同时使用多个摄像头画面

注意：本页介绍的是 Camera2 软件包。除非您的应用需要 Camera2 中的特定低级功能，否则我们建议您使用 CameraX。CameraX 和 Camera2 都支持 Android 5.0（API 级别 21）及更高版本。

相机应用可以同时使用多个帧流。在 在某些情况下，不同的视频流甚至需要不同的帧分辨率或像素 格式。一些典型用例包括：

• 视频录制：一个视频流用于预览，另一个视频流正在编码和保存 创建一个文件
• 条形码扫描：一个用于预览的流，另一个用于条形码检测。
• 计算摄影：一个数据流用于预览，另一个用于人脸/场景 检测。

处理帧时会产生非常重要的性能开销， 会成倍增加。

CPU、GPU 和 DSP 等资源或许能够利用 框架的再处理 但内存等资源将线性增长。

每个请求多个目标

多个摄像头视频流可合并为单个视频流 CameraCaptureRequest。 以下代码段说明了如何使用 用于相机预览的流和用于图片处理的流：

Kotlin

val session: CameraCaptureSession = ...  // from CameraCaptureSession.StateCallback

// You will use the preview capture template for the combined streams
// because it is optimized for low latency; for high-quality images, use
// TEMPLATE_STILL_CAPTURE, and for a steady frame rate use TEMPLATE_RECORD
val requestTemplate = CameraDevice.TEMPLATE_PREVIEW
val combinedRequest = session.device.createCaptureRequest(requestTemplate)

// Link the Surface targets with the combined request
combinedRequest.addTarget(previewSurface)
combinedRequest.addTarget(imReaderSurface)

// In this simple case, the SurfaceView gets updated automatically. ImageReader
// has its own callback that you have to listen to in order to retrieve the
// frames so there is no need to set up a callback for the capture request
session.setRepeatingRequest(combinedRequest.build(), null, null)

Java

CameraCaptureSession session = …;  // from CameraCaptureSession.StateCallback

// You will use the preview capture template for the combined streams
// because it is optimized for low latency; for high-quality images, use
// TEMPLATE_STILL_CAPTURE, and for a steady frame rate use TEMPLATE_RECORD
        CaptureRequest.Builder combinedRequest = session.getDevice().createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);

// Link the Surface targets with the combined request
        combinedRequest.addTarget(previewSurface);
        combinedRequest.addTarget(imReaderSurface);

// In this simple case, the SurfaceView gets updated automatically. ImageReader
// has its own callback that you have to listen to in order to retrieve the
// frames so there is no need to set up a callback for the capture request
        session.setRepeatingRequest(combinedRequest.build(), null, null);

如果您正确配置目标平台，此代码将仅生成 只要视频流达到 FPS 的 StreamComfigurationMap.GetOutputMinFrameDuration(int, Size) 和 StreamComfigurationMap.GetOutputStallDuration(int, Size)。 实际性能因设备而异，不过 Android 提供了一些 可保证根据三个变量支持特定组合： 输出类型、输出大小和硬件级别。

使用不受支持的变量组合可能会在低帧速率下运行；如果 否则，将触发其中一个失败回调。 createCaptureSession 的文档 说明了保证什么有效

输出类型

“输出类型”是指帧的编码格式。通过 可能的值为 PRIV、YUV、JPEG 和 RAW。有关 createCaptureSession 对它们进行了说明。

在选择应用的输出类型时，如果目标是尽可能增加 兼容性，然后使用 ImageFormat.YUV_420_888 进行帧分析 ImageFormat.JPEG 表示静态图片 图片。对于预览和录制场景，您可能会使用 SurfaceView、 TextureView、 MediaRecorder， MediaCodec 或 RenderScript.Allocation。在 在此类情况下，请勿指定图片格式。为确保兼容性，将被视为 ImageFormat.PRIVATE、 无论内部实际使用的格式如何。查询支持的格式 获得最大收益 CameraCharacteristics、 请使用以下代码：

Kotlin

val characteristics: CameraCharacteristics = ...
val supportedFormats = characteristics.get(
    CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP).outputFormats

Java

CameraCharacteristics characteristics = …;
        int[] supportedFormats = characteristics.get(
CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP).getOutputFormats();

输出大小

所有可用的输出大小均按 StreamConfigurationMap.getOutputSizes()、 但只有 2 个与兼容性相关：PREVIEW 和 MAXIMUM。尺寸 用作上限如果 PREVIEW 大小的某个元素有效，那么任何具有 小于 PREVIEW 的尺寸也是不错的选择。MAXIMUM 也是如此。通过 文档 CameraDevice 介绍了这些尺寸

