Develop Reference

Update Canvas Preserving Old Canvas Details

Hi I'm developing a game in android, for that I have draw 8*8 rectangles using canvas.drawRect() method, now what I need is when user touches on any of the rect, its color has to change. For this I have done as follows.
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;
outerLoop: for (int i = 0; i < 8; i++) {
for (int j = 0; j < 8; j++) {
if (GameView.tiles[i][j].rect.contains(xLocTouch, yLocTouched)) {
touched = true;
xTouched = i;
yTouched = j;
break outerLoop;
}
}
}
}
protected void onDraw(Canvas canvas) {
for (int i = 0; i < ROWS; i++) {
for (int j = 0; j < COLS; j++) {
canvas.drawRect(tiles[i][j].rect, paint);
canvas.drawRect(tiles[i][j].rect, p);
if(touched && i==xTouched && j == yTouched) {
Paint touchedColor = new Paint();
touchedColor.setColor(Color.BLUE);
canvas.drawRect(tiles[i][j].rect, touchedColor);
}
}
}
This code is works properly, but the problem is when I touch first time that rect's color changes, but for the second touch it erases old touches position. I need to keep all the touched rect as different color. Is there any way for this?

A Boolean array that tracks the touch state of each rectangle would do the trick. I can't see the external code, but this could be an additional field in your GameView class, and updated appropriately in your handleTouches method.
A perhaps less efficient, less elegant solution would be to just not set the color of the rectangle in onDraw if it's already Color.BLUE (modify your if statement appropriately). You can get the color of the touched pixel using this SE answer, but be forewarned you have to first turn the canvas into a bitmap in order to sample the color (hence the inefficiency).

How to avoid ghost drawing after clearing SurfaceView canvas

Hi I am working on plotting a real time graph of incoming signals using SurfaceView.
The sampling rate is 128Hz and the target graph refresh rate is 50Zh.
Things run pretty smoothly, the points are drawn real-time properly.
I plot the data in segments of a few points using Path()
for each segment I call path.computeBounds() to get a rect that I will use to call holder.lockCanvas(rect) and draw the path. Using a rect prevents flickering and reduces cpu usage
when the graph reaches the end I lock the entire canvas and clear the background, draw the graph frame and then continue on plotting.
the problem is that at the beginning of each new "page" I get a ghost image from the last page:
I believe this is caused by double buffering / use of a dirty area when plotting.
I have looked for solutions to this problem but none seem adequate for this type of application. Any help is most welcome.
Thanks
Jean-Pierre
Code follows:
private void draw() {
Point point = null;
Canvas canvas = null;
Path path = new Path();
ArrayList<Point> pointArray;
float oldX = -1;
boolean setToClear = false;
boolean isNewSegment = false;
if (samplesInQueue == 0) {
return;
}
pointArray = new ArrayList<Point>((int) samplesInQueue);
for (int i = 0; i < samplesInQueue; i++) {
// take a peek at the point without retrieving it from the point
// queue
point = Points.peek();
// check if first point of segment is the start of a page
if (i == 0) {
if (lastSegmentEndPoint != null) {
if (point.x < lastSegmentEndPoint.x) {
// yes then we will need to clear the screen now
isNewSegment = true;
}
} else {
// yes then we will need to clear the screen now
isNewSegment = true;
}
}
if (point != null) {
if (point.x > oldX) {
// put consecutive points in the path point array
point = Points.poll();
samplesInQueue--;
pointArray.add(point);
oldX = point.x;
} else {
// we have a wrap around, stop and indicate we need to clear
// the screen on the next pass
if (!isNewSegment) {
setToClear = true;
}
break;
}
}
}
// no points, return
if (pointArray.size() == 0) {
return;
}
// fill the path
for (int i = 0; i < pointArray.size(); i++) {
Point p = pointArray.get(i);
if (i == 0) {
if (lastSegmentEndPoint != null) {
if (p.x >= lastSegmentEndPoint.x) {
// if we have the end of the last segment, move to it
// and line to the new point
path.moveTo(lastSegmentEndPoint.x, lastSegmentEndPoint.y);
path.lineTo(p.x, p.y);
} else {
// otherwise just line to the new point
path.moveTo(p.x, p.y);
}
} else {
path.moveTo(p.x, p.y);
}
} else {
path.lineTo(p.x, p.y);
}
}
if (clear || isNewSegment) {
if (clear) {
clear = false;
}
// we need to clear, lock the whole canvas
canvas = holder.lockCanvas();
// draw the graph frame / scales
drawGraphFrame = true;
drawGraphFrame(canvas);
} else {
// just draw the path
RectF bounds = new RectF();
Rect dirty = new Rect();
// calculate path bounds
path.computeBounds(bounds, true);
int extra = 0;
dirty.left = (int) java.lang.Math.floor(bounds.left - extra);
dirty.top = (int) java.lang.Math.floor(bounds.top - extra);
dirty.right = (int) java.lang.Math.round(bounds.right + 0.5);
dirty.bottom = (int) java.lang.Math.round(bounds.bottom + 0.5);
// just lock what is needed to plot the path
canvas = holder.lockCanvas(dirty);
}
// draw the path
canvas.drawPath(path, linePaint);
// unlock the canvas
holder.unlockCanvasAndPost(canvas);
// remember last segment end point
lastSegmentEndPoint = pointArray.get(pointArray.size() - 1);
// set clear flag for next pass
if (setToClear) {
clear = true;
}
}
Draw frame / clear graph code
private void drawGraphFrame(Canvas canvas) {
if (!drawGraphFrame) {
return;
}
if (canvas == null) {
Log.e(TAG, "trying to draw on a null canvas");
return;
}
drawGraphFrame = false;
// clear the graph
canvas.drawColor(Color.BLACK, Mode.CLEAR);
// draw the graph frame
canvas.drawLine(leftMargin, topMargin, leftMargin, mCanvasHeight - bottomMargin, framePaint);
canvas.drawLine(leftMargin, mCanvasHeight - bottomMargin, mCanvasWidth - rightMargin, mCanvasHeight
- bottomMargin, framePaint);
// more drawing
}

