在Android 2D游戏中使对象（位图）在画布上滚动

Question

我们知道，在2D游戏中移动对象非常容易。只需使用SurfaceView并使用Canvas.drawBitmap（）绘制单个位图。

但是当谈到滚动物体时，一个球，我怎么能做到这一点？使用变换矩阵会导致渲染位图的质量很差，不是吗？

Answer 1

如果要使用基于Canvas的绘图来旋转位图，则只需使用变换矩阵，或者通过传递给Canvas.drawBitmap（）的Matrix或直接在Canvas上调用的变换操作。如果你用Paint.setFilterBitmap(boolean)启用位图过滤，质量将会很好，至少按我的标准来看。

下面是一个完整的示例（基于矩阵），您可以随意使用它，特别是查看它对开启和关闭位图过滤的影响（或仅查看下面的截图）。它不使用SurfaceView虽然，只是一个普通的自定义视图，但它应该很容易地移植到SurfaceView：

CustomView.java：

package com.example.android; 

import android.content.Context; 
import android.content.res.Resources; 
import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 
import android.graphics.Canvas; 
import android.graphics.Matrix; 
import android.graphics.Paint; 
import android.util.TypedValue; 
import android.view.View; 

public class CustomView extends View { 
    private static final float DP_PER_SECONDS = 10; 

    private final float mBallCirfumference; 
    private final float mBallRadius; 
    private final Bitmap mBallBitmap; 
    private final Paint mBallBitmapPaint; 
    private final Matrix mBallTransformMatrix = new Matrix(); 
    private final float mPxPerSecond; 

    private long mStartTime = -1; 

    public CustomView(Context context) { 
     super(context); 
     final Resources res = getResources(); 

     // Load the ball bitmap. You probably want to use a better bitmap ;) 
     mBallBitmap = BitmapFactory.decodeResource(res, R.drawable.icon); 

     // We need the radius and circumference of the ball for our calculations 
     // later 
     mBallRadius = mBallBitmap.getHeight()/2; 
     mBallCirfumference = mBallRadius * 2 * (float)Math.PI; 

     // Create ourself a paint object so we can adjust the quality of the 
     // bitmap drawing 
     mBallBitmapPaint = new Paint(); 

     // Significantly improves quality when drawing transformed bitmaps. Compare 
     // with when you disable this, which is the default 
     mBallBitmapPaint.setFilterBitmap(true); 

     // Calculate speed of ball in pixels 
     mPxPerSecond = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, DP_PER_SECONDS, 
       res.getDisplayMetrics()); 
    } 

    @Override 
    protected void onDraw(Canvas canvas) { 
     super.onDraw(canvas); 

     // Calculate how far into the animation we are 
     if (mStartTime == -1) { 
      mStartTime = getDrawingTime(); 
     } 
     long currentTime = getDrawingTime(); 
     float secondsPassed = (currentTime - mStartTime)/1000.0f; 

     // Calculate how far the ball has moved and how many degrees it has been 
     // rotated as a consequence of the movement 
     float movedDistance = secondsPassed * mPxPerSecond; 
     float fullRotationsMade = movedDistance/mBallCirfumference; 
     float rotationInDegrees = fullRotationsMade * 360; 

     // Setup the transformation matrix to simulate a rolling ball 
     mBallTransformMatrix.reset(); 
     mBallTransformMatrix.postRotate(rotationInDegrees, mBallRadius, mBallRadius); 
     mBallTransformMatrix.postTranslate(movedDistance, 0); 
     canvas.drawBitmap(mBallBitmap, mBallTransformMatrix, mBallBitmapPaint); 

     // Force redraw so we get an animation 
     invalidate(); 
    } 
} 

ExampleActivity.java：

package com.example.android; 

import android.app.Activity; 
import android.os.Bundle; 

public class ExampleActivity extends Activity { 
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(new CustomView(this)); 
    } 
} 

带位图过滤的示例屏幕截图启用。如果您运行的例子中，位图将推出像一个球（原谅我的球显卡的选择不当）：

样品截图与位图筛选禁用。