了解Android中的alpha蒙版

Question

我正在学习如何使用Shaders并尝试理解BitmapShader上的old post by Romain Guy。在帖子中，有一个关于他想要实现的内容的演示视频，包含所有源代码。他正在使用这样的模糊图像：

看着面具图像，它看起来在中心有1个α，在我们移出时有0个。这应该意味着当图像缩小时，这个叠加蒙版将隐藏它下面的所有内容，因为它的不透明度。但正好相反。

我无法理解这个遮罩是如何创建效果的，就好像遮罩中心有alpha 0。

帖子中的相关代码：

package com.example.spotlight;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.BitmapShader;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.Shader;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewTreeObserver.OnGlobalLayoutListener;

public class SpotlightView extends View {
    private int mTargetId;

    private Bitmap mMask;
    private float mMaskX;
    private float mMaskY;
    private float mMaskScale;
    private Matrix mShaderMatrix = new Matrix();

    private Bitmap mTargetBitmap;
    private final Paint mPaint = new Paint();

    private AnimationSetupCallback mCallback;

    public interface AnimationSetupCallback {
        void onSetupAnimation(SpotlightView spotlight);
    }

    public SpotlightView(Context context, AttributeSet attrs) {
        super(context, attrs);

        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.SpotlightView, 0, 0);
        try {
            mTargetId = a.getResourceId(R.styleable.SpotlightView_target, 0);

            int maskId = a.getResourceId(R.styleable.SpotlightView_mask, 0);
            mMask = convertToAlphaMask(BitmapFactory.decodeResource(getResources(), maskId));

            android.util.Log.d("Spotlight", "c=" + mMask.getConfig());
        } catch (Exception e) {
            android.util.Log.e("Spotlight", "Error while creating the view:", e);
        } finally {
            a.recycle();
        }
    }

    public float getMaskScale() {
        return mMaskScale;
    }

    public void setMaskScale(float maskScale) {
        mMaskScale = maskScale;
        invalidate();
    }

    public float getMaskX() {
        return mMaskX;
    }

    public void setMaskX(float maskX) {
        mMaskX = maskX;
        invalidate();
    }

    public float getMaskY() {
        return mMaskY;
    }

    public void setMaskY(float maskY) {
        mMaskY = maskY;
        invalidate();
    }

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

        float maskW = mMask.getWidth() / 2.0f;
        float maskH = mMask.getHeight() / 2.0f;

        float x = mMaskX - maskW * mMaskScale;
        float y = mMaskY - maskH * mMaskScale;

        mShaderMatrix.setScale(1.0f / mMaskScale, 1.0f / mMaskScale);
        mShaderMatrix.preTranslate(-x, -y);

        mPaint.getShader().setLocalMatrix(mShaderMatrix);

        canvas.translate(x, y);
        canvas.scale(mMaskScale, mMaskScale);
        canvas.drawBitmap(mMask, 0.0f, 0.0f, mPaint);
    }

    public void setAnimationSetupCallback(AnimationSetupCallback callback) {
        mCallback = callback;
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();

        getViewTreeObserver().addOnGlobalLayoutListener(new OnGlobalLayoutListener() {
            @SuppressWarnings("deprecation")
            @Override
            public void onGlobalLayout() {
                createShader();
                setMaskScale(1.0f);

                if (mCallback != null) {
                    mCallback.onSetupAnimation(SpotlightView.this);
                }

                getViewTreeObserver().removeGlobalOnLayoutListener(this);
            }
        });
    }

    private void createShader() {
        View target = getRootView().findViewById(mTargetId);
        mTargetBitmap = createBitmap(target);
        Shader targetShader = createShader(mTargetBitmap);
        mPaint.setShader(targetShader);
    }

    private static Shader createShader(Bitmap b) {
        return new BitmapShader(b, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
    }

    private static Bitmap createBitmap(View target) {
        Bitmap b = Bitmap.createBitmap(target.getWidth(), target.getHeight(), Bitmap.Config.ARGB_8888);
        Canvas c = new Canvas(b);
        target.draw(c);
        return b;
    }

    private static Bitmap convertToAlphaMask(Bitmap b) {
        Bitmap a = Bitmap.createBitmap(b.getWidth(), b.getHeight(), Bitmap.Config.ALPHA_8);
        Canvas c = new Canvas(a);
        c.drawBitmap(b, 0.0f, 0.0f, null);
        return a;
    }

    public float computeMaskScale(float d) {
        // Let's assume the mask is square
        return d / (float) mMask.getHeight();
    }
}