On 10/05/2010 01:20 AM, David Gowers (kampu) wrote:

> In that case it really is a bug. Neither the input colors or output
> colors contain 255.
> The output color series is:
> 008700 008d00 009300 009800 009d00 00a200 00a700 00ab00 00af00 00b300 00b600 
> ...

But the outputs contain 0's. And that's not because they happened to 
come out to exactly 0, but actually they should be <0 and thus are clipped.

So if expected clipping occurs, it's either to 0 or to 255. If no 
component of the output is 0 or 255 then it wasn't clipped.

That said, the current algorithm has a limited precision. About 1.3% of 
possible RGB values will be off by one after a RGB-Lab-RGB conversion, 
0.003% will be off by two (bright cyans are worst). LCH conversion is 
slightly worse.

So far I haven't found a way to get perfect round-trips without being 
much slower.

I currently have a floating point version which is at least 4 times 
slower. It gives perfect Lab roundtrips, and almost-perfect for LCH (off 
by one for 0.00006% of RGB values). It still needs quite a bit of work, 
but I am hoping to turn that into a viable babl candidate.


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