使用alpha渐变将位图绘制到画布

Question

我想在Bitmap上绘制Canvas，并应用（线性）Alpha渐变。重要的是，我不想用任何其他颜色覆盖图像;背景（来自我View后面View后面的Canvas s应该只是“闪耀”。为了说明，我的目标是这样的（棋盘图案代表后面的View）

有人会认为我可以这样做：

Bitmap bitmap = ...;
Paint paint = new Paint();
paint.setShader(new LinearGradient(0, 0, 100, 0, FROM, TO, Shader.TileMode.CLAMP));
canvas.drawBitmap(bitmap, 0, 0, paint);

但此处LinearGradient的{​​{1}}和FROM参数需要是颜色，而不是alpha值;所以我没有办法指明这一点，例如TO应完全透明，FROM应完全不透明（不应用任何颜色叠加）。

Answer 1

使用ComposeShader，如下所示：

class V extends View {
    Bitmap bitmap;
    Paint paint = new Paint();

    public V(Context context) {
        super(context);
        bitmap = BitmapFactory.decodeResource(context.getResources(), R.drawable.chrome);
        Shader shaderA = new LinearGradient(0, 0, bitmap.getWidth(), 0, 0xffffffff, 0x00ffffff, Shader.TileMode.CLAMP);
        Shader shaderB = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
        paint.setShader(new ComposeShader(shaderA, shaderB, PorterDuff.Mode.SRC_IN));
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawRect(0, 0, bitmap.getWidth(), bitmap.getHeight(), paint);
    }
}

Answer 2

基于this answer about masking，我可以使用辅助屏幕外画布来完成此操作：

Bitmap backing = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), Bitmap.Config.ARGB_8888);
{
    Canvas offscreen = new Canvas(backing);
    offscreen.drawBitmap(bitmap, 0, 0, null);
    Paint paint = new Paint();
    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
    paint.setShader(new LinearGradient(0, 0, 100, 0, 0x00000000, 0xFF000000, Shader.TileMode.CLAMP));
    offscreen.drawRect(0, 0, bitmap.getWidth(), bitmap.getHeight(), paint);
}
canvas.drawBitmap(backing, 0, 0, null);