Gijs Gieskes created this steampunk Image Scan Sequencer – a mechanical scanner that outputs MIDI:

It uses LDRs to measure the gray-scale of specific point of a image, and triggers midi notes from a selected threshold. When the threshold is reached the velocity will be set by the darkness at that point. the darker point the higher the velocity will be.

The sequencer plays the notes as a arpeggiator, i chose for this playback method because i dont have a midi device that can play 24 keys at the same time.. There are 2 different arpeggio modes. One rearranges the playback sequence to the active notes velocitys. And the second mode changes the arpeggio playback speed to the amount of notes that are active. If this mode is not selected the playback speed is set by a potentiometer. These modes can also be combined.

The image plays a modified version of Treatise by Cornelius Cardew. Details, more demo videos and code at Gieskes‘ site.