Android上如何做出毛玻璃模糊的效果？

上图展示了一种很典型的视觉效果——文字的背景不再是固定的，而是将底层的相应区域模糊化，好似盖了一层毛玻璃。

其原理也很简单，分为三步走：

1. 获取底层的view的Bitmap——b1
2. 将b1模糊化，保存为b2
3. 把b2设为文字的背景。

里面涉及到的技术点有两个：

1. 何时访问能获取到底层view的完整Bitmap？
2. 如何模糊化，该采用什么算法？

技术点一：何时访问能获取到底层view的完整Bitmap？

ViewTreeObserver里面有一个监听器为OnPreDrawListener

// 当一个视图树将要绘制时，所要调用的回调函数的接口类
public interface OnPreDrawListener {
    boolean onPreDraw();
}

当它执行时，布局文件经过了measured、laid out、displayed，即将被绘制到屏幕，此时调用它的getDrawingCache()方法可以获得其Bitmap。完整方法如下：

lowerView.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
    @Override
    public boolean onPreDraw() {
        lowerView.getViewTreeObserver().removeOnPreDrawListener(this);
        lowerView.buildDrawingCache();

        Bitmap bmp = lowerView.getDrawingCache();
        return true;
    }
});

技术点二：如何模糊化，该采用什么算法？

方案有两种：

1. 借助Renderscript做高斯模糊，本质上是做卷积计算。
2. 采用StackBlur算法对图片进行模糊处理，相较于高斯模糊，计算量小了很多。

两种方案都可以进行对Bitmap对象的模糊处理，但当模糊半径增大时，StackBlur能够保持较好的性能，且不受Renderscript半径25px的限制。

在GitHub项目有一个项目blurring，其实现了StackBlur算法的Java实现版FastBlur，并给出两种方案效率对比demo。经过测试，

• 在Mac上Genymotion模拟器下运行Android4.4.4，Renderscript耗时42ms，FastBlur耗时82ms
• 在小米4真机上运行Android6.0.1，Renderscript耗时124ms，FastBlur耗时271ms

看起来，Renderscript的性能更好，应该是Android上对Renderscript做了优化。尽管如此，考虑到Android中渲染一帧的时间应该不超过16ms（60fps），这样的性能并不友好。blurring作者想出了另外一种思路：

把bitmap的尺寸先降低然后进行模糊处理，然后再放大尺寸

这时候，效率提升非常明显：

• 在Mac上Genymotion模拟器下运行Android4.4.4，Renderscript耗时5ms，FastBlur耗时1ms
• 在小米4真机上运行Android6.0.1，Renderscript耗时16ms，FastBlur耗时7ms。

生成的模糊图片当然有所不同，但是都是模糊背景，所以对用户而言没有太大差别。

好了，至此Android上制作毛玻璃背景模糊效果的技术都确定了。

源码

我在blurring基础上做了封装，接口如下：

/**
 *
 * @param context 上下文
 * @param lowerView 底层view控件
 * @param upperView 需要模糊背景的控件
 */
public static void doBlur(final Context context, final View lowerView, final View upperView) {
    lowerView.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
        @Override
        public boolean onPreDraw() {
            lowerView.getViewTreeObserver().removeOnPreDrawListener(this);
            lowerView.buildDrawingCache();
            Bitmap bmp = lowerView.getDrawingCache();
            doBlur(context, bmp, upperView, true);
            return true;
        }
    });
}

相对应类有两个：

1. RSBlur

package com.paveldudka.util;
import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.drawable.BitmapDrawable;
import android.os.Build;
import android.renderscript.Allocation;
import android.renderscript.RenderScript;
import android.renderscript.ScriptIntrinsicBlur;
import android.support.v4.app.FragmentActivity;
import android.view.View;
import android.view.ViewTreeObserver;
/**
 * Created by shitianci on 16/8/8.
 */
public class RSBlur {
    /**
     *
     * @param context 上下文
     * @param lowerView 底层view控件
     * @param upperView 需要模糊背景的控件
     */
    public static void doBlur(final Context context, final View lowerView, final View upperView) {
        lowerView.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
            @Override
            public boolean onPreDraw() {
                lowerView.getViewTreeObserver().removeOnPreDrawListener(this);
                lowerView.buildDrawingCache();
                Bitmap bmp = lowerView.getDrawingCache();
                doBlur(context, bmp, upperView, true);
                return true;
            }
        });
    }
    /**
     *
     * @param context 上下文
     * @param bkg 原图
     * @param view 需要模糊背景的控件
     * @param isDownScale 是否降低等级，优化效率（建议开启）
     */
    @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR1)
    public static void doBlur(Context context, Bitmap bkg, View view, boolean isDownScale) {
        float scaleFactor = 1;
        float radius = 20;

        if (isDownScale) {
            scaleFactor = 8;
            radius = 2;
        }

        Bitmap overlay = Bitmap.createBitmap((int) (view.getMeasuredWidth() / scaleFactor),
                (int) (view.getMeasuredHeight() / scaleFactor), Bitmap.Config.ARGB_8888);

