[cairo] 24bit and/or MSB first Xservers

Owen Taylor otaylor at redhat.com
Wed May 11 05:29:44 PDT 2005

On Tue, 2005-05-10 at 15:04 -0600, Peter Dennis Bartok wrote:
> Hi all,
> Has anyone successfully used Cairo on anything other than a Xfree/Xorg X 
> server?
> Two cases where we are seeing problems are Sun's X server and Hummingbirds' 
> Exceed Win32 X server. On both or applications usually die with a BadMatch 
> (in XPutImage or XGetImage). Also, on Servers that have MSB first, cairo 
> treats it as if they are LSB first, thereby swapping red and blue.
> My understanding of the inner workings of cairo and libpixman sadly is quite 
> limited, so I wouldn't know where to start trying to fix this.

No libpixman or Cairo knowledge needed. It's all really shallow inside

For the endianess problems, the code simply needs to byte-swap the
image after calling XGetImage / before calling XPutImage if you 
detect cross-endian. (Image byte order not equal to local byte

(In some cases, there are tricks you can play with calling a 
big endian RGB x888 visual a little endian BGR 888x visual. 
libpixman can draw to a BGR 888x visual but it's going to be 
slow. Certainly simpler to just byte-swap than to worry about
accelerating that.)

It looks to me like the code should work fine on MSB as long
as the client/server endianess matches.

To track down the BadMatch, put a call to 
XSynchronize (display, True) into the program that is producing
the BadMatch, and then get a backtrace. If we know what request
is causing it, it should be pretty simple to track down what is
going on.


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