border_pixels is a list of image pixels in the border, used to define the source-plane border. However, we don't necessarily want to trace image pixels to the source-plane, instead only using sub-pixels (and sparse pixels). So, we need a border_sub_pixels, which gives a list of the sub-pixel in each image pixel which is closest to the border.
border_pixels is a list of image pixels in the border, used to define the source-plane border. However, we don't necessarily want to trace image pixels to the source-plane, instead only using sub-pixels (and sparse pixels). So, we need a border_sub_pixels, which gives a list of the sub-pixel in each image pixel which is closest to the border.