A prototype implementation of a shadow effect inspired by the Google Play Games app (https://play.google.com/store/apps/details?id=com.google.android.play.games).

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  Copyright (c) 2017 Google Inc.

  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  in compliance with the License. You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software distributed under the License
  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
  or implied. See the License for the specific language governing permissions and limitations under
  the License.
  -->
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ProgressBar
        android:id="@+id/loading"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:indeterminate="true" />

    <android.support.v7.widget.RecyclerView
        android:id="@+id/carousel"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:clipToPadding="false"
        android:orientation="horizontal"
        android:paddingStart="96dp"
        android:scrollbarStyle="insideOverlay"
        android:scrollbars="horizontal"
        app:layoutManager="LinearLayoutManager"
        tools:context=".MainActivity" />

</FrameLayout>

app_item.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  Copyright (c) 2017 Google Inc.

  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  in compliance with the License. You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software distributed under the License
  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
  or implied. See the License for the specific language governing permissions and limitations under
  the License.
  -->
<com.example.playgamesshadows.IconView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="224dp"
    android:layout_height="match_parent" />

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  Copyright (c) 2017 Google Inc.

  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
  in compliance with the License. You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software distributed under the License
  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
  or implied. See the License for the specific language governing permissions and limitations under
  the License.
  -->
<resources>
    <declare-styleable name="AppTheme">
        <attr name="iconViewStyle" format="reference" />
    </declare-styleable>

    <declare-styleable name="IconView">
        <attr name="constantShadowTranslationY" format="dimension" />
        <attr name="variableShadowTranslationY" format="dimension" />
        <attr name="scaleDown" format="float" />
        <attr name="bigBlurRadius" format="float" />
        <attr name="smallBlurRadius" format="float" />
    </declare-styleable>
</resources>

IconView.kt

/*
 *  Copyright 2017 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the
 * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the specific language governing
 * permissions and limitations under the License.
 */

package com.example.playgamesshadows

import android.content.Context
import android.graphics.Bitmap
import android.graphics.Bitmap.Config.*
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.ColorMatrix
import android.graphics.ColorMatrixColorFilter
import android.graphics.Paint
import android.graphics.Rect
import android.graphics.RectF
import android.graphics.drawable.Drawable
import android.renderscript.Allocation
import android.renderscript.Element
import android.renderscript.RenderScript
import android.renderscript.ScriptIntrinsicBlur
import android.support.annotation.AttrRes
import android.support.annotation.StyleRes
import android.support.v4.view.animation.FastOutSlowInInterpolator
import android.util.AttributeSet
import android.view.View

class IconView @JvmOverloads constructor(
        context: Context,
        attrs: AttributeSet,
        @AttrRes defStyleAttr: Int = R.attr.iconViewStyle,
        @StyleRes defStyleRes: Int = R.style.Widget_ShadowPlay_IconView)
    : View(context, attrs, defStyleAttr, defStyleRes) {

    var icon: Drawable? = null
        set(value) {
            field = value
            setIconBounds()
            createShadows()
        }

    var offset = 0f
        set(value) {
            field = interp.getInterpolation(value.coerceIn(0f, 1f))
            val scale = scaleDown + (1f - scaleDown) * (1f - field)
            scaleX = scale
            scaleY = scale
            smallBlurShadowAlpha = (field * 255).toInt()
            bigBlurShadowAlpha = 255 - smallBlurShadowAlpha
            shadowTranslate = constantShadowTranslationY + variableShadowTranslationY * (1f - field)
            postInvalidateOnAnimation()
        }

    private var bigBlurShadow: Bitmap? = null
    private var smallBlurShadow: Bitmap? = null
    private var smallBlurShadowAlpha = 255
    private var bigBlurShadowAlpha = 255
    private var shadowTranslate = 0f
    private var constantShadowTranslationY = 8f
    private var variableShadowTranslationY = 16f
    private var scaleDown = 0.85f
    private var bigBlurRadius = 24f
    private var smallBlurRadius = 8f
    private var padding = bigBlurRadius.toInt()
    private val iconBounds = Rect()
    private val shadowBounds = RectF()
    private val shadowPaint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
        colorFilter = ColorMatrixColorFilter(ColorMatrix().apply {
            setScale(SHADOW_SCALE_RGB, SHADOW_SCALE_RGB, SHADOW_SCALE_RGB, SHADOW_SCALE_ALPHA)
        })
    }

