Raw format for high-resolution greyscale

Discussion in 'Digital Photography' started by POHB, Apr 24, 2006.

  1. POHB

    POHB Guest

    As I understand it a Raw image has brightness values for each of the
    sensor elements on the CCD and these sensors are arranged in a grid
    pattern so that a group of 4 make up one colour pixel.
    Would it therefore be possible to process a Raw image file to make a
    greyscale image with twice the resolution of a colour image from the
    same camera?
    If so, is there any software out ther to do this?
     
    POHB, Apr 24, 2006
    #1
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  2. POHB

    tlianza Guest

    Hi,

    I think that you are misunderstanding the process. The interpolation
    process is used to "up-size" the image. It is not true that 4 sensor
    elements are used to generate 1 color pixel. The image is interpolated,
    generally, to a 1:1 representation. The fact that there are half the number
    of red and blue elements than there are green, is a recognition of the lack
    of importance of these colors to our perception of sharpness. The sampling
    rate of the green on most sensor is half the potential rate horizontally and
    vertically. The luminance channel is interpolated from all the color
    information, but the green is weighted very high in this process. It
    doesn't matter if the image is rendered as color or monochrome, the
    resolution would be the same.

    Take care,
    TL.

    --
    Tom Lianza
    Director of Display and Capture Technologies
    GretagMacbeth LLC
    3 Industrial Drive
    Unit 7&8
    Windham, NH 03087
    603.681.0315 x232 Tel
    603.681.0316 Fax


    "POHB" <> wrote in message
    news:...
    > As I understand it a Raw image has brightness values for each of the
    > sensor elements on the CCD and these sensors are arranged in a grid
    > pattern so that a group of 4 make up one colour pixel.
    > Would it therefore be possible to process a Raw image file to make a
    > greyscale image with twice the resolution of a colour image from the
    > same camera?
    > If so, is there any software out ther to do this?
    >
     
    tlianza, Apr 24, 2006
    #2
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  3. POHB

    POHB Guest

    So for example a CCD with 3000x2000 sensor elements produces a raw file
    with 3000x2000 values that can then be processed to produce e.g. a Jpeg
    with 3000x2000 pixels? Sorry if I'm being dumb but I want to make sure
    I've understood right this time.
     
    POHB, Apr 24, 2006
    #3
  4. "POHB" <> wrote in message
    news:...
    > So for example a CCD with 3000x2000 sensor elements
    > produces a raw file with 3000x2000 values that can then
    > be processed to produce e.g. a Jpeg with 3000x2000 pixels?


    That's correct.

    > Sorry if I'm being dumb but I want to make sure
    > I've understood right this time.



    We all had to learn about it before we knew:
    <http://www.dpreview.com/learn/?/key=color+filter+array>
    <http://www.shortcourses.com/how/sensors/sensors.htm>
    <http://www.creativepro.com/story/feature/21600.html?cprose=daily>
    <http://micro.magnet.fsu.edu/primer/digitalimaging/cmosimagesensors.html>

    Bart
     
    Bart van der Wolf, Apr 24, 2006
    #4
  5. "POHB" <> wrote in message
    news:...
    > As I understand it a Raw image has brightness values for each
    > of the sensor elements on the CCD and these sensors are
    > arranged in a grid pattern


    Yes, sofar.

    > so that a group of 4 make up one colour pixel.


    No. Each sensel captures a color filtered part of the light treaching
    that sensel. That produces a fairly accurate luminocity sample for
    that part of the spectrum. The contribution of the other parts of the
    spectrum for that same sensel position is reconstructed, thus of
    potentially lower reliability, from the 8 or more direct neighbors of
    that same sensel.

    > Would it therefore be possible to process a Raw image file
    > to make a greyscale image with twice the resolution of a
    > colour image from the same camera?
    > If so, is there any software out ther to do this?


    It is possible to do that, but it is not without problems. Because the
    Red, Green, or Blue filtered sensels absorb/filter different amounts
    of light, and because there are twice as many Green filtered sensel
    compared to Red or Blue, there has to be a weigthed average for it to
    become something closer to the unfiltered response of a silicon based
    sensor array.

    There is a process called Binning where the values of adjacent sensels
    are added to gain sensitivity, or some weighted average is obtained to
    reduce noise. You typically find those possibilities in applications
    for Astro Photography.

    Bart
     
    Bart van der Wolf, Apr 24, 2006
    #5
  6. "POHB" <> writes:
    >So for example a CCD with 3000x2000 sensor elements produces a raw file
    >with 3000x2000 values that can then be processed to produce e.g. a Jpeg
    >with 3000x2000 pixels? Sorry if I'm being dumb but I want to make sure
    >I've understood right this time.


    Yes, that's right. With most cameras, out of those 6 million sensor
    elements, 3 million are behind green filters, and 1.5 million are behind
    red and blue. The resulting image has a B&W component that generally
    contains just as much fine detail as if you had 6 million monochrome
    sensors, but the colour component of the image has somewhat lower
    resolution (approximately equivalent to 1.5 million 3-colour pixels).

    Dave
     
    Dave Martindale, Apr 24, 2006
    #6
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