Can a graphics card lookup table be read back, or is it write only?

Discussion in 'Photoshop Tutorials' started by Greg, Jan 21, 2004.

  1. Greg

    Greg Guest

    Is there a utility that can unobtrusively *read* the current gamma
    correction lookup tables from the display adaptor?
    I'm using Windows XP.

    Thanks,
    Greg.
     
    Greg, Jan 21, 2004
    #1
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  2. Greg,

    you can download the Vesa-Vbe3 stuff
    http://www.vesa.org/vbelink.html

    but the programming is not a pleasure.

    Best regards --Gernot Hoffmann
     
    Gernot Hoffmann, Jan 21, 2004
    #2
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  3. Greg

    Greg Guest

    Thanks Gernot. I've actually managed to find a utility which will do it -
    it's called Powerstrip.
    http://www.entechtaiwan.net/ps.htm
    This can interrogate the LUT, and save it. (it saves it in the registry, but
    it's easy to export)
    So I have managed to avoid some unpleasurable programming - phew. ;^)

    Greg.
     
    Greg, Jan 22, 2004
    #3
  4. Greg,

    thanks for the feedback.
    My experiences with Matrox Graphics Cards:
    the results are NOT according to the VBE specs.
    Maybe my error.
    I´m going to test your suggested utility.

    Best regards --Gernot Hoffmann
     
    Gernot Hoffmann, Jan 22, 2004
    #4
  5. Greg

    Greg Guest

    Gernot,
    Feel free to drop me an email if you have trouble using it - I found it a
    bit unintuitive. But in the end
    I got it to work properly, and it's a very useful utility.

    Greg.
     
    Greg, Jan 23, 2004
    #5
  6. Greg

    Greg Guest

    Powerstrip actually does not retrieve the LUT on my system, if I have used
    the Windows control panel to change the gamma of the display adaptor. In
    Powerstrip, there is an option "Write directly to palette DAC". I think the
    Windows control panel must use this method to
    write the LUT. There must be some other indirect method of writing the LUT,
    which puts the info in a read/write buffer somewhere.
    When this method is used, Powerstrip is able to read back the LUT data. When
    I use the Eye One display profiler loader to load the LUT (i.e, to write the
    vcgt tag from the ICC file into the LUT), Powerstrip is able to read back
    the LUT data successfully. (of course, I could have simply dumped the data
    directly from the ICC file)

    Greg.
     
    Greg, Jan 24, 2004
    #6
  7. Greg

    Greg Guest

    Ah, Powerstrip does actually retrieve the LUT successfully after I've loaded
    it from the Windows control panel,
    but *only* if Powerstrip is invoked with the "Write directly to palette DAC"
    already checked. If it's not
    checked before having changed the LUT in Windows, Powerstrip seems to reset
    the LUT as soon as
    the setting is checked, to whatever LUT data Powerstrip currently has in
    it's own tables. Very tricky.

    Greg.
     
    Greg, Jan 24, 2004
    #7
  8. Greg, in the moment I don´t have the time to check Powerstrip.
    I had tried to read and write the LUTs according to the Vesa-Vbe
    specs, but this didn´t work for my Matrox cards (it´s assembly
    programming).
    I suspect that these specs are not accurately cast into software,
    because the Windows programmers who know the direct access to the
    graphics card´s registers don´t care anymore about the Vesa specs.
    On the other hand I had success programming the pixel clock
    (vbe-3.0 for Matrox G550).

    See you later ..

    Best regards --Gernot Hoffmann
     
    Gernot Hoffmann, Jan 24, 2004
    #8
  9. Greg

    ozonepark Guest

    Greg,

    Long time. You and I chatted some time ago about something similar.

    I just replaced a Matrox G550 with a P650. The P650 comes with a
    program called Coloreal that can be used to calibrate (via eyeball) a
    dual monitor config. Any, I too am having a bit of a difficult time
    understanding the relationship between AdobeGamma, the AdobeGamma
    Loader, XP and Coloreal. Here is my situation.

    I created a profile with Coloreal which has one monitor very bright
    and the other very dark (for test purposes); lets refer to this
    monitor setup as "DL" for short. I delete the Coloreal profile from
    Properties-Settings-Advanced-Color Management. I reboot and the DL
    appearance returns. I can I add and profile I want via
    Properties-Settings-Advanced-Color Management and reboot and I still
    get the DL appearance. I suspect the LUT value the make up DL are
    being saved in the Registry. I gather you investigated this and found
    that to be true, yes?

    If I run (by hand) the AdobeGamma Loader "AGL" it will change the
    monitor appearance based on the Default monitor profile. I think this
    makes sense. However when I reboot (again), I'm back to DL. I
    gather that running AGL does not update the Registry. Note anything
    about the Registry is pure conjecture on my part. What ar your
    thoughts?

    As a related question, I gather that XP does nothing with the info in
    Properties-Settings-Advanced-Color Management, other than to keep in
    there for any piece of software that cares? Is this consistent with
    you understanding?

    I also notice, that I can change the default profile in
    Properties-Settings-Advanced-Color Management run AGL then change the
    default profile to something else and rerun AGL but this time nothing
    visibly changes. Almost like it will only do something the first
    time. Very strange.

    I'd be interested in you experience with regard to any of this.



    Jerry
     
    ozonepark, Feb 11, 2004
    #9
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