How to scale GdkPixmap or XPixmap via X server?

I'm interested in offloading heavy computations from the X client. I think开发者_StackOverflow中文版 I can do so using XRender in some way, but I can't figure out how.

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GdkPixmap - by using Cairo.

XPixmap - by using next functions from XRender:

• XRenderSetPictureTransform
• XrenderComposite