Android canvas How do I clear delete contents of a canvas bitmaps living in a surfaceView

Question

In order to make a simple game  I used a template that draws a canvas with bitmaps like this   private void doDraw Canvas canvas        for  int i 0 i lt 8 i            for  int j 0 j lt 9 j                for  int k 0 k lt 7 k            canvas drawBitmap mBits allBits i  j  k    i 50 -k 7  j 50 -k 7  null          The canvas is defined in  run      the SurfaceView lives in a GameThread    My first question is how do I clear  or redraw  the whole canvas for a new layout  Second  how can I update just a part of the screen       This is the routine that calls  doDraw   public void run         while  mRun            Canvas c   null          try               c   mSurfaceHolder lockCanvas null               synchronized  mSurfaceHolder                    if  mMode    STATE RUNNING                       updateGame                    doDraw c                        finally               if  c    null                    mSurfaceHolder unlockCanvasAndPost c

User · Answer

Found this in google groups and this worked for me..

Paint clearPaint = new Paint();
clearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
canvas.drawRect(0, 0, width, height, clearPaint);

This removes drawings rectangles etc. while keeping set bitmap..

User · Answer

I tried the answer of  mobistry   canvas drawColor Color TRANSPARENT  Mode CLEAR    But it doesn t worked for me    The solution  for me  was   canvas drawColor Color TRANSPARENT  Mode MULTIPLY    Maybe some one has the same problem

User · Answer

Your first requirement  how to clear or redraw whole canvas - Answer -  use canvas drawColor color Black  method for clearing the screen with a color of black or whatever you specify    Your second requirement  how to update part of the screen - Answer - for example if you want to  keep all other things unchanged on the screen but in a small area of screen to show an integer say counter  which increases after every five seconds  then use canvas drawrect method to draw  that small area by specifying left top right bottom and paint  then compute your counter value using postdalayed for 5 seconds etc   llike Handler postDelayed Runnable Object  5000      convert it to text string  compute the x and y coordinate in this small rect and use  text view to display the changing counter value

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In my case  I draw my canvas into linearlayout  To clean and redraw again      LinearLayout linearLayout   findViewById R id myCanvas       linearLayout removeAllViews     and then  I call the class with the new values      Lienzo fondo   new Lienzo this items       linearLayout addView fondo    This is the class Lienzo  class Lienzo extends View       Paint paint      RectF contenedor      Path path      ArrayList lt Items gt elementos       public Lienzo Context context ArrayList lt Items gt  elementos            super context           this elementos elementos          init               private void init             path new Path            paint   new Paint            contenedor   new RectF            paint setStyle Paint Style FILL                Override     protected void onDraw Canvas canvas            super onDraw canvas            contenedor left   oneValue          contenedor top   anotherValue          contenedor right   anotherValue          contenedor bottom   anotherValue           float angulo   -90    starts drawing at 12 o clock           total  sum of all element values         for  int i 0 i lt elementos size   i                 if  elementos get i  angulo  0  amp  amp  elementos get i  visible                   paint setColor elementos get i  backColor                   canvas drawArc contenedor angulo  float  elementos get i  value 360  total true paint                    angulo   float  elementos get i  value 360  total                            for example

User · Answer

Try to remove the view at onPause   of an activity and add onRestart    LayoutYouAddedYourView addView YourCustomView   LayoutYouAddedYourView removeView YourCustomView    The moment you add your view  onDraw   method would get called   YourCustomView  is a class which extends the View class

User · Answer

canvas drawColor Color TRANSPARENT  Mode MULTIPLY

User · Answer

In my case  creating canvas every time worked for me  even though it s not memory-friendly Bitmap bm   BitmapFactory decodeResource getResources    R drawable image   imageBitmap   Bitmap createBitmap bm getWidth    bm getHeight    bm getConfig     canvas   new Canvas imageBitmap   canvas drawBitmap bm  0  0  null

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I had to use a separate drawing pass to clear the canvas  lock  draw and unlock    Canvas canvas   null  try       canvas   holder lockCanvas        if  canvas    null               exit drawing thread         break            canvas drawColor colorToClearFromCanvas  PorterDuff Mode CLEAR     finally       if  canvas    null            holder unlockCanvasAndPost canvas

User · Answer

use the reset method of Path class   Path reset

User · Answer

How do I clear  or redraw  the WHOLE canvas for a new layout    try at the game      Just call Canvas drawColor Color BLACK   or whatever color you want to clear your Canvas with      And  how can I update just a part of the screen     There is no such method that just update a  part of the screen  since Android OS is redrawing every pixel when updating the screen  But  when you re not clearing old drawings on your Canvas  the old drawings are still on the surface and that is probably one way to  update just a part  of the screen   So  if you want to  update a part of the screen   just avoid calling Canvas drawColor   method

