My game has a stat called Stress, and the player has some missions to do. Each mission has a computed stat called Difficulty with its own base value. So, let's say:

  • Mission 1, Difficulty 1

However, I want to sum to this difficulty an X amount, where X is a % of the player's stress. Turns out I don't want this % to be static, but instead, the higher is the amount of the player's Stress, the higher is the percentage. To illustrate:

  • Stress = 10
  • Mission 1, Difficulty 2
  • Stress = 20
  • Mission 1, Difficulty 2
  • Stress = 30
  • Mission 1, Difficulty 3
  • Stress = 40
  • Mission 1, Difficulty 5
  • Stress = 50
  • Mission 1, Difficulty 8
  • Stress = 100
  • Mission 1, Difficulty 50

The numbers are merely illustrative and don't represent the exact numbers I'd expect.

As you can see, the difficulty doesn't scale linearly.

I don't know the proper keywords to browse for this, so that's why I'm here with this poor vocabulary. Also, I apologize in anticipation if this is duplicated.

  • 1
    \$\begingroup\$ Your current example literally scales linearly. 2 goes into 10 5x. 10 goes into 50 5x. So the linear formula is difficulty = stress / 5. If you are instead referring to the relationship between mission number and difficulty, we cannot tell if it is linear or not, without more samples in series for differentiation. \$\endgroup\$
    – Engineer
    Nov 25 at 0:03
  • \$\begingroup\$ You are right, @Engineer. My bad. I posted more samples. \$\endgroup\$ Nov 25 at 0:11
  • \$\begingroup\$ What are your edge cases (i.e. minimum and maximum stress and difficulty)? In particular I wonder if there is a maximum difficulty that you want to make sure the formula never goes over. I'm assuming from the example you want the curve ramp up slowly. If there is a maximum difficulty, look for a logistic curve. Otherwise an exponential will do (like the one on DMGregory answer). \$\endgroup\$
    – Theraot
    Nov 25 at 1:03

We can throw your example numbers into a spreadsheet and chart them, say using Google Sheets, and get a look at what the progression looks like:

Difficulty vs Stress curve

We can add a trend line to the graph to try to fit a formula to the data. In Google Sheets, this can be found under:

Chart editor > Customise > Series (Difficulty) > Trend Line

I used these settings, and got a pretty decent fit with a polynomial of degree two (a quadratic):

Trend line settings

We could use exactly the equation that the trend line found for us, but it has a dip in the middle which isn't ideal. So let's see if we can make our own version using that as inspiration.

Custom formula

Here I tried squaring the stress value, and dividing by 10000 (bringing a maximum of 100 * 100 stress down to an output value of 1). Then I multiplied the result by 49 and added 1, to get an output between 1 and 50.

That gets pretty close to your numbers, but it over-estimates a little in the middle. Let's try cubing stress instead. Here I'll normalize stress between 0 and 1 by dividing by 100, then cube this normalized value, multiply by 49 and add 1 as before. And then let's take the ceiling (next integer higher than the number we get out). That reproduces your example values perfectly.

Better formula

Moreso than any particular formula though, I hope this demonstrates for you a method that you can use to explore and find formulas on your own.

  • \$\begingroup\$ Geez, that's a good trick! I wonder - is it possible to create a formula from a graph? Asking because our game has lots of formulas and for non-mathematicians, it's quite a nightmare lol. \$\endgroup\$ 2 days ago
  • \$\begingroup\$ That's effectively what the trendline technique I showed above does. \$\endgroup\$
    – DMGregory
    2 days ago
  • \$\begingroup\$ Wait. I guess I missed that part. I see you have =ROUND((B3*B3)*49/10000+1), however I don't know how that formula showed up there - I guess you wrote it manually, right? \$\endgroup\$ 2 days ago
  • \$\begingroup\$ I did. I explained my thinking in the answer - the trendline showed that a quadratic could be a good fit for the data, so I tried squaring the stress, then scaling and shifting it into the range 1-50, and rounding the result. \$\endgroup\$
    – DMGregory
    2 days ago
  • \$\begingroup\$ Oh, I see. Yes, I mean, I guess my question was more like if there was a way to automatically extract the formula for the quadratic graph somehow - not exactly by designing the formula and the numbers myself. But anyways, your approach enlightened my day. Thank you so much! \$\endgroup\$ 2 days ago

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