Alpha Channel Capacity

GA
Posted By
George_Austin
Oct 22, 2006
Views
495
Replies
18
Status
Closed

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CC
Chris_Cox
Oct 22, 2006
You’re confusing bits/pixel and bits/channel.

32 or 24 bits/pixel is only a limitation of some file formats. And that would be 4 or 3 channels, of 8 bits/channel each.

You can have more than 3 or 4 channels in a file (see the TIFF spec.). I think the current record is about 400 channels for spectral satellite data.

The number of channels of data is completely independent of the number of bits/channel.

So, you could have 4 channels of CMYK data, plus transparency, plus 30 alpha channels (masks or spot color plates) at 16 bits/channel – making a total of 560 bits/pixel (showing why bits/pixel is really a useless number) all in one file (not even counting layer data).
GA
George_Austin
Oct 22, 2006
Chris,

Thanks so much for your response.

How’d you calculate 580 bits/pixel? I get (4+1)x8 + 30×16 = 520 ?

Then 32-bit RGB color means R8, G8, B8, opacity 8?

Then CMYK must go to 40 bits—C8, M8, Y8, B8, opacity 8?

And 30 independent Alpha channels @ 16 bits per channel can be added?

Why are 16 bits assigned to each alpha channel when masks are defined by only 100 opacity values, requiring only 7 bits?

George
B
Bernie
Oct 22, 2006
I get (4+1)x8 + 30×16 = 520 ?

(4+1)x16 +30×16 = 560 (which is what Chris mentions, not 580)
GA
George_Austin
Oct 22, 2006
Cybernetic Nomad,

Wow! You caught a double error. Serves me right for hurrying to finish a post with a friend in my driveway honking for me. I note yet another error "…C8, M8, Y8, B8…" where the "B" should be "K". Bad hair day!!

George
CC
Chris_Cox
Oct 22, 2006
32 bit RGB color means nothing.
That could be 4 channels of 8 bits each, or 32 bits/channel. But in most cases it means 4 channels, each 8 bits. The meaning of the fourth channel varies, it could be opacity/transparency or it could be used for any other data.
And, again, the total number of bits is a useless number. What defines the data is the number of channels, the bits per channel, and the meaning of each channel. (normally all channels are the same depth, but in some cases (3D apps) they create some channels with more precision than others).

Again, channel depth is independent.
The channel values are continuous, there is no "100 values" involved.
GA
George_Austin
Oct 23, 2006
Chris,

Good! All of what you have said makes sense and cuts through a lot of amgiguous terminology out there.

As for "100 levels", I was referring to the number of opacity levels to be accomodated in a layer mask. Since opacity is not cut finer than 0-100, an opacity (call it transparency if you like) mask can be fully described within an 8-bit (256-level)channel with data bits to spare. Therefore, I reckon, there must be some other usage for which 16-bit alpha channels are needed. Like what??

George
CC
Chris_Cox
Oct 23, 2006
Opacity/Transparency is a channel, and has 256 levels in 8 bits, 32769 levels in Photoshop’s 16 bit, etc. It’s not limited to 100 levels in any way.
GA
George_Austin
Oct 23, 2006
Chris,

Because a channel has enough bits to cover 256 opacity/transparency levels does not mean they are all needed. In fact, since PS only recognizes 100 opacity levels, 256 exceeds the needed number by 156. An 8-bit channel uses 100 levels plus (I presume) a few for overhead, and the remainder go unused. There’s breathing room, and that’s comfortable. The question is, why would you want 16 bits in a channel that only deals with opacity. Color channels need 16-bits for editing headroom, but I don’t see how transparency mask channels do. We don’t apply Curves and Levels etc. to opacity.
CC
Chris_Cox
Oct 23, 2006
Photoshop doesn’t "recognize" only 100 opacity/transparency levels. No, an 8 bit opacity channel uses all 256 levels.

And you sometimes need more precision for transparency as well as color, so you need more bits.

You seem to be stuck on the idea that only 100 values are used for opacity: and I keep telling you that’s wrong.
B
Bernie
Oct 23, 2006
In fact, since PS only recognizes 100 opacity levels,

Consider the source of that information, then consider the name of the person you are arguing with about it…
GA
George_Austin
Oct 23, 2006
Cyber,

"…Consider the name of the person you are arguing with…"

Snide personal attacks are contrary to forum protocol. How about confining remarks to the issues.

