What's a dead pixel?

Discussion in 'Digital Photography' started by Newbie, Apr 23, 2005.

  1. Newbie

    Newbie Guest

    What's a dead pixel? What else do we test for on a newly bought
    digicam?
    Newbie, Apr 23, 2005
    #1
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  2. Newbie

    Mark² Guest

    "Newbie" <> wrote in message
    news:...
    > What's a dead pixel? What else do we test for on a newly bought
    > digicam?


    A dead pixel is simply a photosite on the sensor that doesn't generate any
    data, and therefore shows up as a black spot...most noticable within smooth,
    bright portions of an image. Most cameras nowadays have built-in algorythms
    that map these out...so that you never know they're there.

    The other thing to check for is the problem of "stuck pixels," which show up
    as conssatntly bright...often of a particular color of the spectrum in all
    images. The easiest way to check for this is to take an exposure with the
    lens cap on the with a shot of a half second or so. Longer exposures will
    usually show hot pixels of some sort on nearly ALL digital cameras, unless
    they have a facility for mapping them out automatically.
    Mark², Apr 23, 2005
    #2
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  3. Newbie

    Patzt Guest

    Patzt, Apr 23, 2005
    #3
  4. Newbie

    GTO Guest

    Not exactly. You forgot to distinguish between stuck pixel and hot pixel.

    See http://www.nikon.com.sg/TechSupp/KB/TOPFAQ/DeadHotRandom.htm for more.

    Gregor

    "Mark²" <mjmorgan(lowest even number here)@cox..net> wrote in message
    news:7Ihae.3087$Zi.1597@fed1read04...
    >
    > "Newbie" <> wrote in message
    > news:...
    >> What's a dead pixel? What else do we test for on a newly bought
    >> digicam?

    >
    > A dead pixel is simply a photosite on the sensor that doesn't generate any
    > data, and therefore shows up as a black spot...most noticable within
    > smooth, bright portions of an image. Most cameras nowadays have built-in
    > algorythms that map these out...so that you never know they're there.
    >
    > The other thing to check for is the problem of "stuck pixels," which show
    > up as conssatntly bright...often of a particular color of the spectrum in
    > all images. The easiest way to check for this is to take an exposure with
    > the lens cap on the with a shot of a half second or so. Longer exposures
    > will usually show hot pixels of some sort on nearly ALL digital cameras,
    > unless they have a facility for mapping them out automatically.
    >
    >
    >
    GTO, Apr 23, 2005
    #4
  5. Newbie

    Mark² Guest

    "GTO" <> wrote in message
    news:14lae.2136$...
    > Not exactly. You forgot to distinguish between stuck pixel and hot pixel.


    Ya, OK.
    Mark², Apr 23, 2005
    #5
  6. Newbie

    Stacey Guest

    Mark² wrote:

    >
    > "Newbie" <> wrote in message
    > news:...
    >> What's a dead pixel? What else do we test for on a newly bought
    >> digicam?

    >
    > A dead pixel is simply a photosite on the sensor that doesn't generate any
    > data, and therefore shows up as a black spot...most noticable within
    > smooth,
    > bright portions of an image. Most cameras nowadays have built-in
    > algorythms that map these out...so that you never know they're there.
    >


    By "built in", you mean most have to be sent in for pixel mapping as very
    few have their own mapping system onboard.
    --

    Stacey
    Stacey, Apr 23, 2005
    #6
  7. Newbie

    Mark² Guest

    "Stacey" <> wrote in message
    news:...
    > Mark² wrote:
    >
    >>
    >> "Newbie" <> wrote in message
    >> news:...
    >>> What's a dead pixel? What else do we test for on a newly bought
    >>> digicam?

    >>
    >> A dead pixel is simply a photosite on the sensor that doesn't generate
    >> any
    >> data, and therefore shows up as a black spot...most noticable within
    >> smooth,
    >> bright portions of an image. Most cameras nowadays have built-in
    >> algorythms that map these out...so that you never know they're there.
    >>

    >
    > By "built in", you mean most have to be sent in for pixel mapping as very
    > few have their own mapping system onboard.


    My understanding is that many have this built in as we speak, but it's not
    widely publicized--since manufacturers have nothing to gain by telling folks
    that tiny portions of their images are not really generated by the shot
    scene...
    I will be happy to shift this understanding if shown otherwise.
    Mark², Apr 23, 2005
    #7
  8. Newbie

    Stacey Guest

    Mark² wrote:

    >
    > "Stacey" <> wrote in message
    >>
    >> By "built in", you mean most have to be sent in for pixel mapping as very
    >> few have their own mapping system onboard.

