Have I misunderstood digital colors?

[Thonord]

I'm trying to understand things like saturation and vibrance, and have found that I need to improve my understanding of digital colors. (Lets stick to RGB and 8 bits)

I have just read, about Prophoto RGB, that it contains colors the human eye cannot see.

I know there are several million possible colors in RGB, and I understand that the human brain cannot distinguish more than 100 000 from each other, but I can "see" any combination of RGB values, except pure white on paper.
,
I do not understand how any color space can have a color, within 8 bit RGB that I cannot see. (except whhite on paper)

 

[MrToM]

If you look at the Prophoto profile...



...some of it does lie outside the visible range.

This would suggest, therefore, that those colours outside the visible range would indeed not be visible to the human eye.

Maybe investigating the Prophoto profile in more depth may help explain exactly what those colours out of visible range actually are.

Regards.
MrToM.

 

[Thonord]

Thanks for the comparison of color sapces. The "visible horseshoe" was most usefull, But - I still don't understand.

Lets take the left corner of the Prophoto triangle. It's Blue, R0 G0 B255. Please explain the difference between Prophoto Blue and aRGB R0, G0, B255

 

[thebestcpu]

Hi Thonard

I think I know what you are asking so I will give it a try and will use analogies in the the process. Note that this is not a totally complete and exhaustive explanation yet enough to get you closer to the explanation you seek.

Two primary pieces in having a known color is the color data (e.g. R0 G0 B255) and the color space (which is effectively a color scale).

As an analogy, if I tell you it is 72 degrees outside, those that use Fahrenheit as a temperature scale might think that is a great temperature yet if you use a Celsius temperature scale then it would be deathly hot. How hot it actually feels to the first order needs the temperature number (data) as well as the temperature scale or you truly have insufficient information on how to dress to be comfortable.

Similar with RGB numbers. If you don't have the scale (color space), then you really don't know the intended visual color with that color number. A fixed R,G,B set of numbers will be a different color in ProPhoto RGB, Adobe RGB, sRGB, on your monitor, and on your printer (just to name a few). It is the Color Management System used with Color Managed applications that keep it all straight. The Color Management System is similar to a universal translator to convert the color data numbers when you move between color scales/spaces or move the data to your monitor or to your printer to keep the color you want to represent / display / print all consistent. Every color space and piece of hardware actually speak a different dialect or color.

There is more to it then that yet I hope this gives you a start.

Now, if you want a color space that does not move around on you, it needs to be what is called a device independent color space. The one that PS supports if "Lab" There are not a multitude of versions of Lab as there are in RGB. Just the single Lab space and the numbers represent a specific color.

Visual examples could be provided if that would be helpful.

 

[Tom Mann]

I'll add one thing to what John said about the difference between the same "numbers" when used in reference to different color spaces:

Take the phrase you quoted, "some of it does lie outside the visible range..." with a grain of salt. It's probably more understandable to say that when you go into the extreme corners of these diagrams, the eye's overall sensitivity decreases, as does its sensitivity to to changes in saturation and hue (ie, everything starts looking alike). Unfortunately, this isn't quite correct, either, but is closer to the truth than the 1st statement. If I get a chance, I'll go into it in more detail later, but I'm super busy with photography work at the moment, so no promises, LOL.

All the best,

Tom M

 

[Tom Mann]

Here is another way to think about it: The human eye has a gamut just like a monitor or a printer, or a camera, or, in the old days, film. If u expose a dslr or film to light whose wavelength is just a bit longer than the camera's normal range of operation, it will still register as red, and the recorded hue and saturation will hardly change, even if u change the wavelength by a lot, assuming it's bright enough. So, someone might informally (but incorrectly) say that the camera (or eye) doesn't "see" these colors.

Of course, go far enough into the IR and it won't be a matter of distinguishing them, you won't see them at all, no matter how bright they are.

Does that help?

Tom M