I’m not sure what the actual word for it is but in SAI if you press H you can mirror an image (it doesn’t actually flip the image, just the view) and it was great for checking how your image looks since it can look fine on one view but distorted mirrored so I used it a lot for anatomy or difficult shapes like circles.
I can’t seem to find the hotkey for this in mypaint though, nor can I find the rotate canvas key (again, doesn’t rotate the image, just the view) so I thought I’d ask what those keys are here.
@Kazekai
You’re looking for our “Mirror Horizontal” and “Mirror Vertical” commands, I think? As you say they’re great for getting proportions right just like the old paper-era trick with a mirror.
You’ll find them in the menus under View → Adjust View, or on the I and U keys by default. Mirror Horizontal is on the toolbar too - it’s one of the “View (Main)” selection.