Recognize physical object identities by decoding touch patterns on capacitive screens.
The code is written to work with .NET Framework >= 4.5 and references the Math.NET Spatial and Math.NET Numerics libraries for calculation.
Given a list of TouchPoint instances from WPF, call this API function:
public Touchcode Check(IList<TouchPoint> touchPoints, bool xMirror = true, int maxY = 1080)The xMirror flag should be set to true if y-coordinates of the screen start counting at the top of the screen. Additionally, maxY tells the system how many pixels the screen height is.
A more general way to check for Touchcodes that does not rely on WPF can be used like this:
public Touchcode Check(IList<Point2D> touchpoints, bool xMirror = true, int maxY = 1080)In both cases, if no Touchcode could be read, Touchcode.None will be returned. If a valid Touchcode is returned, the origin and the angle between the Touchcode and the positive y-axis can be retrieved.
Touchcodes are encoded using a physical 4x4 sized point grid in which points can be either on or off, meaning whether some capacity can be measured by the touch screen at that point (e.g. by inserting a grounded metal pin at that point.)
In order to find a Touchcode, three corner pins must always be on, so that an origin and two virtual axes vx and vy can be found by the system. The 16 points are numbered with an index 0 - 15, going left to right and top to bottom.
Four pins must always be the encoded the same way:
- pin 0:
on(vy) - pin 3:
off - pin 12:
on(origin) - pin 15:
on(vx)
That leaves us with 12 pins that can be used to encode a Touchcode, they are here called the "Touchcode pins". Each Touchcode pin is mapped to a single bit in a 12-bit number, so 4096 Touchcodes can be encoded, numbered 0 - 4095 or 0x000 - 0xFFF in hex.
The following example is the minimum configuration necessary to read a Touchcode (X meaning on and o meaning off):
X----o----o----o
| | | |
o----o----o----o
| | | | Touchcode 0x00 (0)
o----o----o----o (angle = 0°)
| | | |
X----o----o----X
None of the 12 Touchcode pins is on, so the recognized Touchcode will be 0x00.
To encode other Touchcodes, the 12 Touchcode pins can be set to on, with pin 1 encoding the least significant bit and pin 14 the most significant bit.
Some examples:
X----X----o----o
| | | |
o----o----o----o
| | | | Touchcode 0x01 (1)
o----o----o----o (angle = 0°)
| | | |
X----o----o----X
X----o----o----o
| | | |
o----o----o----o
| | | | Touchcode 0x800 (2048)
o----o----o----o (angle = 0°)
| | | |
X----o----X----X
X----X----o----o
| | | |
o----o----o----o
| | | | Touchcode 0x801 (2049)
o----o----o----o (angle = 0°)
| | | |
X----o----X----X
X----X----X----o
| | | |
X----X----X----X
| | | | Touchcode 0xFFF (4095)
X----X----X----X (angle = 0°)
| | | |
X----X----X----X
It is important that all pins have the same vertical and horizontal distance so that the system can recognize the grid.