SurfaceView.lockCanvas()无法正确清除位图缓冲区

Question

我创建了一个扩展SurfaceView的类,以便循环一系列ARGB位图.这主要起作用,除了基础位图的状态(通常但不总是)为每个新帧保留.

换句话说,如果第一帧I显示是不透明的,并且后续帧是透明的,则在绘制新帧时不会清除原始帧中的不透明像素.

这种行为让我感到困惑,因为SurfaceHolder.lockCanvas()的文档特别指出:

"在SurfaceCanvas()和lockCanvas()之间永远不会保留Surface的内容,因此,必须写入Surface区域中的每个像素."

如果我只有一个坚实的背景,那么调用canvas.drawARGB(255,0,0,0)成功将其清除为黑色...但我希望有一个透明的背景,我无法将其清除为透明color,因为canvas.drawARGB(0,0,0,0)没有效果.

import java.util.ArrayList;
import java.util.Random;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

/*
 * Accepts a sequence of Bitmap buffers and cycles through them.
 */

class AnimatedBufferView extends SurfaceView implements Runnable
{
    Thread thread = null;
    SurfaceHolder surfaceHolder;
    volatile boolean running = false;

    ArrayList<Bitmap> frames;
    int curIndex = 0;

    public AnimatedBufferView(ArrayList<Bitmap> _frames, Context context) 
    {
        super(context);
        surfaceHolder = getHolder();
        frames = _frames;
    }

    public void onResume(){
        running = true;
        thread = new Thread(this);
        thread.start();
    }

    public void onPause(){
        boolean retry = true;
        running = false;
        while(retry){
            try {
                thread.join();
                retry = false;
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    @Override
    public void run() 
    {
        // TODO Auto-generated method stub
        while(running)
        {
            if(surfaceHolder.getSurface().isValid())
            {               
                Canvas canvas = surfaceHolder.lockCanvas();

                //clear the buffer?
                //canvas.drawARGB(255, 0, 0, 0);

                //display the saved frame-buffer..
                Matrix identity = new Matrix();
                Bitmap frame = frames.get(curIndex);
                canvas.drawBitmap(frame, identity, null);


                surfaceHolder.unlockCanvasAndPost(canvas);

                curIndex = (curIndex + 1) % frames.size();

                try {
                    thread.sleep( 100 );
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }
}

Answer 1

好的,发现问题是默认的Porter-Duff绘图模式,这使得无法绘制透明颜色.只需改变模式.即

Canvas canvas surfaceView.lockCanvas();
canvas.drawColor(0, Mode.CLEAR);