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When you choose the polygon mode, you left-click into the image to define vertices of the polygon, which are being connected with a line in the order you put them in. You can move existing vertices by clicking and dragging them, or use double-click to delete them. When you're satisfied with the shape, use right-click which joins the last vertex with the first one and fills in the polygon with chosen color. Polygons work in the erase-mode as well.
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