COMBAT: Video Programming 2

Double Buffering

• How to avoid flickering when updating the screen
• You do not want to write directly into "screen"
• Videocards sometimes use conventional RAM as screen buffer
• Solution: Reserve two areas in RAM for Videocard. While one is diplayed update the other one. Change displayed area during horizontal blank.

Problem: Unix/X-Windows keeps hardware protected

But: We have cycles to burn because machines are fast

Our Scheme

• Allocate a (hidden) bitmap on server as big as the window
• Change only this bitmap
• Once in a while flush this bitmap to the actual window
• This is like double buffering where we copy the contents of the hidden buffer 1 to buffer2 which is displayed all the time.
• Copying the entire bitmap is expensive but not too expensive because it represents a rectangular part of the screen.
• We do not want to overload the server so we will keep the number of operations on the bitmap small.

Further Problem: Synchronization