Couble of questions about color...

Discussion in 'Beyond the Basics' started by Senor Hound, Jul 15, 2008.

  1. Senor Hound

    Senor Hound TPF Noob!

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    I heard someone say they have an awesome MF DSLR that records 16 bits of color. That is per channel, right? Cause 16-bit color isn't all that impressive when you're talking about photography. Which also means that DSLRs that record 12 bits are more than sufficient to cover the full sRGB spectrum?

    Also, why is 32-bit color 32 bits, when its only eight bits per channel and there is only three channels? What does the other 8 bits do?

    Edited to add: BTW, how many bits per channel is Adobe RGB?
     
  2. prodigy2k7

    prodigy2k7 No longer a newbie, moving up!

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    There is a site i dont remember the link, it explains that is it useful, but not very visible. Most of the time it helps out in deep shadows. Thats all the information I have.
     
  3. Bifurcator

    Bifurcator TPF Noob!

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  4. Moglex

    Moglex TPF Noob!

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    Internaly a computer may store image data in 32 bit format whilst it is being worked upon so that no matter from where in an image you start, data will always be aligned on a word boundary and pixels can be accessed in one operation whereas if the data is stored in 24 bits it will, oddly, require at least two.
     
  5. Helen B

    Helen B TPF Noob!

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    Er, that's not what '32 bit' colour means in this case. References to 8-bit, 16-bit and 32-bit colour are all references to bits per channel, as Bifurcator's helpful post explains (the one he gave a link to). As Bifurcator mentions, the number of bits per pixel will depend on the number of channels - which can vary because there may be an alpha channel as well as the other channel or channels. '32 bit colour' is not 8 bits per channel, but 32 bits per channel. The uncompressed file formats that photographers tend to use manage to get an 8-bit-per-channel, 3-channel image into 24 bits per pixel, not 32.

    "BTW, how many bits per channel is Adobe RGB?"

    Adobe RGB and sRGB are colour spaces, and they can be described by however many bits per channel you wish - there is no inherent bit depth.

    Edit
    "Which also means that DSLRs that record 12 bits are more than sufficient to cover the full sRGB spectrum?"
    Remember that the number of bits the camera 'records' (ie the number of bits coming from the ADC) should not be considered with respect to the output colour space (eg sRGB), but to the sensor's native colour space which may be much greater than sRGB. In addition, the Raw data is a linear, or close to linear, representation of the brightness of the light falling on the pixel rather than the gamma-translated data of sRGB or Adobe RGB. In general you need higher bit depth in a linear description of the brightness of light than in a logarithmic* for the same tonal smoothness and same dynamic range - mostly because we sense light in more of a logarithmic manner than in a linear manner. That can be explained in greater, er, depth if you wish.

    *Second edit
    In a linear representation, the pixel value is directly related to the brightness. For 8-bit, you have 256 values (0-255). If 100% brightness is 255, 50% brightness is 127. Half of your available values are describing the uppermost stop of your brightness range. This gives great tonal detail in the highlights, but it isn't how we perceive brightness. The famous 18% grey card is about what we think of as 'middle grey', so a perceptual rendering should place 18% at about the half way mark in the data - which is roughly what sRGB and Adobe RGB do. 18% is about two and a half stops down from 100%. Therefore when you go from a linear description of brightness to a more logarithmic one you are stretching out the shadow data and squeezing in the highlight data.

    Just as an aside, I've just read 'Understanding Exposure'. It's aimed at beginners, but it makes an error when describing 'black' as 9% and a very bad error when saying that white is 36%. Maybe a question for another thread, if I can be bothered.

    Best,
    Helen
     
    Last edited: Jul 16, 2008
  6. Moglex

    Moglex TPF Noob!

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    I have to disagree.

    The OP said:

    "Also, why is 32-bit color 32 bits, when its only eight bits per channel and there is only three channels? What does the other 8 bits do?"

    He was very clearly asking what the spare eight bits were used for when 3 x 8 = 24 bit colour is refered to a 32bit.

    Whilst 32bit colour has more than one meaning it's perfectly clear which one the OP was refering to as 32 bit depth is not made up of 3 * 8 and does not have eight mysterious spare, sepearate, bits.
     
  7. Helen B

    Helen B TPF Noob!

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    Can you give a reference for 3 channels of 8-bits per channel being referred to as '32 bit colour'? If there are 4 channels, as there often are (as mentioned by Bifurcator and myself) then it can be called '32 bit', but the reference is to '32 bit colour'. It is a little confusing but suggesting, as you did, that it is called 32 bit colour because the pixel information has to span two words (ie it is padded with one byte of unused space) is misleading, I believe.

    Edit: Perhaps the OP could give a link to a page referring '32 bit colour' that was puzzling - the context would enable this confusion to be cleared up.

    Best,
    Helen
     
    Last edited: Jul 16, 2008
  8. Moglex

    Moglex TPF Noob!

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    If you check the Windows documentation you wil see that DIB's may be 32 bits. I was just explaining why you would want to use 32 bit DIB's.

    There are, of course, other meanings for '32 bit colour', but as they do not apply AFAIAA to DSLR's and computer GUI's (which OP is likey to use) make use of 32 bit DIB's extensively, it seemed a worthwhile definition to explain.

    What you said was: "Er, that's not what '32 bit' colour means in this case. References to 8-bit, 16-bit and 32-bit colour are all references to bits per channel,"

    and

    "'32 bit colour' is not 8 bits per channel, but 32 bits per channel."

    You now seem to be saying something competely different, ie. that references to 32 bit colour refer to something else entirely (i.e. the sum of three eight bit colour channels and one alpha channel).

    You seem to be somewhat confused as to what even you mean by 32 bits within the space of three hours. ;)
     
  9. Moglex

    Moglex TPF Noob!

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    OP.

    I don't think you wanted a long and turgid explanation of 32/48/64/96/128 bit colour so here is a simple precis of the three main things 32 bit colour can mean.

    1) A method of storing 8bits/channel colour data in Windows and other graphical operating systems to optimise processing speeds.

    2) Normal 8 bit/ch colour with the addition of an extra channel (alpha channel) that is used in blending operations.

    3) Very high accuracy 32 bit/ch colour depth.


    Hope that somewhere in there is what you're looking for.
     
    Last edited: Jul 16, 2008
  10. Bifurcator

    Bifurcator TPF Noob!

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    The simple answer as Helen provided is that it's not called "32-bit color" when it's 8-bits per channel. I feel Moglex intends to confuse but it is really just that simple. In the late 80's and early 90's for awhile Targa systems referred to 8-bit per channel color (8x3) files that also contained an 8-bit alpha channel (+8x1) as "32-bit files" but even then not "32-bit color" specifically.
     
  11. Bifurcator

    Bifurcator TPF Noob!

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  12. Moglex

    Moglex TPF Noob!

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    Was that where she said:

    "the number of bits per pixel will depend on the number of channels - which can vary because there may be an alpha channel as well as the other channel or channels. '32 bit colour' is not 8 bits per channel, but 32 bits per channel".

    or where she said:

    "If there are 4 channels, as there often are (as mentioned by Bifurcator and myself) then it can be called '32 bit''"

    That is an outrageous statement.

    I explained one of meanings of 32 bit colour - i.e. a 32 bit colour DIB.

    As with 'ATM' there are several meanings.

    I then made a precis of the different meanings without all the waffle so that the OP could determine which meaning was the best fit to the context where he heard the expression.
     

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