Android媒体开发指南：视频音频播放与录制

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"Pro Android Media" 是一本专注于Android多媒体开发的书籍，由Shawn Van Every撰写。本书涵盖了Android平台上的视频播放与录制、音频播放与录制、图形绘制以及丰富的媒体应用开发技术。书中通过详细的代码解释和逐步指导，为读者提供了一个良好的入门教程，适合想要在Android上构建多媒体应用的开发者。内容概述： 1. **Android Media APIs**：书中详细介绍了Android提供的Media APIs，这是Android系统中用于处理音频、视频和图像的核心组件。开发者可以利用这些API来实现播放、暂停、快进、快退等媒体控制功能，同时支持多种媒体格式的处理。 2. **视频播放与录制**：讲解了如何使用Android的MediaPlayer类进行视频播放，包括设置视频源、控制播放状态、处理视频流等。此外，还介绍了使用Camera API和MediaRecorder类进行视频录制，包括配置摄像头参数、捕获视频帧和保存视频文件。 3. **音频播放与录制**：介绍了如何使用MediaPlayer类和AudioTrack类播放音频，以及使用MediaRecorder类进行音频录制。此外，还包括音频格式转换和音频效果处理等内容。 4. **图形绘制**：探讨了使用Android的Canvas和Bitmap类进行2D图形绘制，以及使用SurfaceView进行实时图形渲染。同时，可能还涉及OpenGL ES，用于创建更复杂的3D图形和游戏。 5. **丰富的媒体应用开发**：这部分内容可能会涵盖如何将视频、音频和图形结合，创建互动式和沉浸式的用户体验，例如音乐播放器、视频编辑应用或游戏。 6. **最佳实践和优化**：书中可能包含关于性能优化、内存管理、设备兼容性测试以及如何利用硬件加速等实用技巧，帮助开发者打造高效、稳定的多媒体应用。 7. **实例代码**：每个主题都配有示例代码，帮助读者理解和应用所学知识，增强实战经验。《Pro Android Media》是一本全面介绍Android多媒体开发的指南，对于希望在Android平台上构建多媒体应用程序的开发者来说，是一本不可多得的参考书。通过阅读和实践，开发者可以掌握创建动态、引人入胜的移动应用所需的技能。

CHAPTER 1: Introduction to Android Imaging

offer image capture capabilities on a par with the Camera application without having to

build their own custom capture routines.

An intent filter is a means for a programmer of an application to specify that their

application offers a specific capability. Specifying an intent filter in the

AndroidManifest.xml file of an application tells Android that this application and, in

particular, the activity that contains the intent filter will perform the specified task, on

command.

The Camera application has the following intent filter specified in its manifest file. The

intent filter shown here is contained within the “Camera” activity tags.

<intent-filter>

</intent-filter>

In order to utilize the Camera application via an intent, we simply have to construct an

intent that will be caught by the foregoing filter.

Intent i = new Intent("android.media.action.IMAGE_CAPTURE");

In practice, we probably don’t want to create the intent with that action string directly. In

this case, a constant is specified in the MediaStore class, ACTION_IMAGE_CAPTURE. The

reason we should use the constant rather than the string itself is that if the string

happens to change, it is likely that the constant will change as well, thereby making our

call a bit more future-proof than it would otherwise be.

Intent i = new Intent(android.provider.MediaStore.ACTION_IMAGE_CAPTURE);

startActivity(i);

Using this intent in a basic Android activity will cause the default Camera application to

launch in still picture mode, as shown in Figure 1–1.

Figure 1–1. The built-in Camera application as called from an intent shown running in an emulator

CHAPTER 1: Introduction to Android Imaging

<ImageView android:id="@+id/ReturnedImageView" android:layout_width="wrap_content"

android:layout_height="wrap_content"></ImageView>

</LinearLayout>

To complete the foregoing example, here are the contents of AndroidManifest.xml.

