern the difference between 1 and 2 seconds, can one just as easily discern the difference between 0.2 and 0.25 seconds? For most people, fractional differences between numbers seem negligible. So, these seemingly tiny values hinder one’s ability to perceive the difference between them.

Re-Framing Our Perception

What do I mean by “frame” exactly? In a time sense, a frame is the time a game takes between drawing a screen and drawing the next screen after it. How many seconds this is depends on the frame rate, but in common practice we are dealing with 60 frames per second. This means we are dividing each second up into 60 parts.

To convert 1 frame at 60 FPS to seconds, the math works out like this:

1 second / 60 frames = ~0.01667 seconds per frame

When we recalibrate our perception to declare 1 as a much shorter unit of time, negligible fractions suddenly become substantial intervals.

Our 1-second shot from earlier is a shot every 60 frames. Our 0.5-second shot is a shot every 30 frames. Now the numbers we are seeing better match the “feeling” of those too-long intervals. In the end, when we settled on 0.125 seconds, those shots were firing about every 8 frames.

Feeling The Difference

(For this section, I got the GIFs from this imgur link, and the frame data from EventHubs)

I’d be remiss if I talked about frame timing without bringing up fighting games. In fact, I’d say my own tendency to think in frames is heavily influenced by being a fighting game fan.

In fighting games, the action unfolds quickly, and a lot can happen over the course of a second. Hardcore players pore over frame data, which is information about how long attacks take, the position of hitboxes and hurtboxes during the attack animations, and how much damage those active hitboxes deal to opponents.

As a rule of thumb in fighting games, slower attacks which take longer to execute tend to do more damage than faster attacks that can be thrown out more rapidly.

In the Street Fighter series, attacks are divided into three grades: Light, Medium, and Hard. As a case study, let’s look at three grades of Sakura’s standing far punch attacks in Street Fighter IV.

Sakura’s Light Punch (LP) animation takes 10 frames and does 25 damage:

Her Medium Punch (MP) animation takes 22 frames and does 75 damage:

Her Hard Punch (HP) animation takes 28 frames and does 110 damage:

Although we are ultimately dealing with a difference of 18 frames (0.3 seconds) across all three attacks, you can feel the difference in speed and weight among them. You can throw two LPs in the time it takes you to throw one MP!

Once you start to tune in to the differences of frame timings, you can start to train your sense of “how long a frame is.” I recommend reading Masahiro Sakurai’s Famitsu column on this subject (Or Source Gaming’s English translation).

Settling The Great Debate

Hopefully now, you can see some benefit to thinking about time in frames if you didn’t before. So now we’re all 100% frame-thinkers, right?

Well of course, nothing is ever that cut and dry. In a practical sense, you may want to take advantage of differences in perceiving seconds vs. frames.

At my studio Evening Star, we accomodate both approaches in our Star Engine. We have a delta-time value programmers can use called Time.deltaSeconds, which is generally 0.01667 when running at 60 FPS. We also have a value called Time.deltaFrames, which is 1.0 when running at 60 FPS. We use either-or to drive timers depending on the context.

For most people, numbers between 1 and 100 are easiest to think with and visualize. To play into this natural inclination, we generally use this rule of thumb: favor expressing “fast” intervals less than 1 second in frames (which avoids fractional second values), and favor expressing “slow” intervals longer than 1 second in seconds (which avoids large frame values). One may show up a lot more often than the other depending on the nature of each game.

Either way, it is useful to develop your perception of frames, because they have the power to help you make the most out of timing details!