可用的输出大小取决于所选格式。由于存在 CameraCharacteristics 和格式，您可以查询可用的输出大小，如下所示：

Kotlin

val characteristics: CameraCharacteristics = ...
val outputFormat: Int = ...  // such as ImageFormat.JPEG
val sizes = characteristics.get(
    CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)
    .getOutputSizes(outputFormat)

Java

CameraCharacteristics characteristics = …;
        int outputFormat = …;  // such as ImageFormat.JPEG
Size[] sizes = characteristics.get(
                CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)
                .getOutputSizes(outputFormat);

在相机预览和录制用例中，使用目标类来确定 支持的尺寸格式将由相机框架本身处理：

Kotlin

val characteristics: CameraCharacteristics = ...
val targetClass: Class <T> = ...  // such as SurfaceView::class.java
val sizes = characteristics.get(
    CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)
    .getOutputSizes(targetClass)

Java

CameraCharacteristics characteristics = …;
   int outputFormat = …;  // such as ImageFormat.JPEG
   Size[] sizes = characteristics.get(
                CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)
                .getOutputSizes(outputFormat);

如需获取 MAXIMUM 大小，请按面积对输出大小进行排序，然后返回 一：

Kotlin

fun <T>getMaximumOutputSize(
    characteristics: CameraCharacteristics, targetClass: Class <T>, format: Int? = null):
    Size {
  val config = characteristics.get(
      CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)

  // If image format is provided, use it to determine supported sizes; or else use target class
  val allSizes = if (format == null)
    config.getOutputSizes(targetClass) else config.getOutputSizes(format)
  return allSizes.maxBy { it.height * it.width }
}

Java

 @RequiresApi(api = Build.VERSION_CODES.N)
    <T> Size getMaximumOutputSize(CameraCharacteristics characteristics,
                                            Class <T> targetClass,
                                            Integer format) {
        StreamConfigurationMap config = characteristics.get(CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);

        // If image format is provided, use it to determine supported sizes; else use target class
        Size[] allSizes;
        if (format == null) {
            allSizes = config.getOutputSizes(targetClass);
        } else {
            allSizes = config.getOutputSizes(format);
        }
        return Arrays.stream(allSizes).max(Comparator.comparing(s -> s.getHeight() * s.getWidth())).get();
    }

PREVIEW 是指与设备的屏幕分辨率或 1080p (1920x1080)，以较小者为准。宽高比可能与 与屏幕的宽高比完全一致，因此您可能需要对 剪裁到信息流，以全屏模式显示。要获得正确的 将可用的输出尺寸与显示尺寸进行比较 并考虑到显示屏可能会旋转。

以下代码定义了一个辅助类 SmartSize，用于设置大小 更轻松地进行比较：

Kotlin

/** Helper class used to pre-compute shortest and longest sides of a [Size] */
class SmartSize(width: Int, height: Int) {
    var size = Size(width, height)
    var long = max(size.width, size.height)
    var short = min(size.width, size.height)
    override fun toString() = "SmartSize(${long}x${short})"
}

/** Standard High Definition size for pictures and video */
val SIZE_1080P: SmartSize = SmartSize(1920, 1080)

/** Returns a [SmartSize] object for the given [Display] */
fun getDisplaySmartSize(display: Display): SmartSize {
    val outPoint = Point()
    display.getRealSize(outPoint)
    return SmartSize(outPoint.x, outPoint.y)
}

/**
 * Returns the largest available PREVIEW size. For more information, see:
 * https://d.android.com/reference/android/hardware/camera2/CameraDevice
 */
fun <T>getPreviewOutputSize(
        display: Display,
        characteristics: CameraCharacteristics,
        targetClass: Class <T>,
        format: Int? = null
): Size {

    // Find which is smaller: screen or 1080p
    val screenSize = getDisplaySmartSize(display)
    val hdScreen = screenSize.long >= SIZE_1080P.long || screenSize.short >= SIZE_1080P.short
    val maxSize = if (hdScreen) SIZE_1080P else screenSize

    // If image format is provided, use it to determine supported sizes; else use target class
    val config = characteristics.get(
            CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)!!
    if (format == null)
        assert(StreamConfigurationMap.isOutputSupportedFor(targetClass))
    else
        assert(config.isOutputSupportedFor(format))
    val allSizes = if (format == null)
        config.getOutputSizes(targetClass) else config.getOutputSizes(format)

