Heuristics using display EDIDs

Does anybody know any heuristics to work out if a connected display EDID data blob is a projector, CRT or LCD? For the new GNOME color control panel we want to select the correct device type by default, but allow the user to change it if the heuristic is wrong or the EDID data is crazy. Ideas welcome. Thanks.

9 responses to “Heuristics using display EDIDs”

  1. Stefan Brüns

    How about an open database where everyone (?) can upload EDID data? There should be enough different devices available to the community.

    This database can be used to create the heuristics, check heuristics, create black-/whitelists …

    Regards, Stefan

  2. Christoph Brill

    Did you try to ask the usual X.org/graphics suspects Adam Jackson, Dave Airlie or Keith Packard? From what I know the should know most of the nasty EDID tricks.

    There was a possibility to upload to hal-info? I didn’t know that!
    How about adding the upload functionality to gnome-control-center, so that if the heuristics didn’t match and the user changed it he will be suggested to “share with the community”.

  3. Dan Nicholson

    Adam Jackson (ajax) is generally the EDID guru in X land. I definitely think you should contact him, although I suspect he’ll discourage you based on how much broken EDID he’s seen over the years. :) I.e., I think randr would likely export that information already if people felt it could be done reliably. Maybe I’m wrong, though.

  4. Éric Piel

    Maybe have a look at the Horizontal & Vertical image size bytes (bytes 21 & 22). Wikipedia says projectors will often report 0 while others (LCD/CRT) should report numbers >0.

  5. Chris Halse Rogers

    Projectors are relatively easy to herusticly determine – they’ll have a physical size of 0x0.

    I’ve absolutely no idea how I’d try to go about distinguishing between CRT and LCD panels, though.

    1. Tobias

      Available refresh rates, maybe? I don’t know much about EDID, but wouldn’t the range of refresh rates for LCD panels be much narrower (e.g. only 50, 60 and 120) then for conventional displays?

  6. Chris Murphy

    There are people still using CRTs? New ones?

    I think the question that also needs to be answered, is the stability of xy as the phosphors decay. I know Y changes, but I’m not totally sure if xy change significantly. Because if xy changes much, the EDID color data is next to useless after about 6 months, since that’s roughly the 1/2 life for the phosphors. I know xy changes between some and a lot for CCFL phosphors in displays, which if it’s a lot will wreak havoc on any colorimeter that doesn’t have an adjustable calibration matrix to account for the changing spectral power distribution of the light source. And LED can be even worse.

    So I’m thinking we need to go back to CRTs and candlesticks, honestly.

    1. Chris Murphy

      I dug a bit more into this ancient world. Turns out the primaries of a CRT do decay primarily in Y, but minimally in xy. The issue though, is that they don’t decay at the same rate. So the Y for each phosphor changes differently, resulting in a white point shift, mostly toward magenta since it’s the green phosphor that changes the most.

      For an old CRT, the EDID xy data for primaries might be valid, but the xy data for the white point probably isn’t. Correcting for this without measurements is tricky, but I’m not sure that’s what you have in mind, once you’ve learned if something is a projector, LCD, or CRT?

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