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

android:versionCode="1"

android:versionName="1.0" package="com.apress.proandroidmedia.ch1.cameraintent">

<activity android:name=".CameraIntent"

android:label="@string/app_name">

<intent-filter>

</intent-filter>

</activity>

</application>

<uses-sdk android:minSdkVersion="4" />

</manifest>

In this example, the image is returned from the Camera application in an extra passed

through the intent that is sent to our calling activity in the onActivityResult method. The

name of the extra is "data" and it contains a Bitmap object, which needs to be cast from

a generic Object.

// Get Extras from the intent

Bundle extras = intent.getExtras();

// Get the returned image from that extra

Bitmap bmp = (Bitmap) extras.get("data");

In our layout XML (layout/main.xml) file, we have an ImageView. An ImageView is an

extension of a generic View, which supports the display of images. Since we have an

ImageView with the id ReturnedImageView specified, in our activity we need to obtain a

reference to that and set its Bitmap through its setImageBitmap method to be our

returned image. This enables the user of our application to view the image that was

captured.

To get a reference to the ImageView object, we use the standard findViewById method

specified in the Activity class. This method allows us to programmatically reference

elements specified in the layout XML file that we are using via setContentView by

passing in the id of the element. In the foregoing example, the ImageView object is

specified in the XML as follows:

<ImageView android:id="@+id/ReturnedImageView" android:layout_width="wrap_content"

android:layout_height="wrap_content"></ImageView>

To reference the ImageView and tell it to display the Bitmap from the Camera, we use

the following code.

imv = (ImageView) findViewById(R.id.ReturnedImageView);imv.setImageBitmap(bmp);

When you run this example, you’ll probably notice that the resulting image is small. (On

my phone, it is 121 pixels wide by 162 pixels tall. Other devices have different default

CHAPTER 1: Introduction to Android Imaging

NOTE: The foregoing code snippet for creating the URI to the image file could be simplified to the

following: imageFileUri =

Uri.parse("file:///sdcard/myfavoritepicture.jpg");

In practice, though, using the method shown will be more device-independent and future-proof

should the SD card–naming conventions or the URI syntax for the local filesystem change.

Displaying Large Images

Loading and displaying an image has significant memory usage implications. For

instance, the HTC G1 phone has a 3.2-megapixel camera. A 3.2-megapixel camera

typically captures images at 2048 pixels by 1536 pixels. Displaying a 32-bit image of

that size would take more than100663kb or approximately 13MB of memory. While this

may not guarantee that our application will run out of memory, it will certainly make it

more likely.

Android offers us a utility class called BitmapFactory, which provides a series of static

methods that allow the loading of Bitmap images from a variety of sources. For our

needs, we’ll be loading it from a file to display in our original activity. Fortunately, the

methods available in BitmapFactory take in a BitmapFactory.Options class, which

allows us to define how the Bitmap is read into memory. Specifically, we can set the

sample size that the BitmapFactory should use when loading an image. Indicating the

inSampleSize parameter in BitmapFactory.Options indicates that the resulting Bitmap

image will be that fraction of the size once loaded. For instance, setting the

inSampleSize to 8 as I do here would yield an image that is 1/8 the size of the original

image.

BitmapFactory.Options bmpFactoryOptions = new BitmapFactory.Options();

bmpFactoryOptions.inSampleSize = 8;

Bitmap bmp = BitmapFactory.decodeFile(imageFilePath, bmpFactoryOptions);

imv.setImageBitmap(bmp);

This is a quick way to load up a large image but doesn’t really take into account the

image’s original size nor the size of the screen. It would be better if we scaled the image

to something that would fit nicely on our screen.

The segments of code that follow illustrate how to use the dimensions of the display to

determine the amount of down sampling that should occur when loading the image.

When we use these methods, the image is assured of filling the bounds of the display as

much as possible. If, however, the image is only going to be shown at 100 pixels in any

one dimension, that value should be used instead of the display dimensions, which we

obtain as follows.

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