Chris,

I do appreciate your expertise and patience with me on this subject. But I’m STILL stuck on the 100 opacity levels thing, and here’s why: The PS Info Palette gives opacity as a percentage with only integral values between 0 and 100. The layer opacity slider and all other opacity sliders are calibrated in percent, and manual opacity entries can only be made as integers up to 100. The bologna doesn’t appear to be sliced any thinner. If there are 256 opacity levels, where are the other 156 levels hiding. If the user can’t access them, of what use are they?
B
Bernie
Oct 23, 2006
Snide personal attacks are contrary to forum protocol. How about confining remarks to the issues.

It was NOT an attack, I was telling you to look at who you are arguing with. How you understand this to be an attack I cannot understand.

And, if you don’t know who Chris Cox is, I recommend reading the splash screen next time you boot Photoshop.

The PS Info Palette gives opacity as a percentage with only integral values between 0 and 100

It’s a well documented fact that while PS display ink percentages values for CMYK and greyscale modes, PS uses all 256 levels in each channel anyways.
Y
YrbkMgr
Oct 23, 2006
I admit that I don’t know a lot about this (read: anything), but this caught my attention.

Are we sure, George, that we aren’t confusing percent with absolute?

IOW, 50% of 256 levels is 128. Maybe I’m not understanding but the palette isn’t reporting 100 levels, it’s reporting percent of the levels.

So I don’t see how you come to the conclusion that 156 levels are missing…

<shrug>
GA
George_Austin
Oct 23, 2006
Cyber,

Ah! That’s better. I should have had more faith in you. Instead of advising Chris to beware of me as a source you were advising me to respect Chris as an authority for which, BTW, I need no prompting.

Tony,

Either way, 0-100 absolute values or 0-100 percentages of 256, you end up with only 100 distinctions.
JR
John_R_Nielsen
Oct 23, 2006
You can access all 256 transparency values using Quick Mask mode. First, open the Info Palette menu > Palette Options, and make sure one of the readouts is set to "RGB Color". Make a new document. Add a new layer, and fill it with something so it’s completely opaque. Enter Quick Mask mode, making sure "Color indicates selected areas". Open the Color Picker (NOT the Color Palette), and enter the desired value in the R, G, and B fields,and press Enter.

Fill the Quick Mask channel with this color. Go to Standard Mode, and press Delete. Every pixel in the layer now has the desired transparency.

To see this, Select > Load Selection > Layer One Treansparency, with the ‘Invert’ box checked. Go to Quickmask Mode, and run the Eyedropper over the image. It should read the same as the value you put in the Color Picker fields.

Add another filled layer, and do as above, only instead of filling with a color, use a Black-to-White Gradient. Now, when you examine with the Eyedropper, you’ll see the values range from zero to 255.
Y
YrbkMgr
Oct 23, 2006
Either way, 0-100 absolute values or 0-100 percentages of 256, you end up with only 100 distinctions.

That may be true but that doesn’t mean that "156 are missing". Ones ability or inability to address distinct levels doesn’t go to the notion that PS doesn’t support them.

I still don’t see how you can equate percent with absolute.
CC
Chris_Cox
Oct 23, 2006
George – The readouts are 0 to 100% because that’s how most people think of it. It’s just a UI convention.
The same goes for CMYK reading 0-100%, when it’s still a full 8 or 16 bits underneath.
GA
George_Austin
Oct 24, 2006
John,

While I got tangled up in my underwear trying to follow your demo, its essence is clear. The opacity at any point in a masked layer equals the gray level of the mask. Since that gray level can be any of 255 values, the resulting opacity of the masked layer also has 255 values. Got it! Thanks for turning the light bulb on.

Tony,

Where your opacity entries are limited to integral percentages of 255, you can only get opacities of 0, 2.55, 5.10, 7.15, 10.20,… Since you can’t have fractioal values, these round out to 0, 3, 5, 7, 10,… That leaves out levels 1, 2, 4, 6, 8, 9,…

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