Image Masking
Image masking involves using an image to determine which parts of the minimap insert should be visible. It is often useful to pair an image mask with an overlay border to ensure the map image does not show outside of the border.
When using an image mask, you can use a grayscale image to specify the opacity of a given pixel of the minimap -- with black being wholly transparent and white being wholly opaque.
Image masks are sized in the same way as an Overlay image, and use the same size settings to do so.
When creating a custom mask for an overlay image, I've found it effective to use an image editor that supports layers and use the overlay image as a guide when actually drawing the mask. Having some overlap in visible area with the actual overlay image is not a problem, and can help avoid issues with transparency on the edges of the overlay image.
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|---|---|---|
| Overlay image | Mask | Layered |