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#1
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Bits per channel
Digicams sensors have 8 bits per channel to record represent voltage
per pixel. So luminescence is represented by a number between 0 and 255. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? Things seem to be at 8-bit for pretty long now. - Siddhartha |
#2
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On 6 Jan 2005 05:06:34 -0800, "Siddhartha Jain"
wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. State your source. Most DSLRs already use 12 bits per channel. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? We already did. Things seem to be at 8-bit for pretty long now. Then get a better camera. But, before you get too fret up, remember that your graphics card can't even display 11 bits per channel. So, if you want 32 bits per channel you'll never see the difference. -- Owamanga! |
#3
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Siddhartha Jain wrote:
Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? Things seem to be at 8-bit for pretty long now. - Siddhartha You get an analog value from the sensor that can be represented with about 12 bits, i.e. 4096 grey levels, with a linear scaling of grey level to luminance. This is what you get in RAW files. This is the situation inside the camera, before conversion to 8-bits takes place. In image files, 8-bit JPEG or TIFF files, the digital value does not directly represent light level, but a value nearer to the log of the light level. A gamma-correction of approximately 0.45 power law is used to convert the 0..4095 range of the sensor data into the 0..255 range of the 8-bit image data. In practice, perhaps only value 0..2047 are converted, the remaining values in the RAW file representing the extra "headroom" which people mention. The effect of the gamma correction is to reduce the number of light levels which can be separately represented at the bright end of the range. I.e. the eye cannot distinguish between light levels of 2045 and 2046, so they are both mapped to "254", for example. Light levels at the low end of the 0..4095 range (for example 1 or 2) are much more accurately represented in the 8-bit JPEG/TIFF image, so shadow detail is preserved. The display device typically has a gamma around 2.2, i.e. it is rather non-linear between the drive voltage in and the light level out. The combination of a 0.45 * 2.2 gamma (camera and display) result in an approximately linear net transfer between light into the sensor and light out of the display. This is all a simplification, but should help you understand why 8-bit data is adequate (just) for normal usage. Personally, I would like to see rather more than 8-bits, perhaps 10-bit or 12-bit JPEGS, so that all of the sensor range and more could be used for subsequent processing steps. Cheers, David |
#4
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Owamanga wrote:
On 6 Jan 2005 05:06:34 -0800, "Siddhartha Jain" wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. State your source. Most DSLRs already use 12 bits per channel. Ok, correction. 12-bits. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? We already did. I don't think that answers the question. But, before you get too fret up, remember that your graphics card can't even display 11 bits per channel. Ok, so why not? What I am trying to understand is that are there no significant benefits in moving to a broader bus? |
#5
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Owamanga wrote:
On 6 Jan 2005 05:06:34 -0800, "Siddhartha Jain" wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. State your source. Most DSLRs already use 12 bits per channel. Thanks. Correction, 12-bits. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? We already did. I don't think that answers the question. But, before you get too fret up, remember that your graphics card can't even display 11 bits per channel. So, if you want 32 bits per channel you'll never see the difference. What I am trying to understand is are there no significant advantages to pushing the number of bits upwards? - Siddhartha |
#6
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Siddhartha Jain wrote:
[] What I am trying to understand is that are there no significant benefits in moving to a broader bus? "significant" is the operative word. Tests have show that the eye has problems in using more than 8-bit data when applied to a gamma-corrected monitor as I described, although you can set up some special cases which show that for colour slightly more may be required. Prior to conversion to 8-bits for display, though, there may be a slight advantage working in the linear 12/16-bit domain. My guess is that it will be like CDs - for domestic use 16-bit 44KHz audio is adequate, for production studios have moved to 24-bit/96/192Khz audio. Cheers, David |
#7
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Owamanga wrote:
