Conceptually similar, there was also the ANS synthesizer [1] designed by the Soviet engineer Evgeny Murzin. Unlike the Mellotron, it was opto-electronic, giving the composer more direct access and control over the "samples". Waveforms were drawn on glass plates, and pressing one of the keys would scan across the associated plate, feeding the generated signal through various filters, etc.
It features prominently in the soundtrack by Eduard Artemyev, for the historical film Siberiade from 1979. [2]
I remember my dad telling me years and years ago about an arbitrary function generator that they used at university, where you had an oscilloscope with a cardboard mask representing the function's graph in front of the screen, and a light sensor in a light-tight funnel that controlled the spot height. When light shone on the sensor it would reduce the input voltage, and so the spot would trace the top of the cardboard mask.
Interestingly enough the documentary "Sisters with Transistors" show some British pioneers adopt a similar approach, but drawing on film rather than glass plates, in the 40s and 50s.
That was Daphne Oram, who was a pioneer and all-round innovator. You can find her book online and it's a fascinating insight into a different cultural universe than the one we have today.
But her Oramics system was more like an automation controller hand-painted onto film strip than optical waveform synthesis.
It was a direct ancestor of the automation in all modern DAWs.
That sounds similar to how the Hammond organ worked, I think. I believe it had spinning wheels with a wavy edge; light shone past the edge of a wheel generated an electrical waveform.
It features prominently in the soundtrack by Eduard Artemyev, for the historical film Siberiade from 1979. [2]
[1] https://en.m.wikipedia.org/wiki/ANS_synthesizer
[2] https://youtube.com/watch?v=sJj9y4t9UnU