Problems with coordinate ranges when doing conversion from LCH to LAB to RGB

Matteo matteo.niccoli at
Mon Nov 16 10:03:30 EST 2015

I moved the notebook with the color conversion tests from RGB to LAB to LCH 
then back to LAB and to RGB to a stable location 
on GitHub.

On Friday, October 3, 2014 at 8:13:54 AM UTC-6, Matteo wrote:
> Hi Stefan (and Juan)
> I run this test last night. Nothing fancy, essentially I created a 16x16x3 
> RGB azure image (RGB 0,153,255 or 0,0.6,1) and took it for a walk from RGB 
> to LAB to LCH then back to LAB and RGB again.
> Feel free to use this if you like as an example of color conversion. I 
> will be adding it to my GitHub anyway
> As you can see the loop of transformations closes precisely.
> When I tried with pure red (RGB 1,0,0 or 255,0,0) the final RGB values are 
> all e-16 numbers, some negative. Not sure if that qualifies as deficiencies.
> Certainly it points to me to the need to include documenntation on the 
> coordinate ranges as Juan observed.
> From this I conclude that:
> r,g, and b are in the range (0 1)
> L is in the range (0 100) as Juan pointed out (already evident from my 
> color evaluation notebook), however
> a and b must be in the range (-100 100) since a is small but positive and 
> b is large but negative (as expected) in my example
> chrima c must be in the range (0 100) because it is the distance from the 
> polar axis so it can't be negative
> h is in the range (0 2pi) as specified already in the documentation
> I hope this is useful. I'l lbe checking my original example with the new 
> ranges tonigth.
> Cheers
> Matteo
> On Thursday, October 2, 2014 4:02:54 PM UTC-6, Stefan van der Walt wrote:
>> Hi Matteo 
>> On 2014-10-01 20:56:09, Matteo <matteo.... at> wrote: 
>> > A good test would be to convert a single  colour, say red, from RGB to 
>> LAB, 
>> > to LCH, then back to LAB and RGB and check the values at each step. 
>> I'll 
>> > try tomorrow and post my results back for your information. 
>> Did you have any luck with that?  Also, if you come up with any good 
>> test cases that point out deficiencies in the code, we'd be happy to 
>> include them in the test suite. 
>> Thanks for the link to the article--it's very enjoyable to learn more 
>> about color map perception in such a vividly illustrated way. 
>> Regards 
>> Stéfan 
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