Re: UMAX transparency adapter / averaging

From: Oliver Rauch (oliver.rauch@Wolfsburg.DE)
Date: Sun Dec 10 2000 - 07:35:06 PST

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    Marko Cebokli wrote:
    > Hello!
    > These ae some of my observations (and wishes) concernig the
    > transparency adapter for the UMAX Astra 1220S.
    > When I scan a normally exposed slide, it comes out quite dark, and
    > I must stretch the histogram in XSANE (GAMA=1.3 B=120 C=120 for example)
    > This causes 'holes' in the output histogram (the dark values go 0, 7,
    > 11, 15...). So it may be desirable to scan internally with 12
    > bits/color, even if the output is only 8 bits - to provide 'meat' for
    > histogram stretching.

    Hi Marko,

    the preview is always done with 8/24bpp.
    If you select "use custom gamma table" the real scan is done with full
    bit depth (12/36 bpp for the Astra 1220S) and the gamma correction
    is done with a transformation from 36->24bpp in the scanner.

    > Increasing the exposure time would help here (in the dark areas) but
    > would probably cause overflow in the bright pixels.
    The Astra 1220S does not support change of the exposure time.

    > 1. at each scanhead position take several readings and average them
    > before moving the scanhead on.
    This can not be done in the sane backend because the scanner controls
    the movement of the scanhead.

    > 2. run several full scans (running the scanhead back and forth across
    > the scan area). (smpling only when moving forward)
    This would not be a backend function, this should be done in a frontend.

    I will add it as "may be one day" to the end of the todo list of xsane.
    But I am not sure if it will ever come.

    But a suggestion:

    xsane is able to auto-increase the file name counter.

    Scan an image n times as image-01.pnm to image-nn.pnm

    Write a tool that reads image-01.pnm to image-nn.pnm
    and does average and save it to image.pnm


    E-Mail:		mailto:Oliver.Rauch@Wolfsburg.DE

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