I wanted to test how good I was at drawing shapes with horizontal symmetry.
Instead of just doing a visual inspection, I tried to do it with the help of an image editor. After I scanned the drawing, I aligned each shape around the horizontal axis (horizontal axis of the shape coinciding with the horizontal axis of the page). Next, I copied the resulting layer, and mirrored it horizontally. Then I set the layer mode to display the difference between the original and the mirrored layer. This gave me a mostly black image, so I laid a white layer on top, with the layer mode set to display the difference between it and the layers below.
You can see I made a mistake in the fifth line (forgot the H).
Of course, normally you would use the mirror page horizontally feature to check for proper horizontal symmetry in an image editor. However, you can’t export that into a static image.
That is all.