#279 G400 last line of buffer in strange mode

Rendering Error
open
MGA OpenGL (56)
5
2003-05-12
2001-01-09
No

Hard to explain this one ...

Sometimes the last line of the screen in a direct-rendering mode looks funky. As an object passes over it, colours get written in, but selectively, as though a Z-buffering algorithm were being applied or something.

Certain xlock modes show this quite clearly e.g. gears, where now again one or more of the gears touches the bottom of the screen, or the 3D bubbles where a streak of colour is left at the bottom.

Now, when the screensaver mode changes, sometimes when another GL mode kicks in, the garbage in the bottom line is still there, but others will clear it correctly e.g. pipes.

Very occasionally I've seen a similar effect in the rightmost column of the screen, though I havn't seen it often enough or peered hard enough to see if it's just garbage caused by an off-by-one clear area, or if it is behaving in the same odd way as the bottom line.

I'm convinced the bottom line is somehow being treated as both part of the display and part of the depth buffer, but have no way to verify this. I don't know enough to code up tests for that yet :( although I could/can probably figure out in time.

/me makes note to learn GL in his alleged copious quantities of free time.

Discussion

  • Bill Crawford

    Bill Crawford - 2001-02-19

    This is still happening with the recently merged mga-1-0-0 branch, looks exactly the same.

    I'm pretty certain that a Z-buffer algorithm is being applied to the pixels in the last line/column of the screen as bright blue overwrites bright red on the gears demo (this would have a higher value in little-endian 32 bits). It's pretty consistent behaviour, some demos clear that line and it stays black until something passes over it.

     
  • Ian Romanick

    Ian Romanick - 2003-05-12

    Logged In: YES
    user_id=423974

    Have you been able to reproduce this on recent XFree86 or
    DRI? I remember seeing problems like this a long time ago,
    but my texmem changes seem to have helped clear it up.

     
  • Ian Romanick

    Ian Romanick - 2003-05-12
    • assigned_to: nobody --> idr
     
  • Bill Crawford

    Bill Crawford - 2003-09-25

    Logged In: YES
    user_id=14762

    I'm still occasionally seeing this problem; Red Hat 9, packages:
    XFree86-Mesa-libGL-4.3.0-2
    kernel-2.4.20-20.9

    Please let me know if I should test with something more recent?

    Otherwise no problems at all apart from the hardware
    limitations of the old G400 *g*

     

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