convert to grey: 4 x the spatial resolution?

Discussion in 'Digital Photography' started by digiboy@mailinator.com, Feb 19, 2005.

  1. Guest

    Hi

    If I have a color image and convert to grey, do I get 4 x the spatial
    resolution?

    TA!

    DG
    , Feb 19, 2005
    #1
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  2. writes:

    >If I have a color image and convert to grey, do I get 4 x the spatial
    >resolution?


    No. You'd get a greyscale image with the same resolution as the colour
    one.

    Actually, if you look closely, not all colour images are the same.

    Uncompressed colour images from a 3-CCD camera, or a flatbed scanner,
    or a film scanner, can have full-resolution red green and blue
    components. If you convert this into a luminance plus colour
    difference space (i.e. LUV, or Lab), the two colour channels have the
    same resolution as the luminance channel. When you convert to
    greyscale, you're basically just keeping the luminance and discarding
    the two colour difference channels.

    If your images are in JPEG format, they've already been converted to a
    luminance/colour form internally, and the colour information has almost
    certainly been downsampled by a factor of 2. So the colour information
    is already only half the resolution of the luminance information. This
    works well in practice for images destined for viewing, because the
    human eye has much lower resolution for colour changes than luminance
    changes.

    If you have a digital camera with a Bayer-type filter, it inherently
    generates images where the colour resolution is half that of the
    luminance information. This matches JPEG output pretty well, but even
    if you use RAW output files the colour information is lower resolution
    than luminance.

    But in all of these cases, the actual spatial resolution of the image is
    comparable to the spatial resolution of the image file.

    Dave
    Dave Martindale, Feb 19, 2005
    #2
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