Develop Reference

How to draw on canvas when touched

I am unable to get the canvas object in ontouch().Without the canvas I cannot draw a circle when touched.How can I draw any shape or image when touched
public class Board extends View implements View.OnTouchListener {
public Board(Context context) {
super(context);
Paint paint1 = new Paint();
paint1.setTextSize(50);
paint1.setColor(Color.WHITE);
View view=this;
view.setOnTouchListener(this);
}
#Override
public void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawRGB(200, 100, 0);
}
#Override
public boolean onTouch(View v, MotionEvent event) {
final int action = MotionEventCompat.getActionMasked(event);
int pointer = MotionEventCompat.getActionIndex(event);
if (action == MotionEvent.ACTION_DOWN) {
canvas.drawCircle(70, 1100, 50, paint1);
}
return false;
}

To draw on canvas wherever you touch, you need a path to keep track of your touch points and path. Using the path object you can draw on canvas. See this answer Draw on touch

canvas not drawing images in another method

I have extended View to use canvas. I have draw basic drawings in onDraw() method, when user touches in the canvas I have to draw an image there, for that I have used canvas inside onTouchEvent() method,it is not drawing anything there, the code is given below, what is the problem and how can i resolve this
public class ScreenView extends View(){
static Canvas canvas;
Bitmap bm;
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
this.canvas = canvas;
bm = BitmapFactory.decodeResource(getResources(),
R.drawable.ic_launcher);
canvas.draw.......
......
...........
}
public boolean onTouchEvent(final MotionEvent event) {
handleTouches(event.getX(), event.getY());
return false;
}
public void handleTouches(float x, float y) {
xLocTouch = (int) x;
yLocTouched = (int) y;
Paint paint = new Paint();
paint.setColor(Color.BLACK);
canvas.drawBitmap(bm, xLocTouch ,yLocTouched , paint);
}
}

You should call invalidate() method inside onTouchEvent, then your onDraw() method will be called, and you just should store your x and y coordinates, and then draw bitmap to this coordinates, like this:
public class ScreenView extends View {
int xLocTouched;
int yLocTouched;
Bitmap bm;
protected void onDraw(final Canvas canvas) {
super.onDraw(canvas);
bm = BitmapFactory.decodeResource(getResources(),
R.drawable.ic_launcher);
//your basic drawings also should depends on xLocTouched and yLocTouched.
Paint paint = new Paint();
paint.setColor(Color.BLACK);
canvas.drawBitmap(bm, xLocTouched ,yLocTouched , paint);
}
public boolean onTouchEvent(final MotionEvent event) {
xLocTouched = (int) event.getX();
yLocTouched = (int) event.getY();
invalidate();
return false;
}
}

Draw circle on touch with own class

I want to place red circles where I touch an image and have a class that listens to the touch and sends the coordinates to an other class:
public class Report extends Fragment {
private Context activity;
private Point point = new Point();
private DrawingCrl imgCircle = new DrawingCrl();
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.report, container,
false);
activity = this.getActivity();
//Where I'm doing the touching and respond to that:
final View touchView = rootView.findViewById(R.id.ImageC);
touchView.setOnTouchListener(new View.OnTouchListener() {
#Override
public boolean onTouch(View v, MotionEvent event) {
point.x = Float.valueOf(event.getX());
point.y = Float.valueOf(event.getY());
touchView = imgCircle.DrawCircle(Point point);
return true;
}
});
return rootView;
public class Point {
float x;
float y;
}
}
When I call the DrawCircle I want the class DrawingCrl to do just that, draw a circle for me on the point I send to the method:
public class DrawingCrl {
private Bitmap mBitmap;
private Paint paint;
private Canvas canvas = new Canvas();
public void DrawCircle(Point point) {
mBitmap = Bitmap.createBitmap(400, 800, Bitmap.Config.ARGB_8888);
paint = new Paint();
paint.setColor(Color.RED);
paint.setStyle(Style.FILL);
canvas.drawCircle(point.x, point.y, 50, paint);
}
}
I have read both getting X and Y coordinates and drawing circle and Draw Circle on touch but don't get how to do the drawing of the circle.
I'm pretty new to android so sorry my many noob failures. I hope you can help me get this working! Thanks! :)

