Re: 15bit + 1 convention. Here is one explanation:
If an 8-bit channel consists of 256 levels, a 10-bit channel consists of 1,024 levels, and a 12-bit channel consists of 4,096 levels, doesn’t it follow that a 16-bit channel should consist of 65,536 levels?
Well, that’s certainly one way that a 16-bit channel could be constructed, but it’s not the way Photoshop does it. Photoshop’s implementation of 16 bits per channel uses 32,769 levels, from 0 (black) to 32,768 (white). One advantage of this approach is that it provides an unambiguous midpoint between white and black (useful in imaging operations such as blending modes) that a channel comprising 65,536 levels lacks.
To those who would claim that Photoshop’s 16-bit color is really more like 15-bit color, we simply point out that it takes 16 bits to represent, and by the time capture devices that can actually capture more than 32,769 levels are at all common, we’ll all have moved on to 32-bit floating point channels rather than 16-bit integer ones.
from:
http://www.peachpit.com/articles/article.aspx?p=1709190&seqNum=2and this:
Photoshop's internel representation is 0..32768. This gives a midpoint to the range (very useful for blending), and allows for faster math because we can use bit shifts instead of divides.
That is not a bug, just a design decision to make 16 bit/channel run faster.
A search of the forums will reveal more information about that decision.
from:
https://forums.adobe.com/message/3472269Fair enough.
For giggles, try this experiment.
Open a raw image in your favorite converter and render it to a 16bit image in Photoshop.
Do the following:
1) Add a Levels adjustment layer. Pull the Output White Point (the white slider under the gradient, not the white slider under the histogram in the Levels dialog) from 255 to 1 (or type "1" in the right Output field that originally says 255). This will make the image black.
2) Flatten the layer stack (gulp! look at that histogram!!!).
3) Add a Levels adjustment layer. Pull the white point (not the Output white point) from 255 to 2 (it will only go to 2).
4) Because you can only drag the white point slider to "2", add another Levels adjustment layer above the first levels layer and pull that white point to 128.
5) Stamp or flatten the layer stack. Refresh the histogram if need be.
Voila! Take a look at the resulting histogram - there are gaps throughout the histogram, but the resulting image looks pretty good compared to the original. What went on in this exercise?
Now, try the same experiment only, after step two, change the document mode from 16bit to 8bit. Then complete steps 3 - 5.
Why is there is difference?
Try the above exercise again by making a blank 16bit document and filing it with a pure white bar, a 50% gray bar and a pure black bar. Also, you can try a black to white gradient. Once you finish step 2 above, use the color sampler and look at the Info palette with the RGB color readout in 8bit mode and then in 16bit mode.
kirk
