How can I crop black corners from a scanned image?

Question

Scans/photocopies often miss the corner which comes out black. How can these black corners be made white using python with numpy, pillow or skimage?

Answer 1

1. Create a mask with all black objects (mask = image < threshold)
2. Remove objects touching the border (new_mask = segmentation.clear_border(mask))
3. Which objects were removed? objs = (new_mask != mask)
4. Fill those objects with white: image[objs] = 1 (or 255, if dtype int).

If you need to make sure that the objects replaced by white are triangles, you can use skimage.measure.regionprops to further examine each one.