        Canvas canvas = new Canvas(overlay);

        canvas.translate(-view.getLeft() / scaleFactor, -view.getTop() / scaleFactor);
        canvas.scale(1 / scaleFactor, 1 / scaleFactor);
        Paint paint = new Paint();
        paint.setFlags(Paint.FILTER_BITMAP_FLAG);
        canvas.drawBitmap(bkg, 0, 0, paint);

        RenderScript rs = RenderScript.create(context);

        Allocation overlayAlloc = Allocation.createFromBitmap(
                rs, overlay);

        ScriptIntrinsicBlur blur = ScriptIntrinsicBlur.create(
                rs, overlayAlloc.getElement());

        blur.setInput(overlayAlloc);

        blur.setRadius(radius);

        blur.forEach(overlayAlloc);

        overlayAlloc.copyTo(overlay);

        view.setBackground(new BitmapDrawable(context.getResources(), overlay));

        rs.destroy();
    }
}

2. FastBlur

package com.paveldudka.util;
import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.drawable.BitmapDrawable;
import android.os.Build;
import android.view.View;
import android.view.ViewTreeObserver;
/**
 * Created by paveld on 3/6/14.
 */
public class FastBlur {
    /**
     *
     * @param context 上下文
     * @param lowerView 底层view控件
     * @param upperView 需要模糊背景的控件
     */
    public static void doBlur(final Context context, final View lowerView, final View upperView) {
        lowerView.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
            @Override
            public boolean onPreDraw() {
                lowerView.getViewTreeObserver().removeOnPreDrawListener(this);
                lowerView.buildDrawingCache();
                Bitmap bmp = lowerView.getDrawingCache();
                doBlur(context, bmp, upperView, true);
                return true;
            }
        });
    }

    /**
     *
     * @param context 上下文
     * @param bkg 原图
     * @param view 需要模糊背景的控件
     * @param isDownScale 是否降低等级，优化效率（建议开启）
     */
    @TargetApi(Build.VERSION_CODES.JELLY_BEAN)
    public static void doBlur(Context context, Bitmap bkg, View view, boolean isDownScale) {
        float scaleFactor = 1;
        float radius = 20;
        if (isDownScale) {
            scaleFactor = 8; //bitmap的尺寸缩小到原图的1/8
            radius = 2;
        }
        Bitmap overlay = Bitmap.createBitmap((int) (view.getMeasuredWidth()/scaleFactor),
                (int) (view.getMeasuredHeight()/scaleFactor), Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(overlay);
        canvas.translate(-view.getLeft()/scaleFactor, -view.getTop()/scaleFactor);
        canvas.scale(1 / scaleFactor, 1 / scaleFactor);
        Paint paint = new Paint();
        paint.setFlags(Paint.FILTER_BITMAP_FLAG);
        canvas.drawBitmap(bkg, 0, 0, paint);

        overlay = FastBlur.doBlur(overlay, (int)radius, true);
        view.setBackground(new BitmapDrawable(context.getResources(), overlay));
    }

    public static Bitmap doBlur(Bitmap sentBitmap, int radius, boolean canReuseInBitmap) {

        // Stack Blur v1.0 from
        // http://www.quasimondo.com/StackBlurForCanvas/StackBlurDemo.html
        //
        // Java Author: Mario Klingemann <mario at quasimondo.com>
        // http://incubator.quasimondo.com
        // created Feburary 29, 2004
        // Android port : Yahel Bouaziz <yahel at kayenko.com>
        // http://www.kayenko.com
        // ported april 5th, 2012

        // This is a compromise between Gaussian Blur and Box blur
        // It creates much better looking blurs than Box Blur, but is
        // 7x faster than my Gaussian Blur implementation.
        //
        // I called it Stack Blur because this describes best how this
        // filter works internally: it creates a kind of moving stack
        // of colors whilst scanning through the lowerView. Thereby it
        // just has to add one new block of color to the right side
        // of the stack and remove the leftmost color. The remaining
        // colors on the topmost layer of the stack are either added on
        // or reduced by one, depending on if they are on the right or
        // on the left side of the stack.
        //
        // If you are using this algorithm in your code please add
        // the following line:
        //
        // Stack Blur Algorithm by Mario Klingemann <mario@quasimondo.com>

        Bitmap bitmap;
        if (canReuseInBitmap) {
            bitmap = sentBitmap;
        } else {
            bitmap = sentBitmap.copy(sentBitmap.getConfig(), true);
        }

        if (radius < 1) {
            return (null);
        }

        int w = bitmap.getWidth();
        int h = bitmap.getHeight();

        int[] pix = new int[w * h];
        bitmap.getPixels(pix, 0, w, 0, 0, w, h);

        int wm = w - 1;
        int hm = h - 1;
        int wh = w * h;
        int div = radius + radius + 1;

        int r[] = new int[wh];
        int g[] = new int[wh];
        int b[] = new int[wh];
        int rsum, gsum, bsum, x, y, i, p, yp, yi, yw;
        int vmin[] = new int[Math.max(w, h)];

