参数'Paint'在Android的Canvas.drawBitmap（）方法中的作用

Question

I was trying to read sample FingerPaint in Android SDK where the method canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint) called within onDraw() .

I was confused by the parameter mBitmapPaint . It seemed to have no effect when I changed the settings of mBitmapPaint . The style of the bitmap painted on the screen only depends on the settings of mPaint defined somewhere else. The Google's Android Reference says that the parameter paint may be null, does it mean paint here is useless? Why put a paint parameter in this method?

It's declared as:

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)

Here is part of the code:

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

The paint for a Bitmap is pretty much useless, I think the only parameter of the paint object that applies for a Bitmap is the AntiAliasing mode wich is turned on by default. I think you can also use the paint for a color filter such as black and white, but for most cases you can just use null as well.

Answer 2

It is mainly used to set antialiasing and bilinear sampling:

• setAntiAlias(boolean)
• setFilterBitmap(boolean)

Answer 3

Actually, Canvas.drawBitmap(Bitmap, Rect, Rect, Paint) works properly with paint, BUT only if Bitmap is in Bitmap.Config.ALPHA_8 format. It can be easily converted on the go with Bitmap.copy(Bitmap.Config.ALPHA_8, true/false) . It's especially useful with Paint, that has a BitmapShader applied.

It's not mentioned enough, so I thought I'll add an answer to this old question.