C - function to return curved result

Question

I have a general programming challenge for you all. I've been scratching my head trying to figure out the best way to do this...

I'm working in C and I've got a "speed" value that is passed to my program. It ranges from 0-255. This speed value needs to correspond to a millisecond time delay built into the program.

    unsigned char currentSpeed; //this is passed into my function, range 0-255
    unsigned long SlowestSpeedMillis = 3000; //in milliseconds
    if (currentSpeed > 0)
    {
       unsigned long Interval = ((256 - currentSpeed)*SlowestSpeedMillis)/255;

    }else{ //currentSpeed = 0
       //When Interval is zero, the program will freeze
       Interval = 0;
    }

Here's some sample results:
currentSpeed of 1 results in 3000 milliseconds (slowest)
currentSpeed of 128 results in ~1500 milliseconds (exactly half)
currentSpeed of 255 results in ~11 milliseconds (fastest)

The problem is that I would like it to be a curved result where it stays in the lower numbers and then quickly goes to 3000 at the end... I want the user to have the choice to make the program go as slow as 3000, but most values leading up to it aren't as important. Such as this:
currentSpeed of 1 results in 3000 milliseconds (slowest)
currentSpeed of 128 results in ~750 milliseconds (1/4th of potential)
currentSpeed of 255 results in ~11 milliseconds (fastest, anything close to zero)

Any ideas how to do this programmatically? Perhaps with some kind of equation?

There are many equations that start slow and grow quickly, such as Exponential, Quadratic, Tangent.... Since you don't seem picky on the details, pick one and use it until it *isn't* appropriate anymore. — abelenky, Oct 18 '13 at 16:00
Why not just use a lookup table of 256 precalculated values? Then you can use whatever function you like to generate them. Storing `currentSpeed` as an unsigned char would make your function more stable. — r3mainer, Oct 18 '13 at 16:06

score 3 · Accepted Answer · answered Oct 18 '13 at 16:07

3

Normally in math you might do something like:

(currentSpeed / 255) * 3000

for linear, and to get a bit of a curve use a power:

((currentSpeed / 255) * (currentSpeed / 255)) * 3000

But, in integer math that doesn't work because (currentSpeed / 255) will almost always be zero.

To do this without floating point you have to scale up first before division.

((3000 * currentSpeed) * currentSpeed) / (255 * 255)

answered Oct 18 '13 at 16:07

joeking

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18
29

This is almost perfect... except I need the inverse of it so that when currentspeed is 255, the result is almost 0. What do you think? `currentSpeed of 1 results in 3000 milliseconds (slowest) currentSpeed of 128 results in ~750 milliseconds (1/4th of potential) currentSpeed of 255 results in ~11 milliseconds (fastest, anything close to zero)` – sscode Oct 18 '13 at 17:09
Just invert it then ... "int x = 255 - currentSpeed", then use "x" in the equation instead of currentSpeed. You can scale currentSpeed a litte like "x = RANGE*(255-currentSpeed)+N" to make it invert range from N to RANGE+N. You can also adjust "3000" a bit, like "((3000-11)*x)*x/(255*255)+11" - this gives your desired result of 11 to 3000 milliseconds. – joeking Oct 18 '13 at 23:21

damienfrancois · Answer 2 · 2013-10-18T21:34:32.223

0

You can use

unsigned char currentSpeed; // range 0-255
unsigned long SlowestSpeedMillis = 3000; // in milliseconds
unsigned long Interval;
if (currentSpeed > 0)
{
   Interval = SlowestSpeedMillis/currentSpeed;

} else {
   Interval = 0;
}

1 is mapped to 3000, and 255 is mapped to 11.

edited Oct 18 '13 at 21:34

answered Oct 18 '13 at 16:28

damienfrancois

52,978
9
96
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C - function to return curved result

2 Answers2