1. connect projector
  2. System > Display > Extended

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  1. Open Madmapper
  2. Add an image and a rectangle

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  1. click this

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  1. Select Rect, in destination, choose projector (not your PC)

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  1. open output mode

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At this point, your projector should display a full screen view, your PC should be in editor mode, and you can adjust your projection area in PC.