Works best on desktop. Tap the background to show the control points, and change the image by dragging these points around. Scale the display area with the pinch-to-zoom gesture or with CTRL-[mouse wheel], and move around the image by dragging the background.
Keyboard & Mouse Controls
space: start/stop the drawing process
backspace: when a function is selected, remove that function
cmd/meta + drag: duplicate a function
modifier key actions
When dragging function system control points holding down different modifier keys will change the reulting behaviour, effects are as follows;
translation vector:
alt: constrain to nearest point lying on either horizontal, vertical, or diagonal
ctrl: snap to grid with 0.1 increments
individual basis vectors
alt: constrain basis to its original span
shift: allow rotation only
ctrl: snap to grid with 0.1 increments
combined basis vectors
no modifier: keeps original angle between basis and ratio of lengths, allows rotation and proportional scaling