Am 01.12.2010 14:00, schrieb Stephen Klebs:
> clipka<ano### [at] anonymousorg>  wrote:
>
>> Photoshop is a liar when it comes to color maths.
>>
> You've obviously never worked as a graphics artist. Eventually, everything goes
> through Photoshop at one point or another. If Photoshop is a "liar", then the
> lie is the truth. Us kids have to play with a lot of multi-colored graphics
> tools - raster and vector and tracers and converters and raytracers and scanline
> and cad, etc. - but we can't play nice together without some supervision and,
> like it or not, Momma Photoshop holds the ruler. Otherwise, we can just play

As a graphics artist, you're probably familiar with color profiles, and 
will be well aware of the fact that different devices use different 
color spaces - not only with different primaries and whitepoints, but 
also with different transfer functions. They're not doing it for fun, 
but because that's simply their "native" way of representing colors.

Similarly, there is a "native" way of representing colors in a 
raytracing engine - and that's linear /physical/ light intensity.

You're probably also aware that Momma Photoshop doesn't have any 
"native" way of representing colours: You can work with it in sRGB color 
space, Adobe color space, wide gamut color space, and a ton of others - 
each with its own definition of what pixel value 128 or "50% grey" mean.

So no - "Momma Photoshop" cannot be the ruler for POV-Ray, because it 
doesn't really have fixed rules. It just manages the laws according to 
which you decided to live. The ruler it holds is just the one you 
happened to put in its hand. (I guess there's a policy at the place you 
work, as to what color profile to use by default.)

So if it helps you, think of POV-Ray 3.7 as using its own color profile 
(though it does not /yet/ make such a sophisticated approach at colors; 
for now all it handles is gamma), and you'll realize that you need to 
feed it data adjusted to that color profile to get it to do what you 
want to.

Fortunately for you, during its long beta-phase POV-Ray 3.7 has also 
developed a set of tools to help you work with data that doesn't fit its 
"native" color profile, to save you a lot of work converting your "raw 
material" to POV-Ray's color space (and converting its output back),

For input images, there are automatic conversions in place for the most 
typical cases in the amateur world (POV-Ray 3.7 will presume sRGB for 
most file formats, and honor PNG sRGB or gAMA chunks), and if you need 
something more uncommon you can always use the "gamma" keyword.

For color literals (which are input, too), POV-Ray 3.7 has no sane 
stable and consistent choice than to require them to be specified in its 
"native" format, but will offer you the "gamma" keyword there, too, to 
make life a little easier for you.

For output images, POV-Ray 3.7 will again do what amateurs are most 
likely to expect: Output will be close to sRGB (currently still 
defaulting to a power-law gamma of 2.2 though if I'm not mistaken); 
however, the "File_Gamma" INI file option will again allow you to 
generate data that better suit your needs.

Likewise, POV-Ray 3.7 allows you to tune the preview window output to 
your display's working color profile via the "Display_Gamma" keyword.


(You may note that I'm talking about color profiles here when in reality 
it's just a small aspect of it - gamma (or, more generally, the transfer 
function). That's because in the long run I hope to be able to make 
POV-Ray fully color-space aware, to even better interface with the 
outside world.)


As for your overused gradient example, I challenge you to make the test: 
Open the image I just posted in "p.b.images" ("Gamma challenge image"), 
and tell Photoshop /not/ to convert it to whatever your working colour 
profile happens to be (you may need to change your colour settings so 
that Photoshop will ask), and then try to draw a smooth gradient.

So much for Momma's Holy Ruler. And it's not necessarily tied to an 
image: You can even use similar settings as your default working colour 
profile.

Pardon my french, but Momma Photoshop is a bitch.

Post a reply to this message