Scribble is a fast and lightweight app for hand drawings and sketches. It offers some tweaks like painting on background images, adjusting colors / strokewidth or two-finger clicks for straight lines. You may even disable system gestures, if they get in your way while drawing. A small lock symbol reminds you to re-enable them in order to close the app later.
Please note, if you add a photo to draw over, you won't be able to switch to another app before saving. If you switch, you'll lose composition with the background image.
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