    >
    > My understanding is that many have this built in as we speak, but it's not
    > widely publicized--since manufacturers have nothing to gain by telling
    > folks that tiny portions of their images are not really generated by the
    > shot scene...


    > I will be happy to shift this understanding if shown otherwise.


    I've read canon dSLR owners have to send theirs in for "pixel mapping" so
    don't think most do this automatically. I know on my E300 is has a menu
    item for the camera to do this and it takes a while for it to run though
    this procedure. I don't see how they could do this "on the sly" so people
    wouldn't know? That and most people do consider this a positive feature for
    a camera to have so it doesn't seem likely they would hide that their
    camera has this feature.

    --

    Stacey
    Stacey, Apr 23, 2005
    #8
  9. Newbie

    Mark² Guest

    "Stacey" <> wrote in message
    news:...
    > Mark² wrote:
    >
    >>
    >> "Stacey" <> wrote in message
    >>>
    >>> By "built in", you mean most have to be sent in for pixel mapping as
    >>> very
    >>> few have their own mapping system onboard.

    >>
    >> My understanding is that many have this built in as we speak, but it's
    >> not
    >> widely publicized--since manufacturers have nothing to gain by telling
    >> folks that tiny portions of their images are not really generated by the
    >> shot scene...

    >
    >> I will be happy to shift this understanding if shown otherwise.

    >
    > I've read canon dSLR owners have to send theirs in for "pixel mapping" so
    > don't think most do this automatically. I know on my E300 is has a menu
    > item for the camera to do this and it takes a while for it to run though
    > this procedure. I don't see how they could do this "on the sly" so people
    > wouldn't know? That and most people do consider this a positive feature
    > for
    > a camera to have so it doesn't seem likely they would hide that their
    > camera has this feature.


    As I say...I may be wrong here.
    Mark², Apr 23, 2005
    #9
  10. Stacey <> wrote:
    : Mark? wrote:

    : >
    : > "Stacey" <> wrote in message
    : >>
    : >> By "built in", you mean most have to be sent in for pixel mapping as
    : >> very few have their own mapping system onboard.
    : >
    : > My understanding is that many have this built in as we speak, but
    : > it's not widely publicized--since manufacturers have nothing to gain
    : > by telling folks that tiny portions of their images are not really
    : > generated by the shot scene...

    : > I will be happy to shift this understanding if shown otherwise.

    : I've read canon dSLR owners have to send theirs in for "pixel mapping" so
    : don't think most do this automatically. I know on my E300 is has a menu
    : item for the camera to do this and it takes a while for it to run though
    : this procedure. I don't see how they could do this "on the sly" so people
    : wouldn't know? That and most people do consider this a positive feature for
    : a camera to have so it doesn't seem likely they would hide that their
    : camera has this feature.

    Maybe the term is being shared by more than one procedure, or some camera
    makers have multiple levels of "mapping". But to clarify the need for
    mapping (as I understand it), EVERY CCD device has a few pixels that are
    not responding correctly (dead, always on, weak, etc). Manufacturing any
    semiconductor device involves a certain percentage of failure of
    individual components on the chip. The more complex, or more densly packed
    the parts the more likely that a failure will impact the circuit
    adversely. If all CCDs were rejected if even a single pixel failed, the
    cost of the chips would be astronomical! So there is an "acceptable
    percentage" of failures for each chip manufacturer and proposed use.

    Now, why map? When a chip is about to be installed in a camera a test of
    each pixel is run and rated for performance. Then a custom "compensation
    function" is generated and burned into the computer in the camera to
    minimize the effects. On the cheapest cameras this is probably where the
    whole thing stops. With slightly more expensive cameras there may be some
    kind of automatic monitoring of ongoing function of the camera which will
    further adjust compensation over time. Then for some high end cameras
    (which could demand much higher quality) a periodic factory mapping, under
    lab conditions (which has to be much more precise than an on the fly
    test) and recompensation would be offered.

    The compensation may take many forms. It could be as simple as copying one
    of the adjacent pixels, or an averaging of the surrounding pixels, all the
    way up to a complex formula calculating the most likely value of the
    missing pixel. Unless you are taking lots of images that require
    contrasting details as small as a single pixel, most "compensated" pixels
    will never be noticed by MOST users. (notice I am specifically saying
    "most" as there will always be those here and elsewhere that will not find
    ANY variation "acceptable".)