Okay, here's how I would do it but I'm going to change your code a bit.
First I would change your DrawingCrl class to just store location (and possibly Paint object but this would probably fit better as a static shared by all DrawingCrls) like this:
public class DrawingCrl {
public Point myLoc;
public Paint paint;
public void DrawCircle(Point point) {
paint = new Paint();
paint.setColor(Color.RED);
paint.setStyle(Style.FILL);
myLoc = point;
}
}
Save a Vector of DrawingCrl objects created in your Activity and create new ones in the onTouch event handler.
This goes in the Report class:
java.util.Vector<DrawingCrl> myCircles = new java.util.Vector<DrawingCrl>();
Then in 'onTouch' create a new circle object and save it:
#Override
public boolean onTouch(View v, MotionEvent event) {
point.x = Float.valueOf(event.getX());
point.y = Float.valueOf(event.getY());
// create a new Circle here and save it
DrawingCrl c = new DrawingCrl(point);
myCircles.add(c);
return true;
}
Then I would override the 'onDraw' method of your View 'touchView'.
The 'onDraw' will give you the Canvas to draw to, so don't create your own like you are doing in your DrawingDMG class right now.
#Override
protected void onDraw(Canvas c)
{
for (DrawingCrl c : myCircles)
{
c.drawCircle(c.myLoc.x, c.myLoc.y, 50, c.paint);
}
}

how to get ontouch event in bitmap inside canvas?

i have a bitmap inside a canvas.the class implements ontouchlistener.i need to hide the image while touch on the image.
class Panel extends View implements View.OnTouchListener {
Paint linepaint=new Paint();
public Panel(Context context) {
super(context);
}
#Override
public void onDraw(Canvas canvas) {
Bitmap imgtable = BitmapFactory.decodeResource(getResources(), R.drawable.table_01);
canvas.drawBitmap(imgtable, centrex, centrey, null);
public boolean onTouch(View v, MotionEvent event) {
// TODO Auto-generated method stub
return false;
}
}

Make on touch listener for the view where you draw the image. And there change the visibility of the view. Not in onDraw();
Edit:
If you set the Bitmap image for example on a ImageView, then onTouchListener() must be applied on the ImageView. Bitmap image is only a data that is supplied to the view(s).

Erase bitmap parts using PorterDuff mode

I try to erase parts of a bitmap in my Android application by using Porter-Duff Xfermodes.
I have a green background which is overlayed by a blue bitmap. When I touch the screen a "hole" in the overlaying bitmap is supposed to be created making the green background visible. Instead of a hole my current code produces a black dot.
Below is my code. Any ideas, what I am doing wrong here?
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN);
setContentView(new DrawView(this));
}
public class DrawView extends View implements OnTouchListener {
private int x = 0;
private int y = 0;
Bitmap bitmap;
Canvas bitmapCanvas;
private final Paint paint = new Paint();
private final Paint eraserPaint = new Paint();
public DrawView(Context context) {
super(context);
setFocusable(true);
setFocusableInTouchMode(true);
this.setOnTouchListener(this);
// Set background
this.setBackgroundColor(Color.GREEN);
// Set bitmap
bitmap = Bitmap.createBitmap(320, 480, Bitmap.Config.RGB_565);
bitmapCanvas = new Canvas();
bitmapCanvas.setBitmap(bitmap);
bitmapCanvas.drawColor(Color.BLUE);
// Set eraser paint properties
eraserPaint.setAlpha(0);
eraserPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
eraserPaint.setAntiAlias(true);
}
#Override
public void onDraw(Canvas canvas) {
bitmapCanvas.drawColor(Color.BLUE);
bitmapCanvas.drawCircle(x, y, 10, eraserPaint);
canvas.drawBitmap(bitmap, 0, 0, paint);
}
public boolean onTouch(View view, MotionEvent event) {
x = (int) event.getX();
y = (int) event.getY();
invalidate();
return true;
}
}