    // Get available sizes and sort them by area from largest to smallest
    val validSizes = allSizes
            .sortedWith(compareBy { it.height * it.width })
            .map { SmartSize(it.width, it.height) }.reversed()

    // Then, get the largest output size that is smaller or equal than our max size
    return validSizes.first { it.long <= maxSize.long && it.short <= maxSize.short }.size
}

Java

/** Helper class used to pre-compute shortest and longest sides of a [Size] */
    class SmartSize {
        Size size;
        double longSize;
        double shortSize;

        public SmartSize(Integer width, Integer height) {
            size = new Size(width, height);
            longSize = max(size.getWidth(), size.getHeight());
            shortSize = min(size.getWidth(), size.getHeight());
        }

        @Override
        public String toString() {
            return String.format("SmartSize(%sx%s)", longSize, shortSize);
        }
    }

    /** Standard High Definition size for pictures and video */
    SmartSize SIZE_1080P = new SmartSize(1920, 1080);

    /** Returns a [SmartSize] object for the given [Display] */
    SmartSize getDisplaySmartSize(Display display) {
        Point outPoint = new Point();
        display.getRealSize(outPoint);
        return new SmartSize(outPoint.x, outPoint.y);
    }

    /**
     * Returns the largest available PREVIEW size. For more information, see:
     * https://d.android.com/reference/android/hardware/camera2/CameraDevice
     */
    @RequiresApi(api = Build.VERSION_CODES.N)
    <T> Size getPreviewOutputSize(
            Display display,
            CameraCharacteristics characteristics,
            Class <T> targetClass,
            Integer format
    ){

        // Find which is smaller: screen or 1080p
        SmartSize screenSize = getDisplaySmartSize(display);
        boolean hdScreen = screenSize.longSize >= SIZE_1080P.longSize || screenSize.shortSize >= SIZE_1080P.shortSize;
        SmartSize maxSize;
        if (hdScreen) {
            maxSize = SIZE_1080P;
        } else {
            maxSize = screenSize;
        }

        // If image format is provided, use it to determine supported sizes; else use target class
        StreamConfigurationMap config = characteristics.get(
                CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);
        if (format == null)
            assert(StreamConfigurationMap.isOutputSupportedFor(targetClass));
        else
            assert(config.isOutputSupportedFor(format));
        Size[] allSizes;
        if (format == null) {
            allSizes = config.getOutputSizes(targetClass);
        } else {
            allSizes = config.getOutputSizes(format);
        }

        // Get available sizes and sort them by area from largest to smallest
        List <Size> sortedSizes = Arrays.asList(allSizes);
        List <SmartSize> validSizes =
                sortedSizes.stream()
                        .sorted(Comparator.comparing(s -> s.getHeight() * s.getWidth()))
                        .map(s -> new SmartSize(s.getWidth(), s.getHeight()))
                        .sorted(Collections.reverseOrder()).collect(Collectors.toList());

        // Then, get the largest output size that is smaller or equal than our max size
        return validSizes.stream()
                .filter(s -> s.longSize <= maxSize.longSize && s.shortSize <= maxSize.shortSize)
                .findFirst().get().size;
    }

检查支持的硬件级别

如需确定在运行时可用的功能，请检查支持的硬件 使用 CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL。

使用 CameraCharacteristics 对象，您可以使用一个语句来检索硬件级别：

Kotlin

val characteristics: CameraCharacteristics = ...

// Hardware level will be one of:
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_LEGACY,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_EXTERNAL,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_LIMITED,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_FULL,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_3
val hardwareLevel = characteristics.get(
        CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL)

Java

CameraCharacteristics characteristics = ...;

// Hardware level will be one of:
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_LEGACY,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_EXTERNAL,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_LIMITED,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_FULL,
// - CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_3
Integer hardwareLevel = characteristics.get(
                CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL);