    Such a compensation is not uncommon in computers. Have you ever looked at
    the "details" while doing a HD defrag? Some of the "locked" sectors are
    simply sections of the disk that are not functioning properly and they are
    locked out of future use. A majority of these "bad sectors" are found and
    locked at the time of manufacture. And over time, wear may cause the
    computer to lock out more sectors as they become damaged (more prevalent
    in computers that are moved around or jostled a lot). So the same process
    in CCDs is not suprizing and not normally spelled out in literature. It
    would be like making a "major selling point" on a new brand of car depend
    on the fact that it has "a device for lifting the car for replacement of a
    flat tire" as standard. :)

    So to sumarize this (overly) long posting, some form of pixel mapping is
    likely to be done at the time of manufacture and/or be done on an ongoing
    basis within the camera software. But a "lab quality" mapping done at the
    manufacturer may be offered for those who require a higher level of
    precision. The former would be "built in" and the latter would be "an
    optional service".

    Does this help?

    Randy

    ==========
    Randy Berbaum
    Champaign, IL
    Randy Berbaum, Apr 23, 2005
    #10
  11. "Newbie" <> wrote in message
    news:...
    > What's a dead pixel? What else do we test for on a newly bought
    > digicam?
    >

    A Pixel is a cross between a PIXie and an AngEL. So a dead one is just
    that - dead, deceased, expired, passed away, passed on, etc, etc.
    Noah Fingawtoo, Apr 23, 2005
    #11
  12. In article <d4d289$vfn$>,
    Randy Berbaum <> wrote:
    >Such a compensation is not uncommon in computers.


    >A majority of these "bad sectors" are found and
    >locked at the time of manufacture. And over time, wear may cause the
    >computer to lock out more sectors as they become damaged (more prevalent
    >in computers that are moved around or jostled a lot). So the same process
    >in CCDs is not suprizing and not normally spelled out in literature.


    The difference is that serious disk manufacturers document how to access
    bad sector lists (for example the SCSI mode pages). A bad sector does
    not affect the performance of the disk much if it is allocated on
    the same track as it was located originally.

    On the other hand, bad pixels have (potentially) much more impact than
    bad sectors, but camera manufactures treat their professional customers
    as simple consumers who are allowed to play with their toys and don't
    need any technical documentation.


    --
    That was it. Done. The faulty Monk was turned out into the desert where it
    could believe what it liked, including the idea that it had been hard done
    by. It was allowed to keep its horse, since horses were so cheap to make.
    -- Douglas Adams in Dirk Gently's Holistic Detective Agency
    Philip Homburg, Apr 23, 2005
    #12
  13. > The compensation may take many forms. It could be as simple as copying one
    > of the adjacent pixels, or an averaging of the surrounding pixels, all the
    > way up to a complex formula calculating the most likely value of the
    > missing pixel.


    With Bayer sensor cameras the process that generates an RGB pixel array
    from the Bayer scheme is very appropriate to handle missing pixel, as it
    interpolates the sensor positions to the pixel position anyway. So I'm
    sure that this algorithm is used for that purpose.

    That brings up the question how missing pixels are handled with RAW
    files. Does the camera provide a missing pixel list within the file or
    does it try (much less appropriately) to replace the missing pixels by
    means of the neighbors of the same color in the Bayer scheme ?

    -Michael
    Michael Schnell, Apr 23, 2005
    #13
  14. Newbie

    Ron Hunter Guest

    Stacey wrote:
    > Mark² wrote:
    >
    >
    >>"Stacey" <> wrote in message
    >>
    >>>By "built in", you mean most have to be sent in for pixel mapping as very
    >>>few have their own mapping system onboard.

    >>
    >>My understanding is that many have this built in as we speak, but it's not
    >>widely publicized--since manufacturers have nothing to gain by telling
    >>folks that tiny portions of their images are not really generated by the
    >>shot scene...

    >
    >
    >>I will be happy to shift this understanding if shown otherwise.

    >
    >
    > I've read canon dSLR owners have to send theirs in for "pixel mapping" so
    > don't think most do this automatically. I know on my E300 is has a menu
    > item for the camera to do this and it takes a while for it to run though
    > this procedure. I don't see how they could do this "on the sly" so people
    > wouldn't know? That and most people do consider this a positive feature for
    > a camera to have so it doesn't seem likely they would hide that their
    > camera has this feature.
    >


    It may be that the procedure is done at the factory, and stored in
    non-volatile memory.
    I doubt that even the most particular among us would be able to find 4
    or 5 dead pixels among 4 to 8 million on the sensor since the firmware
    would disguise them by averaging the pixels adjacent to them. This is
    somewhat analogous to the mapping of bad sectors on hard drives,
    although with hard drives, this process doesn't result in loss of space.



