An encoder to represent spectral data for the Web

SPD means "spectral power distribution", and it's commonly used to record data about a light source or reflectance of a material.

This library aims to be the equivalent of "web colors" for spectra. You might write "red" as #FF0000, but how would it look with 40 components? It turns out we can make SPDs relatively small too, so this library provides a simple way to compress and decode these values.

Here's how we compress the spectrum recorded from an x-rite meter from 380-730nm, using 10nm spacing:

var spdurl = require("spdurl");

// make an SPD and encode/decode roundtrip:
var spd = spdurl.SPD();

spd.base = 380;
spd.delta = 10;
spd.data = [1.039693,1.039379,1.039198,1.564554,1.696937,1.583408,1.778512,1.970525,1.930359,1.800388,1.725509,1.659314,1.651000,1.587592,1.506774,1.541956,1.536947,1.536743,1.488346,1.409579,1.326508,1.219819,1.163692,1.117009,1.068008,1.037550,1.015638,0.944459,0.889883,0.855271,0.801936,0.759832,0.904105,1.111251,1.211360,1.453921];

var enc = spdurl.encodeSPD(spd);
console.log(enc);

The result is 90 bytes:

spd1,380,10,uwi,4,uJuIuI4m68488W_h-38t7c6S6J5A3i4M4G4G3N1u0Hx-w0v0uwuFtmr-qsp2ohncrBvsxz2j

Decoding this string is easy, giving the original data back:

var spdagain = spdurl.decodeSPD(enc);

Metadata is optional, so here is a date, location, and a name (now 132 bytes):

spd.date = Math.floor(new Date().valueOf() / 1000);
spd.loc = [134,-118.5];
spd.name = "i1Studio Sample";

spd1,380,10,uwi,4,uJuIuI4m68488W_h-38t7c6S6J5A3i4M4G4G3N1u0Hx-w0v0uwuFtmr-qsp2ohncrBvsxz2j,d1601573778,ni1Studio%20Sample,l34:-118.5

• Value encoding:
  • Stored with 12 bits of precision (they use two bytes once base64 encoded)
  • Gamma-encoded to enhance the precision of smaller values
  • Use a shared exponent to set the scale
• Samples must be uniformly spaced by wavelength, but any wavelength range is allowed (e.g., visible light is 380-780nm)
• Web-safe base64 is used (RFC 4648)
• No limits on size are enforced, however some browsers will discard more than 2KB.

• Required
  • The "kind" of SPD must be indicated using a dictionary of types (the default is "uW/cm^2" and is abbreviated "uwi")
  • A version number for future revisions
• Optional metadata:
  • Name of sample
  • Date (in UTC unix time)
  • Location (lat, long)

Compared with sophisticated compression algorithms (e.g., zstd), this library produces encodings that can be half the size.

For instance, given 401 values from an LED (380-780nm):

• Compressing 401 values with zstd: 2120 bytes (base64 encoded), which doesn't work in all browsers
• This method: 804 bytes, which is a valid URL

This library is provided for Javascript and node.js, but we are happy to take contributions for other languages as well.