Photoshop's 50% gray

[mcdronkz]

I've got a few questions about Photoshop's 50% gray (128, 128, 128).

255 / 2 = 127.5. Why does Photoshop use 128 and not 127? It does not matter all that much to me but I would be interested in the reason behind this decision nonetheless.

Is 50% gray really 50% gray? It looks different using different color profiles. Is 50% gray always 128, 128, 128 because blend modes such as Overlay, Soft Light and Hard Light rely on it?

When converting to a different color profile, does Photoshop evaluate a flattened version of the image? I don't experience any shifts related to layers on Overlay/Soft Light/Hard Light blend mode. :-)

[Shoku]

Re: Photoshop's 50% gray

The scale begins with 0, not 1. so there are actually 256 shades, not 255, Black to white. Half of 256 is 128.

From wiki: No matter what pixel depth is used, the binary representations assume that 0 is black and the maximum value (255 at 8 bpp, 65,535 at 16 bpp, etc.) is white, if not otherwise noted.

[mcdronkz]

Re: Photoshop's 50% gray

D'oh! I feel stupid now.

[John Wheeler]

Re: Photoshop's 50% gray

Hi mcdronkx - Here is my take.

- You left out the most important question: Is it "gray" or is it "grey"

- In 8 bit mode 128 is used because with the computer math used 127.5 rounds to 128.

- 50% gray in PS just means the mathematical halfway point which is 128 (rounded) in 8 bit mode and 16384 in 16 bit mode (rounded and actually 15 bit mode with PS color picker). So that absolute RGB value is not the same level of perceived gray just as the same exact RGB color numbers would look different in the different color spaces.

I suspect that the reasoning is just legacy issues from day one of PS before color profiles were included in the software.

When you change color profiles with the Convert to Profile there is a checkbox option at the bottom of the panel for "Flatten Image to Preserve Appearance." If you check the box then your Layer Stack is flattened and you have the best preservation of appearance given the limits of gamut and tone limitations between the two color spaces. If you do not check this box, you can create undesired shifts. In many cases this is subtle and in other cases it is not. You are running without seat belts on if you take an entire Layer Stack and shift between color profiles. If your question is purely academic, then hope this answer helps. If you have an actual work flow where this is important, I could provide more information on the issues.

Not sure this answers all your questions yet hope this helps.

[Shoku]

Re: Photoshop's 50% gray

Hi mcdronkx - Here is my take.


- In 8 bit mode 128 is used because with the computer math used 127.5 rounds to 128.
.

John - not so. Read my post above.

[John Wheeler]

Re: Photoshop's 50% gray

Hi Shoku - I was typing while your were posting so did not see your post.

So please help me with my confusion.

Levels "0 to 255" is the 256 levels

Levels "0 to 127" is 128 shades of gray

Levels "128 to 255" is 128 shades of gray

Level "128" has 127 shades of gray above it and 128 shades of gray below it.

Level "127" has 128 shades of gray above it and 127 shades of gray below it.

How can either level "127" or level "128" be in the middle given that neither has an equal number of grays shades above and below that level?

And yes this is an academic question but am curious.

[DianaPolitical]

Re: Photoshop's 50% gray

D'oh! I feel stupid now.

Don't worry, I used to think the same thing. My 6 year old son was the one who pointed out that the scale started at 0 to me, haha! You never know when they'll surprise you

[Shoku]

Re: Photoshop's 50% gray

And yes this is an academic question but am curious.

128 is the middle shade whether you count down to 0 or up to 255. On either side are 127 shades, which makes it the middle.

127 +1 = 128

128 +127 = 255

[John Wheeler]

Re: Photoshop's 50% gray

Hi Shoku - I was trying to understand your perspective yet we are not starting with the same basic understanding.

I agree that in 8 bit mode the RGB numbers can go from 0 to 255 (not 256) yet since we are counting level "0" those numbers represent 256 levels of gray/tone (that includes black = 0 and white =255.

Level "0" = black
Level "255" = white

Reference: http://helpx.adobe.com/photoshop/using/color-modes.html

I don't totally agree with you last post.

I do agree that the number of levels above 128 is "127 levels" to get to 255 (white)

I disagree with your other statement that there is an equivalent number of levels below level "128".

Note: If there were just 127 levels above and below level "128" you would have 2 x 127 = 254 levels plus the level for "128" making a total of 255 levels. Yet there are 256 levels form 0 to 255.

As in my previous post the number of levels below level "128" or from 0 to 127 is 128 levels.

So since we don't agree on that basic, I can understand why we don't agree on the questions around middle gray. The importance of the question was also not high so I will leave this as my last post for this thread. Thanks for the discussion though.

[Shoku]

Re: Photoshop's 50% gray

H

So since we don't agree on that basic, I can understand why we don't agree on the questions around middle gray. The importance of the question was also not high so I will leave this as my last post for this thread. Thanks for the discussion though.

Well here's an answer anyway: Shade 128 is the 128th shade of grey, the middle gray. Since half of 256 is 128 it is the exact middle shade of all levels of the grayscale, including the zero shade. That's why Photoshop has it equal to the 50% point when the curves dialog is changed from Light to Pigment, and why the dialog displays those choices: Light (0-255), Pigment /ink %.

Also, consider this just for fun: there are only 127 shades of gray in the grayscale before shade 128 because shade zero is black, which is the absence of all light, and so cannot be a shade of gray.

[A Grade]

Re: Photoshop's 50% gray

The scale does start at 0, so once you've done 256 / 2 = 128 don't you have minus the 1 making it 127 so it fits back into the 0 to 255 scale?

[klev]

Re: Photoshop's 50% gray

The scale does start at 0, so once you've done 256 / 2 = 128 don't you have minus the 1 making it 127 so it fits back into the 0 to 255 scale?

What do you mean? You have an even number of bits, so you lack a specific middle level. They have to designate one as invariant for that blending mode, and it was determined to be 128. If you used either 257 or 255 total levels, you would have a distinct middle value, not that it really matters. You lose more precision to other things.

[andrewrodney]

Re: Photoshop's 50% gray

Is 50% gray really 50% gray? It looks different using different color profiles.

Sure does. Just make a few documents in different color spaces and examine RGB values when you set Lstar to 50 (which really is middle gray). Otherwise, 128 is just a number that's halfway between the two extremes (0-255). RGB numbers don't tell us what a color looks like, they are a partial ingredient and we need an assigned color space to tell us what's what. Lab where Lstar is 50 is a different story. That does define middle gray as we would see it. John is correct:
- 50% gray in PS just means the mathematical halfway point which is 128