    --
    Ron Hunter
    Ron Hunter, Apr 23, 2005
    #14
  15. Newbie

    Ron Hunter Guest

    Philip Homburg wrote:
    > In article <d4d289$vfn$>,
    > Randy Berbaum <> wrote:
    >
    >>Such a compensation is not uncommon in computers.

    >
    >
    >>A majority of these "bad sectors" are found and
    >>locked at the time of manufacture. And over time, wear may cause the
    >>computer to lock out more sectors as they become damaged (more prevalent
    >>in computers that are moved around or jostled a lot). So the same process
    >>in CCDs is not suprizing and not normally spelled out in literature.

    >
    >
    > The difference is that serious disk manufacturers document how to access
    > bad sector lists (for example the SCSI mode pages). A bad sector does
    > not affect the performance of the disk much if it is allocated on
    > the same track as it was located originally.
    >
    > On the other hand, bad pixels have (potentially) much more impact than
    > bad sectors, but camera manufactures treat their professional customers
    > as simple consumers who are allowed to play with their toys and don't
    > need any technical documentation.
    >
    >

    While it is quite true that a dead pixel is more serious than a bad
    sector (which can be corrected by spare sector use) since the bad pixel
    can't be recovered by merely taking the value from a set of 'spares'.
    Still, I don't think many people, even very picky professionals, would
    detect a few such compensated pixels from the 12 million in the sensor.


    --
    Ron Hunter
    Ron Hunter, Apr 23, 2005
    #15
  16. Newbie

    Guest

    In message <Dhnae.3131$Zi.1498@fed1read04>,
    "Mark²" <mjmorgan(lowest even number here)@cox..net> wrote:

    >As I say...I may be wrong here.


    The Canon 10D and 20D have an internal list of bad pixels, as tested at
    the factory. The RAW data interpolates these pixels (and I assume that
    they are interpolated from the closest pixels with the same color
    filter). There is no facility for the user to update this list, but
    using DCRAW you can give it a list of newly bad pixels, and it is
    trivial to write an app that does the same to an intermediate .DNG file
    if you want; if you convert to an uncompressed .DNG, the data after the
    header is straight-forward 12-bit RAW data which you can overwrite
    before submitting it to ACR or RSE, or any converter that supports .DNG.
    --

    <>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
    John P Sheehy <>
    ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
    , Apr 23, 2005
    #16
  17. Newbie

    Guest

    In message <rztae.9087$>,
    Ron Hunter <> wrote:

    >While it is quite true that a dead pixel is more serious than a bad
    >sector (which can be corrected by spare sector use) since the bad pixel
    >can't be recovered by merely taking the value from a set of 'spares'.
    >Still, I don't think many people, even very picky professionals, would
    >detect a few such compensated pixels from the 12 million in the sensor.


    There's a lot more than "a few" in a typical sensor. Literally
    thousands. With the Canon 20D, you will have 1/2 of the mapped-out
    pixels easily identified in any ISO 3200 image's RAW data. Any odd
    value other than 4095 is interpolated, since the data is actually an
    under-exposed ISO 1600 image, with the numbers 0 through 2047 doubled,
    and anything greater than 2048 clipped to 4095. If you do this with one
    file, you'll get about half the bad pixels; if you do 2, 3/4, if you do
    3, 7/8, etc.

    I don't know if this data is explicitly included in the RAW file's
    header.
    --

    <>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
    John P Sheehy <>
    ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
    , Apr 23, 2005
    #17
  18. Newbie

    Guest

    In message <d4df7l$hsb$03$-online.com>,
    Michael Schnell <> wrote:

    >That brings up the question how missing pixels are handled with RAW
    >files. Does the camera provide a missing pixel list within the file or
    >does it try (much less appropriately) to replace the missing pixels by
    >means of the neighbors of the same color in the Bayer scheme ?


    It couldn't possibly use the neighbors with different color pixels; that
    would cause pixels that stand out, defeating the purpose. The only time
    the levels in the three color channels are roughly the same is when a
    subject is cyan or cyan-green, with most CFAs.
    --

    <>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
    John P Sheehy <>
    ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><
    , Apr 23, 2005
    #18
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