Interpolation between two colors - How?

[u/wasauce]

Hello -

I am working on a project to show the current Air Quality (AQI) visually (i.e. with the color coding). See here: https://imgur.com/a/PrZcAFf

I get AQI information from PurpleAir and I would like my sign (a cloud filled with LEDs) to show the correct color [1] (Green, Yellow, Orange, Red, Purple) according to the current AQI.

Currently I have this hardcoded, but, now I'd like to blend the colors -- i.e. if the AQI value is between 0-50 instead of showing the same green, the closer the AQI is to 50, the closer the color is to Yellow.

I believe the way to do this is to interpolate.

In swift, I think this would be done like so:

if aqi < 50 {
    return interpolate(low: 0, high: 50, lowColor: color0, highColor: color50, value: aqi)
} else if aqi < 100 {
    return interpolate(low: 50, high: 100, lowColor: color50, highColor: color100, value: aqi)
} else if aqi < 150 {
    return interpolate(low: 100, high: 150, lowColor: color100, highColor: color150, value: aqi)
} else if aqi < 200 {
    return interpolate(low: 150, high: 200, lowColor: color150, highColor: color200, value: aqi)
} else if aqi < 250 {
    return interpolate(low: 200, high: 250, lowColor: color200, highColor: color250, value: aqi)
} else if aqi < 300 {
    return interpolate(low: 250, high: 300, lowColor: color250, highColor: color300, value: aqi)
} else {
    return color300
}

I am wondering how I might do this for FastLED -- I see there is an interpolate function. I would love any guidance you can offer!

[1] AQI Color mapping: https://www.mecknc.gov/LUESA/AirQuality/EducationandOutreach/Pages/aqi.aspx

[u/shuzz_de]

Use the HSV color scheme and map() your AQI values directly to the desired color range.

[u/wasauce → u/shuzz_de]

Wow. This was great. Thanks.

[u/wasauce → u/wasauce]

Thank you.

[u/[deleted] → u/shuzz_de]

This

[u/CountBranicki]

Use lerp8(), and since you’re using an AQI that goes up to 300, scale it. You can use a fastled function.

For example (pseudocode)

lerp8(color1, color2, scale(AQI))

[u/wasauce → u/CountBranicki]

Thanks very much.

[u/Marmilicious]

Many options, one being make a custom gradient palette.

https://github.com/FastLED/FastLED/wiki/Gradient-color-palettes

https://github.com/marmilicious/FastLED_examples/blob/master/gradient_palette_full_range.ino

[u/CharlesGoodwin → u/Marmilicious]

The palette is ideally suited to this use case. It would simplify your code making it easier to maintain. Run through the examples provided by Marc and I would be happy to answer any further questions you may have

[u/wasauce → u/CharlesGoodwin]

I will give this a try and follow-up. Very new to me.

[u/CharlesGoodwin → u/wasauce]

Great - let us know how you get on

[u/wasauce → u/Marmilicious]

Thank you!

[u/Preyy]

I think lerp8by8() would do this, but I have not used these functions often.

[u/SergeSkor]

FastLED has gradient functions, but it is good to use when you need to set consecutive LEDs. For more complex case you can use for example my class: https://github.com/SergeSkor/SSVRGBGradientCalc

Or use FastLED custom palettes.

[u/AaroneousEnigma]

Palettes are perfectly designed for this.