Your problem is quite straight forward.. your only locking the new portion of the canvas that the new path covers. So the best thing to do is to make your path and dirty rect's private members of your class. Then at the start of your draw method get the path's current bounds (the old bounds) in your dirty rect. Now call path.rewind(); and start modifying your path. After do a union on the dirty rect with the new bounds. Now your dirty rect covers the old and new rect's. So your clear will remove the old path. This also reduces overhead because you don't want to be allocating 100+ objects per second for rect's and path's. Now since your drawing an oscilloscope then you probably want to adjust the old bounds to only be a portion of the width of the view. The same amount your new portion covers.
Hope that's cleared things up.

My simple answer is just using this function clear_holder() wherever you want to clear the canvas. I copy and paste 3 line for 3 times because it need 3 times clear to leave holder blank.
After clearing holder, you should draw any new thing you want!
This link give me this source code!
private void clear_holder(SurfaceHolder holder){
Canvas c = holder.lockCanvas();
c.drawColor( 0, PorterDuff.Mode.CLEAR );
holder.unlockCanvasAndPost(c);
c = holder.lockCanvas();
c.drawColor( 0, PorterDuff.Mode.CLEAR );
holder.unlockCanvasAndPost(c);
c = holder.lockCanvas();
c.drawColor( 0, PorterDuff.Mode.CLEAR );
holder.unlockCanvasAndPost(c);
}

It looks like you are clearing the canvas so, it's not double buffering problem. I think it's related to your path been reused.
Try adding adding the next line when starting new page.
path.reset();

