Re: Photoshop Lab values
Re: Photoshop Lab values
- Subject: Re: Photoshop Lab values
- From: Marco Ugolini <email@hidden>
- Date: Mon, 9 Jul 2007 11:18:41 -0700 (GMT-07:00)
The way I see it (and I realize it's not a very scientific way to put it -- my apologies to Graeme), the blue primary in ProPhoto RGB is *outside* CIELAB for reasons of pure and simple mathematics. This causes that end of the ProPhoto RGB gamut actually to get *clipped* when its colors are represented in CIELAB.
You can see that clipping very clearly in ColorThink, where the blue end of the profile's gamut looks like it has hit a wall and got flattened by the impact.
That seems to cause the RGB blue value at the extreme of the numerical scale (R 0, G 0, B 255) to sort of "fall to the floor", and return a nonsensical L* value near 0 for a color whose appearance is actually quite bright and saturated.
I'm not sure how the math is done and why a color with a value of L* 0 doesn't look pitch black, as it should in theory. I still haven't figured that one out.
Marco Ugolini
-----Original Message-----
>From: "Fleisher, Ken" <email@hidden>
>Sent: Jul 9, 2007 7:17 AM
>To: colorsync user <email@hidden>
>Subject: Re: Photoshop Lab values
>
>Okay, thank you Thomas and Graeme for the explanation. I was aware of the
>non-real colors possible in Lab in ProPhoto, but I didn't realize that the
>blue colorant has such a low L* value. I would have expected that the full
>range of RGB would have been mapped into real Lab colors. Interesting...
>
>Thanks again.
>
>Ken
>
>
>On 7/9/07 9:39 AM, "Graeme Gill" <email@hidden> wrote:
>
>> Fleisher, Ken wrote:
>>> Hello. I have encountered a strange result that I don't understand.
>>>
>>> 1) Create an RGB document in Photoshop in ProPhoto RGB color space.
>>> 2) Fill some blocks with solid colors where R=G=0
>>> and B = [0, 17, 34, 51, ..., 255]
>>> 3) Read Lab values with the color picker (single pixel)
>>
>>> What confuses me is that L* has a value of zero for all color patches.
>>> Shouldn't L* increase as B increases? It doesn't seem logical that an RGB of
>>> 0, 0, 255 would have an L* of zero.
>>
>>> What am I missing? Any thoughts?
>>
>> ProPhoto has an extreme Blue colorant value. The blue colorant in fact
>> has an almost zero Y value, hence pure blues have an almost zero
>> L* value. In this respect, since ProPhoto has a non-real world
>> blue colorant value, the colorspace itself shouldn't be treated
>> as a real world space anymore than XYZ or L*a*b* are.
>>
>> [The blue colorant has a D50 L*a*b* value of about 0.08 90.25 -172.3].
>>
>> Graeme Gill.
>
>--
>Ken Fleisher
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