[Pixman] [cairo] RFC: More accurate color conversion

James Cloos cloos at jhcloos.com
Tue Oct 8 13:05:26 PDT 2013

>>>>> "CW" == Carl Worth <cworth at cworth.org> writes:

CW> But then you simply assert that the conversion from integer to
CW> floating-point must be f(i) = i/65535.0. What's the justification for
CW> this?

When I looked into this many moons ago, I concluded that the best option
is to multiply (with saturation) by IMAX+1 and divide by IMAX.  Ie, what
cairo currently does.

Dividing by IMAX generates an infinitely repeating value which best
places the integer value w/in the interval which rounds to said integer.

This is akin to mapping {0,...,9} to {0/9,...,9/9} generating the
decimal expansions 0, .111..., .222..., .333,... et cetera.

It is widely accepted that when expanding an 8bit value to 16 bit, one
should duplicate the bits, eg by multiplying by 257.  Dividing by IMAX
generates a floating point value which accomplishes that same goal.

James Cloos <cloos at jhcloos.com>         OpenPGP: 1024D/ED7DAEA6

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