bug#54514: incorrect arithmetic in color-lighten-hsl?

[Noah Friedman]

(Any branch or release; this function in color.el looks to be unchanged 
since it was originally introduced)

I might be mistaken, but I think color-lighten-hsl is wrong.  It's
increasing luminance by an absolute percent of 1, not the present
luminance.


Suppose you want to lighten the color "red":

        (color-name-to-rgb "red")
        => (1.0 0.0 0.0)

        (color-rgb-to-hsl 1.0 0.0 0.0)
        => (0.0 1.0 0.5)

If you try to lighten this by, say, 50%, currently the result is

        (color-lighten-hsl 0.0 1.0 0.5  50)
        => (0.0 1.0 1.0)

Which is clearly wrong.  50% lighter red is not white!
The result I would have expected to see is 

        => (0.0 1.0 0.75)


Currently, the function reads:

        (defun color-lighten-hsl (H S L percent)
          ...
          (list H S (color-clamp (+ L (/ percent 100.0)))))

I think it should be:

        (defun color-lighten-hsl (H S L percent)
          ...
          (list H S (color-clamp (+ L (* L (/ percent 100.0))))))


Could someone sanity check me?  My color theory background is pretty weak.