java - Android的Canvas.drawBitmap()方法中参数 ‘Paint’的作用

Question

我试图阅读 Android SDK 中的示例 FingerPaint，其中方法 canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint) 在 中调用onDraw().

我被参数mBitmapPaint搞糊涂了。当我更改mBitmapPaint的设置时似乎没有效果。位图在屏幕上绘制的样式只取决于在别处定义的mPaint 的设置。 Google的Android Reference说参数paint可能为null，是不是说明这里的paint没用？为什么要在这个方法中放一个 paint 参数？

声明为:

public void drawBitmap(Bitmap bitmap,float left,float top,Paint paint)

Draw the specified bitmap, with its top/left corner at (x,y), using the specified paint, transformed by the current matrix.

Parameters

• bitmap The bitmap to be drawn
• left The position of the left side of the bitmap being drawn
• top The position of the top side of the bitmap being drawn
• paint The paint used to draw the bitmap (may be null)

部分代码如下:

private Bitmap  mBitmap;
private Canvas  mCanvas;
private Path    mPath;
private Paint   mBitmapPaint;

mPaint = new Paint();
mPath = new Path();
mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
mCanvas = new Canvas(mBitmap);

/*
 * Set the styles of mPaint here
 * ......
 */

// Settings of mBitmapPaint seem to make no difference
mBitmapPaint = new Paint(Paint.DITHER_FLAG);

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawColor(0xFFAAAAAA);
    canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
    canvas.drawPath(mPath, mPaint);
}

Answer 1

Bitmap 的绘制几乎没有用，我认为适用于 Bitmap 的绘制对象的唯一参数是 AntiAliasing 模式，默认情况下是打开的。我认为您也可以将颜料用于滤色器，例如黑色和白色，但在大多数情况下，您也可以只使用 null。