[B2-devel] Re: Visuals?

[Brian J. <bjj...@us...>]

On 9 Jul 2001 Brian Johnson wrote:
> On 6 Jul 2001 Christian Bauer wrote:
> > 
> > I've committed some code that tries to select an optimal visual
> > for the requested Mac color depth. However, I was only able to
> > test it with X servers that support exactly one depth so it is
> > possible that some logic in the code is wrong. I have tried it
> > with the 8+24 overlay mode of my Matrox card with XFree86 4.0.3
> > but that gives me a BadMatch error when trying to open a 24-bit
> > window, for no apparent reason.
> 
> I see similar behavior on my SGI box, with a 24-bit default visual.
> I forget what my default Mac video settings are... probably 8 bit.
> B2 starts booting, but appears to try to change the video modes at
> some point, and dies with a BadMatch error: ...

Here's some interesting info from the VkVisual man page which may be
relevant:

REVIEW OF X11 VISUAL INFORMATION
     Many developers do not quite understand how X and Xt deal with
     information that is related to X11 visuals.  This information is
     important if one is going to put part or all of an application's GUI in a
     non-default visual.  Following is a summary of some of the more important
     points.

   X11 Visual Attributes
     X11 does not attach any semantic meaning to a visual.  For example, there
     is no concept of an "overlay visual".  There is, however, a semi-standard
     convention that has been adopted by SGI and by some other workstation
     vendors:

     o    A visual's level is the framebuffer level the visual is associated
          with.  This is a hardware-related term.  It has nothing to do with X
          window stacking order.

     o    Levels less than zero are underlays.  As of this writing (8/96), SGI
          has no hardware that has underlay planes that are supported by the X
          server.

     o    Level zero is the normal planes.  The default visual is generally
          (but not necessarily) in the normal planes.

     o    Levels greater than zero are overlay planes.

     o    Each X11 visual is associated with exactly one level.

     o    SERVER_OVERLAY_VISUALS is a property on the root window, relating
          each X11 visual to its level.

     An X11 window has several attributes that need to be consistent when the
     window is created.  If an application sets these values inconsistently,
     or if it allows an inconsistent value to be inherited, the X server will
     return a fatal BadMatch error.

     o    XCreateWindow(3X) must be passed a consistent visual and depth.

     o    Certain fields in the XSetWindowAttributes structure that is passed
          to XCreateWindow(3X) must be consistent with the visual and depth.
          The significant fields are:

          background pixmap - must be NULL or of the stated depth.

          background pixel - is used if the background pixmap is NULL.  The
          pixel value must not exceed the colormap size.

          border pixmap - must be NULL or of the stated depth.

          border pixel - is used if the border pixmap is NULL.  The pixel
          value must not exceed the colormap size.

          colormap - must match the visual.

     The depth and visual cannot be changed after the window is created.  The
     XSetWindowAttributes values can  be changed later.

						Brian

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