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Painting of the mask works the same way as in the Surface Detection module, there are two painting modes available.
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Basic painting into the image by a brush of selected size. To change the brush size or switch between paint- and erase - mode, you can either use the sliders at the top, or you can use shortcuts, which can be found in the Help section under the ‘?’ symbol.
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When you choose the polygon mode, you left-click into the image to define the 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. Polygons work in the erase - mode as well.