Bit depth vs gamut

[gordo]

Could some one explain this in lay person's terms?

Assume we're editing images in PShop.

The ProPhoto profile has a larger gamut (more colors) than the Adobe 1998 RGB profile.

But you're editing the image at a bit depth of 8 bits per channel - i.e. 24 bit RGB.

How does gamut relate to bit depth?

thx, gordo

[mattbeals]

At the risk of sounding like a complete idiot I always thought of it this way; Gamut describes the range of colors and bit depth referred to the amount of information (but not resolution).

[gordo]

Does this analogy work?

Gamut is like a balloon - it has volume. You can blow more air (color) into the balloon and it gets larger (bigger gamut).
Bit depth is like drawing circles around the balloon's surface. For 8 bit that would be 256 circles. As you inflate the balloon it gets larger so the distance between the circles gets larger - but there's still only 256 circles.
Because the spaces between the circles is larger there is more chance of seeing the steps - i.e. shadestepping.

Something like this:



best, gordo

[mattbeals]

yeah, you're right. that actually makes sense.

[Lukas Engqvist]

Loveley analogy

I have used a picture to explain different profiles the analogy of tents. A family tent and a hiking tent. In the hiking tent i can't stand up…*but it's light and i can take it anywhere, this is like the sRGB. AdobeRGB is like a family tent, I can stand up and move freely, there is more space. (Guessing ProPhoto is a circus tent here ;P )
I use this picture to visualise that if you use the wrong profile then it will be like mixing the tent poles. Using an Adobe profile on an sRGB will make it "break at the seams", and using an sRGB on an AdobeRGB immage will make it sagg/drab. (I'm now thinking circus tent on a hiking tents poles)

Now this analogy does not explain what gordo does so nicely with the balloon…*and I probably will borow that analogy in the future.

[TerryWyse]

Good analogy Gordo.....although I've never liked the smell of balloons.....and don't even get me started on clowns.

"bit depth" has nothing whatsoever to do with either gamut or dynamic range....it only has to do with the number of steps used to encode a given color gamut (or image really as most RGB color spaces/profiles are already 16bits...it's the image bit depth we're talking about here)

Your analogy shows why bit depth DOES matter though when it comes to gamut volume and image editing. In the small balloon (sRGB?), 8 bits-per-channel (bpc) might be perfectly adequate to use as an editing space since the steps are relatively close together and are less likely to result in banding/posterization as these steps are "pulled apart" as a result of a curve edit or other adjustment.

However with a space/gamut volume as large as ProPhotoRGB, 8bpc would reveal banding pretty quickly since the steps are farther apart than something like sRGB or even AdobeRGB.....the reason many professionals in the photography world (where ProPhoto is more prevelant) would recommend working with 16bpc images when using ProPhotoRGB as the color space "container" or "balloon".

So...the larger the gamut volume/color space, the more bit depth you'd need to avoid visible stepping/banding in an image.


Music recording/playback is a good analogy as well.....the higher the sampling rate (bit depth) the smoother the recorded sound is upon playback.....as you start dropping the sampling rate, the music will start sounding "grainy" or "noisy" (banding/stepping in images) because of the "gaps" left in the music from the too-low sampling rate. As in music as it is in images, the degree to which this becomes noticeable is largely a function of the source material. Rock-and-roll music or other forms that may apply a degree of distortion would likely tolerate lower bit depths/sampling rates than something with pure tones and a wider dynamic range such as classical music.

Regards,
Terry

[TerryWyse]

Does this analogy work?

Gamut is like a balloon - it has volume. You can blow more air (color) into the balloon and it gets larger (bigger gamut).

Hey gordo, how about a squeezebox/accordion as an analogy?

Your accordion has 256 pleats in it (8 bits-per-channel)......when it's closed/small, you barely notice the pleats in the bellows.....but open it up to get a BIG sound (larger gamut), all of a sudden one can see all the pleats in the bellows. If you had 16,384 pleats in the bellows (16 bits-per-channel), you probably wouldn't notice them as much and you could concentrate on the monkey holding the tin cup.

:-)

Terry

[meddington]

Your accordion has 256 pleats in it (8 bits-per-channel)......when it's closed/small, you barely notice the pleats in the bellows.....but open it up to get a BIG sound (larger gamut), all of a sudden one can see all the pleats in the bellows. If you had 16,384 pleats in the bellows (16 bits-per-channel), you probably wouldn't notice them as much and you could concentrate on the monkey holding the tin cup.

That's just sheer genius Terry, though I doubt very much Dan Margulis would see the value in a 16-bit accordion. You'd be making sounds that nobody could hear.

[gordo]

That's just sheer genius Terry, though I doubt very much Dan Margulis would see the value in a 16-bit accordion. You'd be making sounds that nobody could hear.

If it was a polka - that might be a good thing LOL!

Thanks all for elaborating on this topic.

best, gordo

[TerryWyse]

That's just sheer genius Terry, though I doubt very much Dan Margulis would see the value in a 16-bit accordion. You'd be making sounds that nobody could hear.

Perfect Mike....you haven't lost YOUR genius. Just remember, as a "genius" you're only one "false profile" away from madness.

;-)

tw