Siddhartha Jain wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. ...remember that your graphics card can't even display 11 bits per channel. So, if you want 32 bits per channel you'll never see the difference. My desktop display properties indicate 32 bit 'color quality' with an option for 16 bit. I'm not sure if this is the same terminology. My older PC ran a lot slower in 32 bit mode as I recall. |
#8
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On 6 Jan 2005 06:42:55 -0800, "Siddhartha Jain"
wrote: Owamanga wrote: On 6 Jan 2005 05:06:34 -0800, "Siddhartha Jain" wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. State your source. Most DSLRs already use 12 bits per channel. Thanks. Correction, 12-bits. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? We already did. I don't think that answers the question. But, before you get too fret up, remember that your graphics card can't even display 11 bits per channel. So, if you want 32 bits per channel you'll never see the difference. What I am trying to understand is are there no significant advantages to pushing the number of bits upwards? That's right - diminishing returns. This whole thing is designed around what the human eye can see. There is no point going crazy with 16 bits, 32 bits, 64 bits, 128 bits per channel when we can't display, print or see the added detail. Can you see the difference between 24 bit mode (8 per channel) and 32 bit mode (10.5 per channel) on your graphics card ? I am sure if someone switched mine down to 24 bits one morning, I'd probably never even notice it had happened. The only argument for 48 bit scanners (16 per channel) and the like is because it allows slightly more scope for exposure correction (ie, you get to choose later which 8 bits per channel you want to keep) ...same with digital audio. Take CDs - 44.1Khz at 16 bits per sample is plenty good enough for our human ears. Since that invention 20 years ago, many subsequent digital audio standards have actually been much worse and the best one is only just over double that sampling rate and nobody is buying it. We have reached a plateau. -- Owamanga! |
#9
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"Siddhartha Jain" writes:
Owamanga wrote: On 6 Jan 2005 05:06:34 -0800, "Siddhartha Jain" wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. State your source. Most DSLRs already use 12 bits per channel. Ok, correction. 12-bits. Is there something to gain by moving to 16-bit or 32-bit? If yes, when are we moving? We already did. I don't think that answers the question. Well, 12 bits is 50% more than 8 bits, last I checked; so the benefit of going all the way to 16 from 12 is less than the benefit of going to 12 from 8. But, before you get too fret up, remember that your graphics card can't even display 11 bits per channel. Ok, so why not? Because your eyes can't distinguish that many colors. What I am trying to understand is that are there no significant benefits in moving to a broader bus? One limitation is the human visual system. Now, it's useful to capture more than that in the initial shot -- but it has to be reduced to what humans can see to work as a print for humans. Like a negative being printed. And, as people said, we *are* moving to broader buses. When the web was new, it was rare to see pictures in more than 256 colors. Now 24-bit color is pretty much the baseline; and better cameras and scanners produce 12 bits or more per channel. That sounds like exactly the movement you're asking about, to me. -- David Dyer-Bennet, , http://www.dd-b.net/dd-b/ RKBA: http://noguns-nomoney.com/ http://www.dd-b.net/carry/ Pics: http://dd-b.lighthunters.net/ http://www.dd-b.net/dd-b/SnapshotAlbum/ Dragaera/Steven Brust: http://dragaera.info/ |
#10
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On Thu, 06 Jan 2005 07:16:11 -0800, paul wrote:
Owamanga wrote: Siddhartha Jain wrote: Digicams sensors have 8 bits per channel to record represent voltage per pixel. So luminescence is represented by a number between 0 and 255. ...remember that your graphics card can't even display 11 bits per channel. So, if you want 32 bits per channel you'll never see the difference. My desktop display properties indicate 32 bit 'color quality' with an option for 16 bit. I'm not sure if this is the same terminology. My older PC ran a lot slower in 32 bit mode as I recall. 32 bits per pixel. Split this into the three color components of Red, Green and Blue and you've got theoretical 10.6 bits per channel. In fact, most (if not all) are using 32 bits just to pad 24 actual bits into something that fits neatly into 4 bytes - this is for performance and design simplicity reasons. So, these modes are actually only displaying 8 bits per channel. 16,776,215 discrete colors. 24/32 bit modes will be slower because they use 4 bytes of card memory per pixel instead of 2 bytes in 16 bit mode (65,536 colors) or 1 byte in 8 bit (256 color) mode. Someone might correct me and tell me there is now available a true 10.6 bit, 12 or 16 bit per channel graphics card out there - anything is possible I am sure. -- Owamanga! |
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