User · Answer

With the following approach  you can clear the whole canvas or just a part of it  Please do not forget to disable Hardware acceleration since PorterDuff Mode CLEAR doesn   t work with hardware acceleration and finally call setWillNotDraw false  because we override the onDraw method    view s constructor setWillNotDraw false   setLayerType LAYER TYPE SOFTWARE  null      view s onDraw Paint TransparentPaint   new Paint    TransparentPaint setXfermode new PorterDuffXfermode PorterDuff Mode CLEAR    canvas drawRect 0  0  width  height  TransparentPaint

User · Answer

mBitmap eraseColor Color TRANSPARENT    canvas drawBitmap mBitmap  0  0  mBitmapPaint

User · Answer

please paste below code on surfaceview extend class constructor               constructor coding      SurfaceHolder holder   getHolder        holder addCallback this        SurfaceView sur    SurfaceView findViewById R id surfaceview       sur setZOrderOnTop true         necessary     holder   sur getHolder        holder setFormat PixelFormat TRANSPARENT     xml coding       lt com welcome panelview PanelViewWelcomeScreen         android id    id one          android layout width  600px          android layout height  312px          android layout gravity  center          android layout marginTop  10px          android background   drawable welcome    gt    try above code

User · Answer

Don t forget to call invalidate     canvas drawColor backgroundColor   invalidate    path reset

User · Answer

Just call   canvas drawColor Color TRANSPARENT

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For me calling Canvas drawColor Color TRANSPARENT  PorterDuff Mode CLEAR  or something similar would only work after I touch the screen  SO I would call the above line of code but the screen would only clear after I then touched the screen  So what worked for me was to call invalidate   followed by init   which is called at the time of creation to initialize the view   private void init         setFocusable true       setFocusableInTouchMode true       setOnTouchListener this        mPaint   new Paint        mPaint setAntiAlias true       mPaint setDither true       mPaint setColor Color BLACK       mPaint setStyle Paint Style STROKE       mPaint setStrokeJoin Paint Join ROUND       mPaint setStrokeCap Paint Cap ROUND       mPaint setStrokeWidth 6        mCanvas   new Canvas        mPaths   new LinkedList lt  gt          addNewPath

User · Answer

Draw transparent color with PorterDuff clear mode does the trick for what I wanted   Canvas drawColor Color TRANSPARENT  PorterDuff Mode CLEAR

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Here is the code of a minimal example showing that you always have to redraw every pixel of the Canvas at each frame   This activity draw a new Bitmap every second on the SurfaceView  without clearing the screen before  If you test it  you will see that the bitmap is not always written to the same buffer  and the screen will alternate between the two buffers   I tested it on my phone  Nexus S  Android 2 3 3   and on the emulator  Android 2 2    public class TestCanvas extends Activity        Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView new TestView this             class TestView extends SurfaceView implements SurfaceHolder Callback        private TestThread mThread      private int mWidth      private int mHeight      private Bitmap mBitmap      private SurfaceHolder mSurfaceHolder       public TestView Context context            super context           mThread   new TestThread            mBitmap   BitmapFactory decodeResource context getResources    R drawable icon           mSurfaceHolder   getHolder            mSurfaceHolder addCallback this               Override     public void surfaceChanged SurfaceHolder holder  int format  int width              int height            mWidth   width          mHeight   height          mThread start                Override     public void surfaceCreated SurfaceHolder holder      Do nothing           Override     public void surfaceDestroyed SurfaceHolder holder            if  mThread    null  amp  amp  mThread isAlive                mThread interrupt               class TestThread extends Thread            Override         public void run                 while   isInterrupted                      Canvas c   null                  try                       c   mSurfaceHolder lockCanvas null                       synchronized  mSurfaceHolder                            c drawBitmap mBitmap   int   Math random     mWidth    int   Math random     mHeight   null                                           finally                       if  c    null                          mSurfaceHolder unlockCanvasAndPost c                                      try                       sleep 1000                     catch  InterruptedException e                        interrupt

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Erasing on Canvas in java android is similar erasing HTML Canvas via javascript with globalCompositeOperation  The logic was similar   U will choose DST OUT  Destination Out  logic   paint setXfermode new PorterDuffXfermode PorterDuff Mode DST OUT      Note  DST OUT is more useful because it can erase 50  if the paint color have 50  alpha  So  to clear completely to transparent  the alpha of color must be 100   Apply paint setColor Color WHITE  is recommended  And make sure that the canvas image format was RGBA 8888   After erased  go back to normal drawing with SRC OVER  Source Over    paint setXfermode new PorterDuffXfermode PorterDuff Mode SRC OVER      Update small area display literally will need to access graphic hardware  and it maybe not supported   The most close for highest performance is using multi image layer

[android] Android, canvas: How do I clear (delete contents of) a canvas (= bitmaps), living in a surfaceView?

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