How to use binary search on an animation curve to find time given value?

Question

I'm using Unity's animation curves which unfortunately only have an Evaluate function that given time gives the value. I want to create a method that does the opposite of that. value gives time. I looked at this post: Unity get time by value but all the answers refer to inverting the curve which I think, and most comments agree, is cumbersome and error prone. I figured a better solution would be to use binary search. However, my current implementation sometimes fails to find the time.

For context the curve represents the velocity of a car as it accelerates. It is always monotonic increasing. So far this is what I've got:

 float GetTimeOnCurve(AnimationCurve c, float speed)
    {
        if (speed < c.keys[0].value)
        {
            return c.keys[0].time;
        }else if (speed > c.keys.Last().value)
        {
            return c.keys.Last().time;
        }

        float min = c.keys[0].time;
        float max = c.keys.Last().time;
        const float elipson = 0.001f;
        while (min <= max)
        {
            float mid = (min + max) / 2;
            if (speed >= c.Evaluate(mid) - elipson && speed <= c.Evaluate(mid) + elipson)
            {
                return mid;
            }else if (speed < c.Evaluate(mid))
            {
                max = mid - elipson;
            }
            else
            {
                min = mid + elipson;
            }
        }
        Debug.LogError("Speed not found");
        return TopSpeed;
    }

What can I do so that the time is always found?

_"sometimes fails to find the time"._ How exactly does it fail? — Abhinav Mathur, Mar 28 '22 at 05:22
What if a certain value is **never** covered by the animation curve? What if a certain value is covered **multiple** times in an animation curve? — derHugo, Mar 28 '22 at 07:55
Besides that ... `and most comments agree, is cumbersome and error prone` .. there is exactly one commenter on the most upvoted answer ^^ For sure there might be some edge cases but honestly I think the inversion is the best you can get if you don't want a plot check like you are doing — derHugo, Mar 28 '22 at 08:26

score 1 · Accepted Answer · answered Mar 28 '22 at 11:25

1

It may fail on a steep curve, you should change the condition to

speed >= c.Evaluate(mid - elipson) && speed <= c.Evaluate(mid + elipson)

answered Mar 28 '22 at 11:25

shingo

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How to use binary search on an animation curve to find time given value?

1 Answers1