Draw random bitmap in the onDraw method

Firstly I am adding my cards to a list like that:
private void initCards() {
for (int i = 0; i < 20; i++) {
Bitmap tempBitmap = BitmapFactory.decodeResource(myContext.getResources(),R.drawable.ic_launcher);
scaledCardW = (int) (screenW/10);
scaledCardH = (int) (scaledCardW*1.28);
Bitmap scaledBitmap = Bitmap.createScaledBitmap(tempBitmap,scaledCardW, scaledCardH, false);
deck.add(scaledBitmap);
}
But when it came to on draw method,my program is transforming a thing like an animation.
My onDraw method is here:
protected void onDraw(Canvas canvas) {
// TODO Auto-generated method stub
super.onDraw(canvas);
int left=newInteger.nextInt(screenW);
int right=newInteger.nextInt(screenH);
for(int i=0 ; i<k ;i++){
canvas.drawBitmap(deck.get(i), left, right, null);
}
}
These bitmaps must have random location and if I change the k from our main Activity different numbers of bitmaps must be drawen.Because of that,ı cannot remove the invalite() function.
Can anybody help me?Unfortunately,another topics about this problem are not providing my request.

Your bitmaps keep moving because you are randomizing their position every time you draw. Try moving these lines:
left = newInteger.nextInt(screenW);
right = newInteger.nextInt(screenH);
into the same function that changes the value of k.

When I try to create animated objects from sprite on Android, they start to be slow

I use followed example (described here) to animate my sprite sheet.
from example I wrote my Object:
public class Sprite {
private int x, y;
private int width, height;
private Bitmap b;
MainGamePanel ov;
int currentFrame = 0;
public Sprite(MainGamePanel mainGamePanel, Bitmap blob) {
this.ov = mainGamePanel;
this.b = blob;
// 1x12
height = b.getHeight();
width = b.getWidth()/12;
x = y = 50;
}
private void update(int dist) {
currentFrame = ++currentFrame % 12;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
//return _b;
}
public void draw(int shift, Canvas canvas, int dist) {
update(dist);
int srcX = currentFrame * width;
Rect src = new Rect(srcX, 0, srcX+width, height);
Rect dst = new Rect(x, y+shift, x+width, y+height+shift);
canvas.drawBitmap(b, src, dst, null);
}
Here every 100 msec I take different part from image (Bitmap) and show it.
1 -> 2 -> 3 -> 4 -> ... -> 12
So my creature flies and moves with wings.
If I show only 1 object, it seems good but when I try to run 20 creatures in loop:
Bitmap blob = BitmapFactory.decodeResource(getResources(), R.drawable.sprite3);
for(int i=0; i<20; i++){
spriteList.add(new Sprite(this, blob));
}
....
for(int i=0; i<spriteList.size(); i++){
sprite = spriteList.get(0);
sprite.draw(canvas, dist);
}
My objects start to be slow according to drawn object count.
It happens I think because of Thread.sleep(100);.
I don't see any performance problem.
Sounds like each object sleep pauses all objects.
For 20 objects this sleep grows to 2 sec.
For sure I use workaround like:
int sleep = 100/spriteList.size();
Thread.sleep(sleep);
But code looks messy.
Can anyone help me how to fix it?
Here is sprite3 image:
[EDIT]
Maybe I need create each object in separate Thread?

You should definitely not sleep while rendering; it greatly reduces the performance, especially, as you add new Animation Clips, etc. Also, don't create objects within your onDraw method and do try to reuse the Rect objects. Creating objects during rendering is very expensive.

Drawing random circles

I am trying to draw a cupola circles at random positions in an Android application.
I draw them on a bitmap and then draw that bitmap on the canvas. This is the function where a draw the circles:
private void drawRandomCircles(int numOfCircles) {
Canvas c = new Canvas(b);
Paint cPaint = new Paint;
cPaitn.setColor(Color.RED);
for(int i = 0; i < numOfCircles; i++) {
int x = Math.Random % 100;
int y = Math.Random % 100;
c.drawCircle(x, y, 20, cPaint)
}
}
The Bitmap b is global.
And after calling this function I just draw the bitmap in the onDraw method.
Now the problem is that I only get one circle drawn on the screen, no matter the size of numOfCircles.
Any clue what is happening here?

That code doesn't even compile. What is new Paint; for instance?
I suggest you log your arguments to drawCircle to make sure you draw them on different locations. If Math.Random for instance is a field, it would change in between reads, which would put the circles on top of each other.
If you intended to write Math.random() the error is that Math.random() returns a value between 0 and 1. You may want to use
Random r = new Random();
// your loop
int x = r.nextInt(100);
int y = r.nextInt(100);