Here is working code... may help somebody
public class ImageDemo extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(new Panel(this));
}
class Panel extends View {
private Bitmap mBitmap;
private Canvas mCanvas;
private Path mPath;
private Paint mPaint;
Bitmap bitmap;
Canvas pcanvas;
int x = 0;
int y =0;
int r =0;
public Panel(Context context) {
super(context);
Log.v("Panel", ">>>>>>");
setFocusable(true);
setBackgroundColor(Color.GREEN);
// setting paint
mPaint = new Paint();
mPaint.setAlpha(0);
mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
mPaint.setAntiAlias(true);
// getting image from resources
Resources r = this.getContext().getResources();
Bitmap bm = BitmapFactory.decodeResource(getResources(), R.drawable.mickey);
// converting image bitmap into mutable bitmap
bitmap = bm.createBitmap(295, 260, Config.ARGB_8888);
pcanvas = new Canvas();
pcanvas.setBitmap(bitmap); // drawXY will result on that Bitmap
pcanvas.drawBitmap(bm, 0, 0, null);
}
#Override
protected void onDraw(Canvas canvas) {
// draw a circle that is erasing bitmap
pcanvas.drawCircle(x, y, r, mPaint);
canvas.drawBitmap(bitmap, 0, 0,null);
super.onDraw(canvas);
}
#Override
public boolean onTouchEvent(MotionEvent event) {
// set parameter to draw circle on touch event
x = (int) event.getX();
y = (int) event.getY();
r =20;
// At last invalidate canvas
invalidate();
return true;
}
}
}

First thought, I'm not sure if setting alpha to 0 on your erase paint object is a good idea. That might make the whole thing ineffective.
Also, you should always use Bitmap.Config.ARGB_8888 if you're dealing with alphas.
If you're having trouble with the PorterDuff stuff, though, I would suggest simplifying your approach to ONLY do that (temporarily). That will help you narrow down the part which isn't working. Comment out everything to do with touch and view updates.
Then you can single out what part of the drawing isn't working right. Set up your constructor like this:
DrawView()
{
/* Create the background green bitmap */
...
/* Create foreground transparent bitmap */
...
/* Draw a blue circle on the foreground bitmap */
...
/* Apply the foreground to the background bitmap
using a PorterDuff method */
...
}
onDraw()
{
/* Simply draw the background bitmap */
...
}
If you set things up like that, you should be able to tell how your PD method is affecting the green bitmap, and change things accordingly.

Here is another advancement for your solution ... See Demo example
public class MainActivity extends Activity {
Bitmap bp;
Canvas bitmapCanvas;
DrawView drawImg;
LinearLayout ln1;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ln1 = (LinearLayout) findViewById(R.id.ln1);
drawImg = new DrawView(this);
ln1.addView(drawImg);
}
public class DrawView extends View implements View.OnTouchListener {
private int x = 0;
private int y = 0;
Bitmap bitmap;
Path circlePath;
Paint circlePaint;
private final Paint paint = new Paint();
private final Paint eraserPaint = new Paint();
public DrawView(Context context){
super(context);
setFocusable(true);
setFocusableInTouchMode(true);
this.setOnTouchListener(this);
// Set background
this.setBackgroundColor(Color.CYAN);
bp = BitmapFactory.decodeResource(getResources(), R.drawable.bg);
// Set bitmap
bitmap = Bitmap.createBitmap(320, 480, Bitmap.Config.ARGB_8888);
bitmapCanvas = new Canvas();
bitmapCanvas.setBitmap(bitmap);
bitmapCanvas.drawColor(Color.TRANSPARENT);
bitmapCanvas.drawBitmap(bp, 0, 0, null);
circlePath = new Path();
circlePaint = new Paint();
circlePaint.setAntiAlias(true);
circlePaint.setColor(Color.BLUE);
circlePaint.setStyle(Paint.Style.STROKE);
circlePaint.setStrokeJoin(Paint.Join.MITER);
circlePaint.setStrokeWidth(4f);
// Set eraser paint properties
eraserPaint.setAlpha(0);
eraserPaint.setStrokeJoin(Paint.Join.ROUND);
eraserPaint.setStrokeCap(Paint.Cap.ROUND);
eraserPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
eraserPaint.setAntiAlias(true);
}
#Override
public void onDraw(Canvas canvas) {
canvas.drawBitmap(bitmap, 0, 0, paint);
bitmapCanvas.drawCircle(x, y, 30, eraserPaint);
canvas.drawPath(circlePath, circlePaint);
}
public boolean onTouch(View view, MotionEvent event) {
x = (int) event.getX();
y = (int) event.getY();
bitmapCanvas.drawCircle(x, y, 30, eraserPaint);
circlePath.reset();
circlePath.addCircle(x, y, 30, Path.Direction.CW);
int ac=event.getAction();
switch(ac){
case MotionEvent.ACTION_UP:
Toast.makeText(MainActivity.this, String.valueOf(x), Toast.LENGTH_SHORT).show();
circlePath.reset();
break;
}
invalidate();
return true;
}
}
}
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