XGetImage returns junk with Xorg/dummy_drv/VTs

Peter Harris peter.harris at hummingbird.com
Mon Nov 24 13:23:32 PST 2008

McDonald, Michael-p7438c wrote:
>>>   I'm only doing XGetImage on the root window.
>> Is your application prepared to handle undefined bits for windows that
>> are not the same depth as the root? (Overlays, for example, but also
>> non-sample servers make multiple depths available).
>   As far as I'm aware, root windows only have one depth. I believe that
> holds for every individual window as well: a window has a single depth,
> which may differ from its parent's and its sibling's depths.

Exactly. And XGetImage always includes inferior windows[1]. When those
inferiors are a different depth, those contents of XGetImage are

Think about what happens when the root is depth 8, and your application
intelligently picks a depth 24 visual for its display. Your monitor will
display the full 24-bit image just fine, but a lossless translation for
XGetImage simply isn't possible. The protocol definition doesn't even
suggest an approximation.

>   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. Consider
Xnest (and Xephyr), which can have its root window obscured by other
windows on the host server.

Consider also a virtual root that is larger than the resolution of your
monitor. Traditionally, the sample implementation has saved the bits
that are not visible, but there is no reason it needs to (other than

Peter Harris

[1] Yes, that was a stupid protocol design decision. But at least the
protocol designers admit it in the docs: "This request is not
general-purpose... It is intended specifically for rudimentary hardcopy


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