[Nexus One General] flarbear:
I tried to leave a comment on your youtube page, but there wasn't enough room to go into details.

Basically, you are confusing "vague dual touch" for multitouch simply because your fingers are close to a right triangle.

The sensor reports up to 2 locations of touching vertically and up to 2 locations horizontally. The software attempts to correlate these 2 pairs of locations by using the history of "when they first appeared" to correlate the 2 sets of values into 2 distinct touches. This is why the sensor gets confused when the 2 touches cross a common axis (horizontally or vertically). With better heuristics they may be able to do a better job of associating the pairs of locations than they are today, but they will never resolve them as true independent pairs of xy coordinates.

Another problem with the sensor is that it lumps 2 touches that are close to each other vertically or horizontally into a single "wider" data point. This is why the rectangle "snaps" into a line when your 2 fingers get close to each other either horizontally or vertically. You can try this on the MT app and see that there is a minimum size to when it reports a rectangle versus reporting just a line when the fingers are any closer. Thus, 2 touches that are within a margin of error of each other vertically or horizontally are reported as a single "wider" sample on that axis.

Now to get back to what you are doing in your video. When your fingers are close to a right triangle the sensor is still only seeing 2 locations vertically and 2 locations horizontally because it lumps your right 2 fingers together vertically and your top 2 fingers together horizontally. It still thinks you are only touching it in 2 places because of this "axis aligned hiding of fingers behind each other" effect. If the app were displaying the distinct xy coordinates of the touches being reported by the driver then it would only draw 2 points that would probably align with your 2 diagonal fingers mostly. The problem with the MT app is that it is only displaying the circle and rectangle both of which fail to distinguish how many of the corners of the rectangle (or which corners) are being registered as touches and which are simply a byproduct of extrapolating the circle and rectangle from 2 touches.

If you move your fingers to a point where it can see 3 distinct vertical locations being touched (i.e. they are not close enough together to its margin of error where it lumps a pair of them into a single touch), then the reported points will freak out and the rectangle and circle will jump to an odd point on the screen. You actually see this happening in your video at around :10 seconds and :14 seconds. The reason the touch graphics are so off at that point is that your fingers have finally moved far enough apart along one of the axes for it to suddenly see 3 samples and it was unable to report them properly. Try this again, but use clearly distinct positions along the length of the display for your 3 fingers and you will never get a sane response out of the app until 2 of the fingers get close enough to each other to appear as one wider finger (when viewed from one of the axes).

Your video is not an example of true Multi-Touch, it is just a confusing artifact of the bizarre way that they implemented dual touch in the 2000 series touch sensor devices.