eglGetDisplay/eglChooseDisplay, CreateContext

[Brian Paul]

Jon Smirl wrote:
> In our model eglGetDisplay(display_name) opens a specific device. So
> if I want to open three screens I need to do:
> eglGetDisplay("display/0");
> eglGetDisplay("display/1");
> eglGetDisplay("display/2");
> 
> And I end up with three display handles.
> 
> With the X server if I XOpenDisplay() I get:
> Display
>      screen0
>      screen1
>      screen2
> 
> With EGL I could also get:
> Display
>      screen0
>      screen1
> Display
>      screen0
> 
> So if I understand things right an EGL display is corresponding to an
> X screen. And there is no equivalent to an X Display in EGL. There is
> also no parallel for EGL screen in X.

I was going on the assumption that an EGL display corresponded to a 
device (a card).  Each card may have 0 or more screens attached to it.

X has screens too.  They're named by an integer starting at 0.


> Another area of confusion I'm having is glXCreateNewContext(display,
> config). The display is not used in DRI code. Instead GLX configs are
> bound to screens and the screen is determined from the config.

Because X is rather screen-oriented internally.


> In eglCreateContext(display, config) my display corresponds to an X
> screen and my config is not bound to a particular EGL screen. So why
> does eglCreateContext() need config as a parameter?

The EGL config (pixel format) sets a variety of parameters/limits for 
the rendering context.  For example, whether or not 
depth/stencil/accum buffers are present, the bit depth of each, 
whether there's 2 or 4 colorbuffers (double buffered, stereo), color 
index mode vs. RGB, etc.

Later, when a rendering context is bound to a drawing surface, we need 
to make sure the context and surface are compatible by comparing their 
EGL configs.

-Brian