How to shared texture just like BMS?

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Recently I look into Lightning’s MFD Extractor source code, want to figure out how BMS team shared HUD & MFD with another API.

I use Free Falcon source code to practice this technical.
Right now I know we should copy the image’s pointer to texture shared memory.(Call:FalconTexturesSharedMemoryArea)
But I stuck on I don’t know what type and structure should let image be.(or how to transformed HUD & MFD DirecDraw to this structure)
Is Free Falcon’s Directx 8.1 can do this same function just like BMS? (Directx 9 I guess)

I ask this because I am very curious on this function.
Hope somebody can explain with me. Thanks:D