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Anatomy of a Game Mechanic

July 29, 2009 Article Start Previous Page 3 of 4 Next
 

Notes on How to Choose the Right Mechanic: Linear or Non-Linear or Combined?

Designing game mechanics is part science, part artistry. Typically you have to make trade-offs between simplicity and accuracy. All sorts of factors should be weighed in a design decision: How accurate of a simulation do you want? How well known is the mechanic you are trying to model? How simple will it be to tune, retune, and balance? Will other team members understand it enough to work on it? Will the end-user (player) understand it? And so on.

Sigman Design Rule 41a: Choose the simplest mechanic that satisfies your design goal.

KISS and all that. This is similar to a very handy axiom in AI Programming. Remember that your goal is not to make the most eloquent/advanced/innovative SYSTEM, but rather to accomplish a DESIGN GOAL with the minimum amount of required work. In other words, your game mechanic only needs to be as clever as it needs to be. A bit Zen-like, but true.

This isn't to say always go simple -- sometimes a design goal can only be achieved with a complex system. But always go as simple as you can get away with, because that will facilitate easier tuning, adjustment, iteration, and understanding.

In order of simplicity of implementation:

  1. Linear mechanic
  2. Non-linear mechanic defined by a single equation (e.g. y = x2)
  3. Segmented Linear mechanic
  4. Non-linear mechanic defined by multiple equations and/or combined linear/non-linear mechanic

However, don't confuse implementation complexity with the complexity that the player sees or experiences!

Sigman Design Rule 41b: Complexity of implementing a mechanic does not always equal the player's complexity of understanding that mechanic.

Non-linear mechanics are usually more complex to implement than linear mechanics, but they aren't typically hard to understand! Like I mentioned earlier, concepts like "diminishing returns" are intuitively familiar to many people. The reason is that we encounter tons of different mechanics in everyday life, and many of them are non-linear.

It is not hard to understand that a car's acceleration varies over its 0-60 run (as gears are changed), or that bunnies multiply exponentially, or that your financial investments can behave any number of ways in between key time periods. People observe mechanics all around them every day, even if they aren't thinking about it.

If you are making a simulation or an arcade action game, studying real-life mechanics is hugely important to game design.

A Few Linear Mechanics in Life

  • Travel speed and distance: if you travel at 120mph in a straight line, you cover twice as much distance per hour as you do traveling 60mph in a straight line. Yes that sounds obvious, but that's a linear mechanic for you!
  • Kinetic energy as a function of mass: a flying object moving at 50 feet per second and weighing 2 lbs. has exactly ½ of the kinetic energy of a flying object moving at the same speed but weighing 4 lbs.
  • Temperature in the troposphere: temperature tends to decrease at a steady rate ("lapse rate") from the surface on up to the tropopause.

A Few Non-Linear Mechanics in Life:

  • US Federal Taxes: thanks to the myriad of factors involved in US taxes (including tax brackets, phased out deductions, and a million other things), someone who earns 100k isn't guaranteed to pay twice as much taxes as someone who earns 50k. But even within your own tax return, you pay more taxes for the last $10 you earn than the first $10 you earn (assuming you make enough to hit the second marginal tax bracket). The core tax bracket system is segmented linear.
  • Weight training: for every hour you do strength training, you don't gain the same amount of strength. Sometimes you have rapid gains, and sometimes you plateau for a time. (non-linear decreasing)
  • Population growth: if you look at human history in 10 year segments, the amount of new people born each decade increases non-linearly. (non-linear increasing)
  • Compounding Interest: if you invest $1,000 in a 5% earning money-market account at age 20, it will be worth $7,040 at age 60. If you had invested the same $1,000 at age 40, it would only be worth $2653 at age 60 (37.7% of the former). If interest compounding was a linear mechanic, you would have expected a 20 year investment to be worth exactly 50% of a 40 year investment. (non-linear increasing)
  • Salary as a function of years of experience (Ok, the Game Developer Salary Survey is always on my mind): someone with 15 years of experience doesn't typically make 3 times what a person with 5 years of experience makes. (non-linear decreasing)
  • Gravity: the attraction of two bodies decreases non-linearly with distance. If you weigh 200 lbs. at Sea Level (~4,000 miles distance from the center of the Earth), you will actually weigh far less than half of that at an altitude of twice that.
  • Kinetic energy as a function of speed: a five-pound object moving at 50 mph has four times as much kinetic energy when moving at 100 mph (non-linear increasing)
  • A normal (aka "bell shaped") distribution of data (see my Statistics article <LINK>): values in the upper 10% band are far more rare than those in the middle 10% band.

Ok, enough about real life. Let's do a couple quick case studies of actual game mechanics.

Game Mechanic #1: Leveling Tables in Typical Role-Playing Games

Quick Description: Characters gain "levels", which correspond to predefined improvements in key attributes and skills. It takes a certain amount of Experience Points (earned through completion of various tasks) to gain levels. Later levels cost vastly more Experience Points to advance than early levels.

Graph It!


Typical Experience Point / Level Chart

Dissect It!

When it comes to an experience point system, there are two main ways of skinning the cat. Method 1, which is by far the most popular, uses an experience point curve like that shown above -- experience to reach the next level increases non-linearly. Only 1,000 may be required to achieve level 2, but you'll need a total of nearly 200,000 to reach level 20.

This system was used in the grandpappy of all RPGs, Dungeons and Dragons. The advantage of this system is that bigger monsters award the player larger XP numbers, so it is easy to compare monsters on an apples to apples basis. Also, the players feel a sense of accomplishment in gaining the larger rewards.

For completeness' sake, the other common system for XP/Levels involves keeping a constant level ramp (say 1,000 XP per level) but adjusting XP awards for monsters to be relative to the players. For example, defeating a kobold with a level 1 character may yield 50 XP, but defeating the same kobold with a level 10 character might only award 5 XP.


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