    init {
        val a = context.obtainStyledAttributes(attrs, R.styleable.IconView, defStyleAttr, defStyleRes)
        constantShadowTranslationY = a.getDimension(R.styleable.IconView_constantShadowTranslationY,
                constantShadowTranslationY)
        variableShadowTranslationY = a.getDimension(R.styleable.IconView_variableShadowTranslationY,
                variableShadowTranslationY)
        scaleDown = a.getFloat(R.styleable.IconView_scaleDown, scaleDown).coerceIn(0f, 1f)
        bigBlurRadius = a.getFloat(R.styleable.IconView_bigBlurRadius, bigBlurRadius)
                .coerceIn(0f, 25f)
        padding = bigBlurRadius.toInt()
        smallBlurRadius = a.getFloat(R.styleable.IconView_smallBlurRadius, smallBlurRadius)
                .coerceIn(0f, 25f)
        a.recycle()
    }

    override fun onSizeChanged(w: Int, h: Int, oldw: Int, oldh: Int) {
        val shadowTop = (h - w) / 2
        val iconTop = shadowTop + padding
        val iconSize = w - padding - padding
        iconBounds.set(padding, iconTop, padding + iconSize, iconTop + iconSize)
        shadowBounds.set(0f, shadowTop.toFloat(), w.toFloat(), (shadowTop + w).toFloat())
        setIconBounds()
        createShadows()
    }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        val saveCount = canvas.save()
        canvas.translate(shadowBounds.left, shadowBounds.top + shadowTranslate)
        shadowPaint.alpha = bigBlurShadowAlpha
        canvas.drawBitmap(bigBlurShadow, 0f, 0f, shadowPaint)
        shadowPaint.alpha = smallBlurShadowAlpha
        canvas.drawBitmap(smallBlurShadow, 0f, 0f, shadowPaint)
        canvas.restoreToCount(saveCount)
        icon?.draw(canvas)
    }

    private fun setIconBounds() {
        icon?.bounds = iconBounds
    }

    private fun createShadows() {
        if (icon == null || shadowBounds.width() == 0f) return
        if (bigBlurShadow == null) {
            bigBlurShadow = Bitmap.createBitmap(
                    shadowBounds.width().toInt(), shadowBounds.height().toInt(), ARGB_8888)
        } else {
            bigBlurShadow?.eraseColor(Color.TRANSPARENT)
        }
        if (smallBlurShadow == null) {
            smallBlurShadow = Bitmap.createBitmap(
                    shadowBounds.width().toInt(), shadowBounds.height().toInt(), ARGB_8888)
        } else {
            smallBlurShadow?.eraseColor(Color.TRANSPARENT)
        }
        val c = Canvas()
        createShadow(bigBlurShadow, c, bigBlurRadius)
        createShadow(smallBlurShadow, c, smallBlurRadius)
    }

    private fun createShadow(bitmap: Bitmap?, canvas: Canvas, blurRadius: Float) {
        canvas.setBitmap(bitmap)
        canvas.translate(padding - iconBounds.left.toFloat(),
                padding - iconBounds.top.toFloat())
        icon?.draw(canvas)
        val rs = getRS(context)
        val blur = getBlur(context)
        val input = Allocation.createFromBitmap(rs, bitmap)
        val output = Allocation.createTyped(rs, input.type)
        blur.setRadius(blurRadius.coerceIn(0f, 25f))
        blur.setInput(input)
        blur.forEach(output)
        output.copyTo(bitmap)
        input.destroy()
        output.destroy()
    }

    companion object {
        private const val SHADOW_SCALE_RGB = 0.85f
        private const val SHADOW_SCALE_ALPHA = 0.6f
        private val interp = FastOutSlowInInterpolator()

        private var rs: RenderScript? = null
        fun getRS(context: Context): RenderScript {
            if (rs == null) {
                rs = RenderScript.create(context.applicationContext)
            }
            return rs!!
        }

        private var blur: ScriptIntrinsicBlur? = null
        fun getBlur(context: Context): ScriptIntrinsicBlur {
            if (blur == null) {
                val rs = getRS(context)
                blur = ScriptIntrinsicBlur.create(rs, Element.U8_4(rs))
            }
            return blur!!
        }
    }
}

