XGetImage returns junk with Xorg/dummy_drv/VTs

[Peter Harris]

Peter Harris wrote:
> McDonald, Michael-p7438c wrote:
>>   In the X protocol, there is no concept of the root window being
>> obscured.
> 
> I don't see why not. If I am running multiple desktops, the root of one
> desktop may be obscured if I am viewing a different desktop.

I forgot to remind you about Xsecurity/Xace. In a properly secured
setup, any attempt to do an XGetImage on the root window will result in
BadAccess (and no image); doing a GetImage on your own bottommost
root-sized window will result in an empty image.

This isn't just theoretical; SSH -X tunnels typically set this up by
default, for example.

In this case, you might as well imagine that the root window is
obscured, at least from the perspective of your application. The
obscuring is artificial, granted, but still present. Your app isn't
allowed to see bits from other applications, so it doesn't.

Peter Harris
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