POV-Ray : Newsgroups : povray.general : strange problem with srgb color in light_source : Re: strange problem with srgb color in light_source Server Time
21 Jun 2021 13:13:33 EDT (-0400)
  Re: strange problem with srgb color in light_source  
From: Kenneth
Date: 7 Apr 2021 07:30:00
Message: <web.606d93daa9b7c959d98418916e066e29@news.povray.org>
Ive <ive### [at] lilysoftorg> wrote:
> Am 4/6/2021 um 17:48 schrieb Kenneth:
> > When picking a color visually in a gamma 2.2 color-picker (like Photoshop),
> > the color appears a certain way there. Let's say I pick a shade of green.
> > The actual values in PS are in the 0-255 range, like <104,230,75>/255.
> > Although the *visual* color I see there is a gamma-bent version of the
> > vector values, the values themselves are linear-- at least, that is what
> > I have always understood them to be.
> And what on earth makes you think so? You can almost always by certain
> that values in byte range (or hexadecimal values) are gamma encoded and
> most probably meant to be sRGB values.
> And the visual color you see is the result of the internal processing of
> P'shop and this in itself depends on the way you did setup the CMS of
> P'shop.

Well! Now we're getting somewhere. There *is* a major misunderstanding, although
I'm not yet sure where. Here's my own breakdown and logic:

I run my old version of Photoshop in it's gamma 2.2 'working gamma space', as
most people do (unless newer versions run in 'srgb'-- but that's a minor

When I want to specify "50% gray" there, I use r-g-b values of <128,128,128> in
the 0-255 range. And the HSB indicators there give B or 'brightness' as 50.  I
have always understood the  visual result there (under 2.2 gamma) to be the
*perceptual* version of 50% gray-- what we would 'expect' to see as pleasing
half black/half white.

But Photoshop allows me to change the working gamma space to 1.0-- in which case
the 50% gray now looks too bright 'perceptually.' However, the <128,128,128>
values there have NOT changed, nor has the B of 50. In effect, all I have done
is to change the working gamma space that the 50%-gray appears in-- the values
themselves are 'what they are.' At least in Photoshop.

Now, in  POV-ray, working in it's assumed_gamma 1.0 universe, I bring in
<128,128,128>/255 and use it as simple RGB, at 'ambient 1.0'-- as a raw 'linear'
color, in other words. (That's what I currently undertstand 'rgb' to mean.) The
visual result is the same as in Photoshop in it's *gamma 1.0 environment*. So it
seems logical to me that the initial <128,128,128> values in PS are 'linear'
values, not sRGB values-- because using them as rgb in POV-ray (divided by 255)
reproduces the identical visual result  in BOTH  gamma-1.0 environments... the
'linear' environments.

Yet it seems that my entire conception is completely wrong-- that PS's
<128,128,128> is *already* SRGB <128,128,128>/255.

So going into in your IC app, and using the OPTIONS/BACKGROUND feature, I see
the r-g-b sliders at '0.5' (by default.) I take that to mean "50% gray" in
whatever sense or gamma environment is meant there...maybe gamma 1.0? So I
compare the appearance of IC's generated 0.5 gray background with PS's 'gamma
1.0' appearance of <128,128,128>, and with POV-ray's   rgb <128,128,128>/255 at
assumed_gamma 1.0. All three *look* identical to me, in their own environments.

Yet IC reports its own '0.5' gray as being  'sRGB 187'. Not 128, as in
Photoshop.  A calculation shows that 187/255  is approximately 128/255 to the
power of 1/2.2. (I'm aware that  2.2 should actually be replaced with the more
complex 'srgb' math equation.) It's not clear to me yet what the discrepancy is
as to why Photoshop reports 50% gray as <128,128,128>-- and *if* indeed those
are supposed to be 'srgb' values rather than 'linear' like I thought--  while
IC reports <187,187,187> instead. And I would guess that other color-picker apps
adhere to PS's way of representing the values.

> At some point even I will loose my patience, but it is not yet reached...

"Patience is a virtue." (so said Confucius... or someone...)  ;-)

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