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Handling multitouch in Java ME
This article explains how to handle multi-touch interactions in Java ME, using the Nokia UI API
The Nokia UI API is a Nokia-proprietary extension to MIDP which offers additional audio and graphics capabilities. One of the useful features it delivers is full support for multitouch interactions, which are available on Series 40 Touch UI and Nokia Asha devices.
This article illustrated how these features can be used to handle multiple touches, and how those touches can be displayed on screen or used to perform specific tasks within a Java app.
The MultipointTouch API
The com.nokia.mid.ui.multipointtouch package contains the two objects that allow to easily implement multitouch handling capability in a Java app:
- the MultipointTouchListener interface is responsible for handling multipoint touch events
- the MultipointTouch class allows to add/remove listeners and to retrieve information about the touch events, such as their coordinates and status
The following sections show how multiple touches can be detected, handled and displayed on a LCDUI Canvas.
Handling multitouch events
Handling touches via tha MultipointTouch API requires the implementation of the MultipointTouchListener interface, that declares a single method:
- pointersChanged, that accepts as argument an int array containing the IDs of the touch events.
The following implementation of the pointersChanged event stores the pointer IDs array into an instance variable, and calls the Canvas repaint method to query a refresh of its user interface, so that the updated state of the touch events can be painted on the screen.
public class TouchTrackingCanvas extends Canvas implements MultipointTouchListener
int currentPointers = null;
public void pointersChanged(int pointerIds)
currentPointers = pointerIds;
Registering the MultipointTouchListener
Once the MultipointTouchListener interface has been implemented, it must be set as listener via the MultipointTouch.addMultipointTouchListener method, as shown by the following code snippet:
MultipointTouch multitouch = MultipointTouch.getInstance();
Displaying touch events
The MultipointTouch class defines 3 static method that can be used to retrieve the details for each of the notifies pointer touches:
- getX returns the X coordinate for the given pointer ID
- getY returns the Y coordinate for the given pointer ID
- getState return the touch state for the given pointer ID, where the state can assume one of the following values:
By using those methods, the Canvas instance defined above can paint the lasted notified touches on the screen as shown in the following code snippet:
protected void paint(Graphics g)
g.fillRect(0, 0, getWidth(), getHeight());
int pointSize = 20;
if(currentPointers != null)
for(int i = 0; i < currentPointers.length; i++)
int touchState = MultipointTouch.getState(currentPointers[i]);
if(touchState == MultipointTouch.POINTER_RELEASED)
int x = MultipointTouch.getX(currentPointers[i]);
int y = MultipointTouch.getY(currentPointers[i]);
g.fillArc(x - pointSize / 2, y - pointSize / 2, pointSize, pointSize, 0, 360);
The above code displays a red dot for each active touch (so, with its state being one of MultipointTouch.POINTER_PRESSED and MultipointTouch.POINTER_DRAGGED), and one gray dot for an inactive touch (with its state being MultipointTouch.POINTER_RELEASED).
Cropping an image with multitouch
This section describes how the multitouch features described above can be used to allow the user of a Java app to dynamically define an image area to be cropped, by adjusting the desired area with two fingers. The cropping feature will then be implemented by using the ImageTransformControl from the AMMS API.
The base Canvas
The starting point is a Canvas instance, that defines the Image to be cropped, and the coordinates of the image mask. The Canvas instance also register itself as a MultipointTouchListener, in order to receive multitouch events.
public class ImageCanvas extends Canvas implements MultipointTouchListener
Image image = null;
int maskX = 0;
int maskY = 0;
int maskWidth = 0;
int maskHeight = 0;
image = Image.createImage("/photo.jpg");
maskWidth = image.getWidth();
maskHeight = image.getHeight();
The Canvas paint method displays on screen both the Image and the mask, displayed as a red rectangle drawn above the image, as shown by the code snippet below.
Adjusting the image mask
The image mask can be adjusted by the user by dragging two fingers on the screen. The technique shown in the following code snippet looks for the minimum and maximum x/y coordinates of the first two active touches (so, excluding touches whose state is POINTER_RELEASED) and, if at least two of those are found, updates the mask coordinates with the new values, calling a repaint of the Canvas to show the updated mask on the screen.
public void pointersChanged(int pointerIds)
int activeTouches = 0;
// initialize maximum and minimum x/y coordinates
int xMin = image.getWidth(), yMin = image.getHeight(), xMax = 0, yMax = 0;
for(int i = 0; i < pointerIds.length; i++)
// check if this is an active touch
if(MultipointTouch.getState(pointerIds[i]) != MultipointTouch.POINTER_RELEASED)
// increment number of found active touches
// retrieve x/y coordinates of the touch event
int x = MultipointTouch.getX(pointerIds[i]);
int y = MultipointTouch.getY(pointerIds[i]);
// update maximum and minimum x/y coordinates
xMin = Math.min(xMin, x);
xMax = Math.max(xMax, x);
yMin = Math.min(yMin, y);
yMax = Math.max(yMax, y);
// if 2 active touches were found, stop looking for more
if(activeTouches == 2)
// if 2 touches were found, update the image mask with the new values
if(activeTouches == 2)
maskX = xMin;
maskY = yMin;
maskWidth = xMax - xMin;
maskHeight = yMax - yMin;
Cropping the image
The new features introduces by the AMMS API on the Asha software platform allow to perform the cropping of an image with few lines of code. The following code grabs a reference to the ImageTransformControl and appropriately sets the source rectangle and target size by using the mask coordinates and size. Done that, the MediaProcessor is initialized with the image input and a ByteArrayOutputStream, the output format is defined via the ImageFormatControl class, and the crop process is finally started.
MediaProcessor mp = GlobalManager.createMediaProcessor("image/raw");
ImageTransformControl transformControl = (ImageTransformControl)mp.getControl("javax.microedition.amms.control.imageeffect.ImageTransformControl");
transformControl.setSourceRect(maskX, maskY, maskWidth, maskHeight);
transformControl.setTargetSize(maskWidth, maskHeight, 0);
outputStream = new ByteArrayOutputStream();
ImageFormatControl fc = (ImageFormatControl)mp.getControl("javax.microedition.amms.control.ImageFormatControl");
The Nokia Asha SDK 1.0 emulator offers support for simulating two-touches interactions, by accessing the Tools menu and selecting Pinch-to-zoom simulation.
Once started, the multitouch tool allows to record two-fingers interactions directly on the simulator screen. Those interactions can then be played back so that multitouch can be properly simulated within the app.
This article illustrates how multitouch interactions can be handled in a Java app by using the features offered by the Nokia UI API, and more specifically by its com.nokia.mid.ui.multipointtouch package.
A real world case is presented to show how those features can be used in real apps to provide a better user experience is various scenarios, if compared to single-touch interactions.
Full source code of the Java app illustrated in this article is available: here: Media:WikiMultitouch.zip