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[css-color-3] HSL examples show incorrect RGB values #3088

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avh4 opened this issue Sep 9, 2018 · 1 comment

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commented Sep 9, 2018

Section 4.2.4. HSL color values provides an algorithm for translating HSL to RGB, and states that the tables in section 4.2.4.1. HSL examples are generated using this algorithm.

The algorithm to translate HSL to RGB is simple (here expressed in ABC [ABC] which was used to generate the tables.)

However, the RGB values in the example tables do not match the output of the algorithm.


For example, in the "0° Reds" example table, Saturation=100% and Lightness=88 is shown to have the RGB value #FFBFBF.

However, running the following ABC program (the algorithm from the spec plus the additional line WRITE hsl.to.rgb(0/360, 1.00, 0.88) produces the output (1.00, 0.76, 0.76), which converts to the [0,255] range as (255.0, 193.8, 193.8), which is the hex RGB value #FFC2C2 (if rounded) (or #FFC1C1, if floored).

ABC program
HOW TO RETURN hsl.to.rgb(h, s, l):
    SELECT:
        l<=0.5: PUT l*(s+1) IN m2
        ELSE: PUT l+s-l*s IN m2
    PUT l*2-m2 IN m1
    PUT hue.to.rgb(m1, m2, h+1/3) IN r
    PUT hue.to.rgb(m1, m2, h    ) IN g
    PUT hue.to.rgb(m1, m2, h-1/3) IN b
    RETURN (r, g, b)

HOW TO RETURN hue.to.rgb(m1, m2, h):
    IF h<0: PUT h+1 IN h
    IF h>1: PUT h-1 IN h
    IF h*6<1: RETURN m1+(m2-m1)*h*6
    IF h*2<1: RETURN m2
    IF h*3<2: RETURN m1+(m2-m1)*(2/3-h)*6
    RETURN m1

WRITE hsl.to.rgb(0/360, 1.00, 0.88)

Of the 540 examples, 251 of them appear not to match the reference algorithm.


I'm assuming the given algorithm is meant to be considered authoritative and the example tables are not--is that a correct assumption?

If so, can the example tables be fixed to have correct RGB values? Or alternatively, at least add a note that the example tables only provide "close" and not accurate values for the purposes of illustration?

@avh4 avh4 referenced this issue Sep 9, 2018

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HSL conversions #8

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@svgeesus svgeesus self-assigned this Sep 10, 2018

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commented Sep 10, 2018

How very odd, especially as the tables are supposed to be direct from the program.

You are correct about the hsl(0, 100%, 88%) testcase which should indeed give #FFC2C2

Thanks for the bug report, will check further and issue an erratum.

Meanwhile the algorithm is authoritative over the example tables (which should still be corrected).

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