Thanks so much for the link to Rosenberg's paper. He does a great job of describing the advantages of the force sensing resistance (FSR) approach. The IFSR technique appears to simply be a very fine mesh FSR array. Sensel Morph (a product from Rosenberg's latest company!) appears to be a very high quality IFSR implementation with an extremely high resolution array and a refined manufacturing technique.
I have no doubt about this at all. It's something that fascinates me though, and I figure I might as well get around to gaining some first-hand experience with it.
That being said, the Sensel Morph is being sold at an extremely affordable price. It's almost inexpensive enough at retail to consider inclusion as-is in a a bespoke musical instrument. That's an amazing accomplishment. But we do need more first-hand experience with it before we can draw conclusions about its musicality.
It's interesting how much more complex @randy's TouchTracker.cpp is from anything I could find in the Sensel repository. Maybe I'm just not looking in the right place.
I'm not at all opposed to the idea of discrete pads. Pretty open right now to any approach I can make work with reasonable musicality. I would very much like to be able to have both quantized and unquantized note modes. My experiences playing the Roli Seaboard compared with experiences with various iPad apps tell me that much of the feeling of "notes" has to do with tactile response from the playing surface. The trick is to create a surface that supports both quantized modes (you can feel the "frets") without interfering with the unquantized mode (you can slide freely). I think Roli went a bit overboard trying to appease keyboard players, leaving them with an overly quantized (in the sense of tactile response) result. By contrast I've heard multiple musicians complain that it can be difficult to hit notes "correctly" on the smooth surface of the Haken Continuum.
But I am guessing that those issues, while not entirely independent of sensor design, are more likely to be more of a factor of playing surface design. As far as touch tracking algorithms go, we are blessed to have a choice of giant's shoulders to stand on. A pretty amazing situation to find ourselves in.
Even without pressure sensing, it's not impossible to get some sense of "force" from blob detection/interpretation in multitouch systems that only track X/Y. However, the sensitivity (and thus the musicality) of such approaches is greatly reduced. I find myself coming back to FSR-like implementations because they have such a direct approach to detecting pressure.
I've hesitated to pursue the Madrona Labs DIY Soundplane approach because I don't happen to have a spare 16 channels of audio I/O laying around. That's a somewhat expensive bit of kit for experimentation.
I've been writing this while reading the Rosenberg paper. As I continue to read I realize IFSR is not simply a very fine grain FSR array. It's more complex than that. Still quite inexpensive to manufacture (obviously, based on the price of the Sensel Morph) but I haven't read enough yet to realize exactly what the process is. However, I did arrive at the point where the algorithm for touch tacking is described (diagram on p.115), and it is of similar complexity to SoundPlane touch tracking.
Just went back and re-read the description of Rosenberg's manufacturing technique (p.70-73) and I must confess I didn't grok it thoroughly enough to understand how to replicate the process. I'll keep mentally digging at that point in hopes of a breakthrough.
Meanwhile, I'm also reading the Getting Started with CapSense guide. CapSense is the technology behind TouchKeys. I can easily imagine using a sandwich of CapSense sensors (for X/Y) and FSR (for pressure). Might be a very quick/easy/cheap way to build a set of discrete keys.
Thanks so much to all of you for your responses so far! I <3 llllllll!