How do I interpret touch screen HID data

[Tariq Ayad]

Hi,

I am trying to track touch screen interaction in my program (the device is Surface Pro 3).

I have been able to successfully register my device for Raw HID input notification and get back WM_INPUT messages when the screen is touched. However I do not know how to interpret this data and get touch screen coordinates.

What i get is a byte array (seems to be 27 bytes long).

As you can see from another post of mine I have had a little bit of success however I am not sure if am on the right path.

For each touch input I get tagRAWHID.dwSizeHid = 27 and tagRAWHID.dwCount = 1;

I assumed that bytes 6 to 10 were the x coordinate and 10 to 14 were the y coordinate. I feel this might be right becaue if I touch the left edge of the screen and drag till the very right edge, the x coordinate goes from 0 to 629155200 and vice versa.

My remaining problem is how do I detect when the right edge has been touched. When i ran the code on my Surface Pro 3, the right edge x-coordinate was 629155200 but running the same code on 7" windows 10 Tab the right edge is 9.


This is what the code looks like. Feel free to review it on github, its a code project sample that i modified to test out the HID touch input


rawtouch.cs

public void ProcessRawInput(IntPtr hdevice)
{
var size = 0;

// Determine Size to be allocated

var ret = Win32.GetRawInputData(hdevice, DataCommand.RID_INPUT, IntPtr.Zero, ref size, Marshal.SizeOf(typeof(Rawinputheader)));

if (ret == -1)
{
Console.WriteLine("error");
return;
}

int sizeToAllocate = Math.Max(size, Marshal.SizeOf(typeof(RawInput_Marshalling)));

IntPtr pData = Marshal.AllocHGlobal(sizeToAllocate);
try
{
//Populate alocated memory
ret = Win32.GetRawInputData(hdevice, DataCommand.RID_INPUT, pData, ref sizeToAllocate, Marshal.SizeOf(typeof(Rawinputheader)));

if (ret == -1)
{
throw new System.ComponentModel.Win32Exception();
}

Rawinputheader header = (Rawinputheader) Marshal.PtrToStructure(pData, typeof(Rawinputheader));

//RAWINPUT starts with RAWINPUTHEADER, so we can do this
RawInputDeviceType type = (RawInputDeviceType)header.dwType;
switch (type)
{
case RawInputDeviceType.RIM_TYPEHID:
{
//As described on page of RAWHID, RAWHID needs special treatement
RawInput_Marshalling raw = (RawInput_Marshalling) Marshal.PtrToStructure(pData, typeof(RawInput_Marshalling));

//Get marshalling version, it contains information about block size and count
RawInput_NonMarshalling raw2 = default(RawInput_NonMarshalling);

//Do some copying
raw2.header = raw.header;
raw2.hid.dwCount = raw.hid.dwCount;

raw2.hid.dwSizHid = raw.hid.dwSizHid;

var numBytes = raw.hid.dwCount * raw.hid.dwSizHid;
byte[] data = new byte[numBytes];

//Allocate array
//Populate the array
IntPtr rawData = new IntPtr(
pData.ToInt64() + Marshal.SizeOf(typeof(Rawinputheader)) + Marshal.SizeOf(typeof(Rawhid_Marshalling)));

Marshal.Copy(rawData, data, 0, (int)numBytes);

// Extract X & Y
byte[] xBytes = new byte[4];
Buffer.BlockCopy(data, 6, xBytes, 0, 4);
byte[] yBytes = new byte[4];
Buffer.BlockCopy(data, 10, yBytes, 0, 4);


int x = BitConverter.ToInt32(xBytes, 0);
int y = BitConverter.ToInt32(yBytes, 0);


Console.WriteLine($"X: {x}\tY: {y}");
if (TouchActivated != null)
{
TouchActivated(this, new RawInputEventArg(x, y));
}
//return raw2;
break;
}
default:
{
//No additional processing is needed
var x = (RawInput_Marshalling)Marshal.PtrToStructure(pData, typeof(RawInput_Marshalling));
break;
}
}
}
finally
{
Marshal.FreeHGlobal(pData);
}
}

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