Very few clients I've seen actually call XShmGetImage to draw on the root window because this is incredibly slow and error prone. Having multiple clients do thi

Very few clients I've seen actually call XShmGetImage to draw on the root window because this is incredibly slow and error prone. Having multiple clients do this at the same time is an almost guaranteed way to cause graphical corruption. On modern hardware with any kind of acceleration you never want to map the framebuffer into userspace like this. The actual suggested use case of XShm is for the client to allocate its own buffer, which then gets copied to the screen. >colorspace conversion, scaling, etc. This is only a tiny subset of the post-processing operating a video player would implement. If you want arbitrary post-proccessing then realistically, you need to use the GPU.