Mapping to OKLab and back significantly changes the color.

[gcr]

hii!

i'm struggling to use this library for some image processing and palette mapping tasks. I've found that transforming to oklab and back significantly brightens and desaturates colors.

import chroma
echo parseHex("F8DB57").to(ColorOklab).to(ColorRGB).color.toHex()
# output:      FCF4BA

Color #F8DB57 turns into #FCF4BA.

Here's some assorted roundtrips:

import chroma
import random
import strformat

proc chip(c: ColorRGB): string =
    ## nice-looking color chip for github-flavored markdown
    let hex = c.color.toHex()
    fmt"`#{hex}` <img valign='middle' alt='blue' src='https://readme-swatches.vercel.app/{hex}'/>"

randomize()
for _ in 1..10:
    let color = ColorRGB(
        r: random.rand(255).uint8,
        g: random.rand(255).uint8,
        b: random.rand(255).uint8,
    )
    let color2 = color.to(ColorOklab).to(ColorRGB)
    echo fmt"{color.chip} -> {color2.chip}"

Output:

#FF1580 -> #1A89CC
#B543B5 -> #E9A8E6
#DBCA21 -> #F1EF99
#28B837 -> #9CE9A0
#E254E2 -> #FBB5F8
#5103BC -> #B06BF8
#3F51B2 -> #A0B0E7
#DB26C5 -> #0196EF
#B44D3E -> #EAAEA1
#803A22 -> #D09E86

Each pair of colors should be identical (before converting to OkLab and after). But for many of these pairs, even the hue is wildly off.

[gcr]

Nevermind - this is fixed at HEAD. I resolved this in my project temporarily by cloning this repository and running nimble install from the root folder.

Once #42 is fixed, this will be too.

Roundtrips at HEAD look good:

#42B5DE -> #42B5DE
#F85C71 -> #F85C71
#249550 -> #249550
#B75EE3 -> #B75EE3
#FBE114 -> #FBE114
#1D31AC -> #1D31AC
#707F97 -> #707F97
#B05B48 -> #B05B48
#97BD88 -> #97BD88
#6C5BD9 -> #6C5BD9

[charles-esterbrook]

You can point nimble to a commit, such as the most recent:

# foo.nimble
# ...
requires "chroma#c19d5d9545164aa1e29c5d8ed82e1dec500b719a"

HTH