        int divsum = (div + 1) >> 1;
        divsum *= divsum;
        int dv[] = new int[256 * divsum];
        for (i = 0; i < 256 * divsum; i++) {
            dv[i] = (i / divsum);
        }

        yw = yi = 0;

        int[][] stack = new int[div][3];
        int stackpointer;
        int stackstart;
        int[] sir;
        int rbs;
        int r1 = radius + 1;
        int routsum, goutsum, boutsum;
        int rinsum, ginsum, binsum;

        for (y = 0; y < h; y++) {
            rinsum = ginsum = binsum = routsum = goutsum = boutsum = rsum = gsum = bsum = 0;
            for (i = -radius; i <= radius; i++) {
                p = pix[yi + Math.min(wm, Math.max(i, 0))];
                sir = stack[i + radius];
                sir[0] = (p & 0xff0000) >> 16;
                sir[1] = (p & 0x00ff00) >> 8;
                sir[2] = (p & 0x0000ff);
                rbs = r1 - Math.abs(i);
                rsum += sir[0] * rbs;
                gsum += sir[1] * rbs;
                bsum += sir[2] * rbs;
                if (i > 0) {
                    rinsum += sir[0];
                    ginsum += sir[1];
                    binsum += sir[2];
                } else {
                    routsum += sir[0];
                    goutsum += sir[1];
                    boutsum += sir[2];
                }
            }
            stackpointer = radius;

            for (x = 0; x < w; x++) {

                r[yi] = dv[rsum];
                g[yi] = dv[gsum];
                b[yi] = dv[bsum];

                rsum -= routsum;
                gsum -= goutsum;
                bsum -= boutsum;

                stackstart = stackpointer - radius + div;
                sir = stack[stackstart % div];

                routsum -= sir[0];
                goutsum -= sir[1];
                boutsum -= sir[2];

                if (y == 0) {
                    vmin[x] = Math.min(x + radius + 1, wm);
                }
                p = pix[yw + vmin[x]];

                sir[0] = (p & 0xff0000) >> 16;
                sir[1] = (p & 0x00ff00) >> 8;
                sir[2] = (p & 0x0000ff);

                rinsum += sir[0];
                ginsum += sir[1];
                binsum += sir[2];

                rsum += rinsum;
                gsum += ginsum;
                bsum += binsum;

                stackpointer = (stackpointer + 1) % div;
                sir = stack[(stackpointer) % div];

                routsum += sir[0];
                goutsum += sir[1];
                boutsum += sir[2];

                rinsum -= sir[0];
                ginsum -= sir[1];
                binsum -= sir[2];

                yi++;
            }
            yw += w;
        }
        for (x = 0; x < w; x++) {
            rinsum = ginsum = binsum = routsum = goutsum = boutsum = rsum = gsum = bsum = 0;
            yp = -radius * w;
            for (i = -radius; i <= radius; i++) {
                yi = Math.max(0, yp) + x;

                sir = stack[i + radius];

                sir[0] = r[yi];
                sir[1] = g[yi];
                sir[2] = b[yi];

                rbs = r1 - Math.abs(i);

                rsum += r[yi] * rbs;
                gsum += g[yi] * rbs;
                bsum += b[yi] * rbs;

                if (i > 0) {
                    rinsum += sir[0];
                    ginsum += sir[1];
                    binsum += sir[2];
                } else {
                    routsum += sir[0];
                    goutsum += sir[1];
                    boutsum += sir[2];
                }

                if (i < hm) {
                    yp += w;
                }
            }
            yi = x;
            stackpointer = radius;
            for (y = 0; y < h; y++) {
                // Preserve alpha channel: ( 0xff000000 & pix[yi] )
                pix[yi] = (0xff000000 & pix[yi]) | (dv[rsum] << 16) | (dv[gsum] << 8) | dv[bsum];

                rsum -= routsum;
                gsum -= goutsum;
                bsum -= boutsum;

                stackstart = stackpointer - radius + div;
                sir = stack[stackstart % div];

                routsum -= sir[0];
                goutsum -= sir[1];
                boutsum -= sir[2];

                if (x == 0) {
                    vmin[y] = Math.min(y + r1, hm) * w;
                }
                p = x + vmin[y];

                sir[0] = r[p];
                sir[1] = g[p];
                sir[2] = b[p];

                rinsum += sir[0];
                ginsum += sir[1];
                binsum += sir[2];

                rsum += rinsum;
                gsum += ginsum;
                bsum += binsum;

                stackpointer = (stackpointer + 1) % div;
                sir = stack[stackpointer];

                routsum += sir[0];
                goutsum += sir[1];
                boutsum += sir[2];

                rinsum -= sir[0];
                ginsum -= sir[1];
                binsum -= sir[2];

                yi += w;
            }
        }

        bitmap.setPixels(pix, 0, w, 0, 0, w, h);

        return (bitmap);
    }
}

参考

1. Android高级模糊技术
2. GitHub项目·blurring
3. 知乎问题·Android 5.0 下毛玻璃（磨砂）效果如何实现？