LICENSE

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MainActivity.kt

/*
 *  Copyright 2017 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the
 * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the specific language governing
 * permissions and limitations under the License.
 */

package com.example.playgamesshadows

import android.app.LoaderManager
import android.content.AsyncTaskLoader
import android.content.Context
import android.content.Intent
import android.content.Loader
import android.graphics.drawable.Drawable
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.support.v7.widget.LinearSnapHelper
import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        carousel.setHasFixedSize(true)
        LinearSnapHelper().attachToRecyclerView(carousel)
        carousel.addOnScrollListener(object : RecyclerView.OnScrollListener() {
            override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
                val center = recyclerView.width / 2
                (0 until recyclerView.childCount).forEach { i ->
                    val child = recyclerView.getChildAt(i) as? IconView ?: return@forEach
                    val cx = (child.left + child.right) / 2
                    val offsetPx = Math.abs(center - cx)
                    child.offset = offsetPx / center.toFloat()
                }
            }
        })

        loaderManager.initLoader(0, Bundle.EMPTY,
                object : LoaderManager.LoaderCallbacks<List<Drawable>> {
            override fun onCreateLoader(id: Int, args: Bundle?) = AppIconLoader(applicationContext)

            override fun onLoadFinished(loader: Loader<List<Drawable>>, icons: List<Drawable>) {
                loading.visibility = View.GONE
                carousel.adapter = AppAdapter(icons)
            }

            override fun onLoaderReset(loader: Loader<List<Drawable>>?) {}
        })
    }

    private class AppIconLoader(context: Context) : AsyncTaskLoader<List<Drawable>>(context) {

        private val icons = mutableListOf<Drawable>()

        override fun onStartLoading() {
            if (icons.isNotEmpty()) {
                deliverResult(icons)
            } else {
                forceLoad()
            }
        }

        override fun loadInBackground(): List<Drawable>? {
            val pm = context.packageManager
            val launchableAppIcons = mutableListOf<Drawable>()
            val launcherIntent = Intent().apply { addCategory(Intent.CATEGORY_LAUNCHER) }
            pm.getInstalledApplications(0).forEach { appInfo ->
                launcherIntent.`package` = appInfo.packageName
                // only show launch-able apps
                if (pm.queryIntentActivities(launcherIntent, 0).isNotEmpty()) {
                    launchableAppIcons += appInfo.loadUnbadgedIcon(pm)
                }
            }
            return launchableAppIcons
        }

        override fun deliverResult(data: List<Drawable>) {
            icons += data
            super.deliverResult(data)
        }
    }

    class AppAdapter(private val icons: List<Drawable>) : RecyclerView.Adapter<AppViewHolder>() {
        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): AppViewHolder {
            return AppViewHolder(LayoutInflater.from(parent.context)
                    .inflate(R.layout.app_item, parent, false) as IconView)
        }

        override fun onBindViewHolder(holder: AppViewHolder, position: Int) {
            holder.iconView.icon = icons[position]
        }

        override fun getItemCount() = icons.size
    }

    class AppViewHolder(val iconView: IconView) : RecyclerView.ViewHolder(iconView)
}

blur_shadows.gif

styles.xml

@MicroRJ Nick it's literally a designer and developer advocate @google for Android. BTW, Thanks Nick for this cool implementation.

@nick hi there ... can you specify this implementation for single image(icon )instead of bunch of icons ?

@nickbutcher Hi! Thank you for sharing this. I have a question because this only applies to drawable (icon), is it possible to use the same method in FrameLayout / CardView?

I'm still a newbie in android development and I need to achieve a CardView with subtle drop shadow like 9-patch.
But 9-patch takes up space inside the card. I need to draw the shadow outside the card boundaries.

Please help out. Thank you :)

Hi @nick thank you for sharing this,
I am trying use this shadow for rectangular icon,But it always showing me square in shape

Please help me out ,Thank you