Round Volume Knob

A round panel is a different layout problem, and worth one example on its own. There are no corners to put anything in, so it is drawn as the thing these panels get used as: a machined knob. What you turn is a dimple in its face - an arc with nothing drawn but its knob, whose background is a picture of a hollow with a shadow in it. A second arc round the outside is the light that shows the level, and the glass in the middle is a button: press the knob to mute. The dial writes the level and reads it back, so the number in the middle is the value the firmware holds - which is the pattern for anything the user sets and something else then acts on. Mute is a separate variable rather than level zero, so unmuting returns to where the dial was rather than to silence.