Android/Java：如何从画布中提取位图？

我混合了两个位图，试图得到一个位图结果。我在画布上获得了很好的效果，但我不知道如何从画布中获取位图，或者更一般地说，如何混合两个位图以获得一个位图

Bitmap bottomImage;
int cnt = 1;
int imageResource = mContext.getResources().getIdentifier("white", "drawable", mContext.getPackageName());
bottomImage = BitmapFactory.decodeResource(mContext.getResources(), imageResource).copy(Bitmap.Config.ARGB_8888, true);
Bitmap topImage;
File imgFile = new  File(LoginActivity.getAppContext().getFilesDir() + "/NTA/APP/icones/" + obj.get("ICONE") + ".png");
if(imgFile.exists()) {
topImage = BitmapFactory.decodeFile(imgFile.getAbsolutePath());
topImage = replaceColor(topImage, Color.WHITE, Color.BLACK);
topImage = replaceColor(topImage, Color.GRAY, Color.BLACK);
topImage = replaceColor(topImage, Color.DKGRAY, Color.BLACK);
topImage = replaceColor(topImage, Color.LTGRAY, Color.BLACK);
topImage = AlphaToBlack(topImage);

int height = (int) (topImage.getHeight());
int width = (int) (topImage.getWidth());

bottomImage = resizeBitmap(bottomImage, width, height);
Canvas comboImage = new Canvas(bottomImage);
comboImage.drawBitmap(topImage, 3f, 0f, null);

如何从画布获取位图？

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提前感谢。

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