iOS Find Color at Point Between Two Colors

Question

I have a problem: I need to be able to take two colors and make a 'virtual gradient' out of them. I then need to be able to find the color at any point on this line. My current approach is this: ``` if (fahrenheit < kBottomThreshold) { return [UIColor colorWithRed:kBottomR/255.0f green:kBottomG/255.0f blue:kBottomB/255.0f alpha:1]; } if (fahrenheit > kTopThreshold) { return [UIColor colorWithRed:kTopR/255.0f green:kTopG/255.0f blue:kTopB/255.0f alpha:1]; } double rDiff = kTopR - kBottomR; double gDiff = kTopG - kBottomG; double bDiff = kTopB - kBottomB; double tempDiff = kTopThreshold - kBottomThreshold; double rValue; double gValue; double bValue; rValue = kBottomR + ((rDiff/tempDiff) * fahrenheit); gValue = kBottomG + ((gDiff/tempDiff) * fahrenheit); bValue = kBottomB + ((bDiff/tempDiff) * fahrenheit); return [UIColor colorWithRed:rValue/255.0f green:gValue/255.0f blue:bValue/255.0f alpha:1]; ``` Variables: - `fahrenheit` is a variable passed into my function that is the number on t…

Accepted Answer

The problem is that you're not subtracting `kBottomThreshold` from `farenheit`.

But let's simplify.

First, we want to map the input temperature to a parameter `t` in the range [0 ... 1]. Then, we want to map `t` to an output in the range [`kBottomR` ... `kTopR`], and also to an output in the range [`kBottomG` ... `kTopG`], and also to an output in the range [`kBottomB` ... `kTopB`].

```
UIColor *colorForDegreesFahrenheit(double fahrenheit) {
    double t = (fahrenheit - kBottomThreshold) / (kTopThreshold - kBottomThreshold);

// Clamp t to the range [0 ... 1].
    t = MAX(0.0, MIN(t, 1.0));

double r = kBottomR + t * (kTopR - kBottomR);
    double g = kBottomG + t * (kTopG - kBottomG);
    double b = kBottomB + t * (kTopB - kBottomB);

return [UIColor colorWithRed:r/255 green:g/255 blue:b/255 alpha:1];
}
```

iOS Find Color at Point Between Two Colors

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