将各部分整合在一起

通过输出类型、输出大小和硬件级别，您可以确定 一组有效的数据流。下图是 具有 CameraDevice 支持的 LEGACY 硬件级别。

目标 1		目标 2		目标 3		应用场景示例
类型	最大大小	类型	最大大小	类型	最大大小
PRIV	MAXIMUM					简单预览、GPU 视频处理或不预览视频录制。
JPEG	MAXIMUM					无取景器静态图片拍摄。
YUV	MAXIMUM					应用内视频/图片处理。
PRIV	PREVIEW	JPEG	MAXIMUM			标准静态成像。
YUV	PREVIEW	JPEG	MAXIMUM			应用内处理加静态拍摄。
PRIV	PREVIEW	PRIV	PREVIEW			标准录制。
PRIV	PREVIEW	YUV	PREVIEW			预览 + 应用内处理。
PRIV	PREVIEW	YUV	PREVIEW			预览 + 应用内处理。
PRIV	PREVIEW	YUV	PREVIEW	JPEG	MAXIMUM	静态拍摄加上应用内处理。

LEGACY 是可能的最低硬件级别。此表显示 支持 Camera2（API 级别 21 及更高级别）的设备最多可输出 使用正确的配置同时在线播放多个视频 限制性能的开销，例如内存、CPU 或散热约束。

您的应用还需要配置目标输出缓冲区。例如， 以硬件级别为 LEGACY 的设备为目标，您可以设置两个目标输出 surface，一个使用 ImageFormat.PRIVATE，另一个使用 ImageFormat.YUV_420_888。这是使用 PREVIEW 大小。使用本主题前面定义的函数，获取 相机 ID 所需的预览尺寸需要以下代码：

Kotlin

val characteristics: CameraCharacteristics = ...
val context = this as Context  // assuming you are inside of an activity

val surfaceViewSize = getPreviewOutputSize(
    context, characteristics, SurfaceView::class.java)
val imageReaderSize = getPreviewOutputSize(
    context, characteristics, ImageReader::class.java, format = ImageFormat.YUV_420_888)

Java

CameraCharacteristics characteristics = ...;
        Context context = this; // assuming you are inside of an activity

        Size surfaceViewSize = getPreviewOutputSize(
                context, characteristics, SurfaceView.class);
        Size imageReaderSize = getPreviewOutputSize(
                context, characteristics, ImageReader.class, format = ImageFormat.YUV_420_888);

它需要使用提供的回调等待 SurfaceView 准备就绪：

Kotlin

val surfaceView = findViewById <SurfaceView>(...)
surfaceView.holder.addCallback(object : SurfaceHolder.Callback {
  override fun surfaceCreated(holder: SurfaceHolder) {
    // You do not need to specify image format, and it will be considered of type PRIV
    // Surface is now ready and you could use it as an output target for CameraSession
  }
  ...
})

Java

SurfaceView surfaceView = findViewById <SurfaceView>(...);

surfaceView.getHolder().addCallback(new SurfaceHolder.Callback() {
            @Override
            public void surfaceCreated(@NonNull SurfaceHolder surfaceHolder) {
                // You do not need to specify image format, and it will be considered of type PRIV
                // Surface is now ready and you could use it as an output target for CameraSession
            }
            ...
        });

您可以通过调用SurfaceView SurfaceHolder.setFixedSize() 或者，您可以采用 AutoFitSurfaceView 来自 Common 单元 GitHub 上提供了相机示例，该示例设置了绝对尺寸， 同时考虑宽高比和可用空间 调整何时触发 activity 更改。

从以下位置设置另一个 surface 所需格式的 ImageReader 为 因为没有需要等待的回调：

Kotlin

val frameBufferCount = 3  // just an example, depends on your usage of ImageReader
val imageReader = ImageReader.newInstance(
    imageReaderSize.width, imageReaderSize.height, ImageFormat.YUV_420_888,
    frameBufferCount)

Java

int frameBufferCount = 3;  // just an example, depends on your usage of ImageReader
ImageReader imageReader = ImageReader.newInstance(
                imageReaderSize.width, imageReaderSize.height, ImageFormat.YUV_420_888,
                frameBufferCount);

使用 ImageReader 等阻塞目标缓冲区时，请丢弃经过训练的 使用它们：

Kotlin

imageReader.setOnImageAvailableListener({
  val frame =  it.acquireNextImage()
  // Do something with "frame" here
  it.close()
}, null)

Java

imageReader.setOnImageAvailableListener(listener -> {
            Image frame = listener.acquireNextImage();
            // Do something with "frame" here
            listener.close();
        }, null);

LEGACY 硬件级别针对的是最低标准 设备。您可以 添加条件分支并对其中一个输出目标使用 RECORD 大小 甚至在硬件级别为 LIMITED 的设备中显示，甚至将该级别提高到 硬件级别为 FULL 的设备的 MAXIMUM 大小。