在上一版本的外景系统 (EVS) 中，IEvsCameraStream 接口定义了一个回调方法，仅用于传递捕获的视频帧。虽然此回调方法简化了 EVS 服务客户端的实现，但它也使客户端难以识别任何流式传输突发事件，因而难以妥善处理相应突发事件。为了增强 EVS 开发体验，AOSP 现在包含一个用于传递流式传输事件的额外回调。
package android.hardware.automotive.evs@1.1;
import @1.0::IEvsCameraStream;
/**
* Implemented on client side to receive asynchronous video frame deliveries.
*/
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
/**
* Receives calls from the HAL each time a video frame is ready for inspection.
* Buffer handles received by this method must be returned via calls to
* IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
* to IEvsCamera::stopVideoStream(), this callback may continue to happen for
* some time as the pipeline drains. Each frame must still be returned.
* When the last frame in the stream has been delivered, STREAM_STOPPED
* event must be delivered. No further frame deliveries may happen
* thereafter.
*
* @param buffer a buffer descriptor of a delivered image frame.
*/
oneway deliverFrame_1_1(BufferDesc buffer);
/**
* Receives calls from the HAL each time an event happens.
*
* @param event EVS event with possible event information.
*/
oneway notify(EvsEvent event);
};
此方法传递的 EvsEventDesc 由三个字段组成：
- 事件的类型。
- 用于标识事件来源的字符串。
- 4 个 32 位字数据，用于包含可能需要提供的事件信息。
/**
* Structure that describes informative events occurred during EVS is streaming
*/
struct EvsEvent {
/**
* Type of an informative event
*/
EvsEventType aType;
/**
* Device identifier
*/
string deviceId;
/**
* Possible additional information
*/
uint32_t[4] payload;
};
此外，为了避免 EVS 与其他 Android 图形组件之间在图形缓冲区说明方面存在任何差异，BufferDesc 已重新定义为使用从 android.hardware.graphics.common@1.2 接口导入的 HardwareBuffer。HardwareBuffer 包含带有缓冲区句柄的 HardwareBufferDescription，后者在 HIDL 中与 Android NDK 的 AHardwareBuffer_Desc 相对应。
/**
* HIDL counterpart of AHardwareBuffer_Desc.
*
* An AHardwareBuffer_Desc object can be converted to and from a
* HardwareBufferDescription object by memcpy().
*
* @sa +ndk libnativewindow#AHardwareBuffer_Desc.
*/
typedef uint32_t[10] HardwareBufferDescription;
/**
* HIDL counterpart of AHardwareBuffer.
*
* AHardwareBuffer_createFromHandle() can be used to convert a HardwareBuffer
* object to an AHardwareBuffer object.
*
* Conversely, AHardwareBuffer_getNativeHandle() can be used to extract a native
* handle from an AHardwareBuffer object. Paired with AHardwareBuffer_Desc,
* AHardwareBuffer_getNativeHandle() can be used to convert between
* HardwareBuffer and AHardwareBuffer.
*
* @sa +ndk libnativewindow#AHardwareBuffer".
*/
struct HardwareBuffer {
HardwareBufferDescription description;
handle nativeHandle;
}
/**
* Structure representing an image buffer through our APIs
*
* In addition to the handle to the graphics memory, need to retain
* the properties of the buffer for easy reference and reconstruction of
* an ANativeWindowBuffer object on the remote side of API calls.
* Not least because OpenGL expect an ANativeWindowBuffer* for us as a
* texture via eglCreateImageKHR().
*/
struct BufferDesc {
/**
* HIDL counterpart of AHardwareBuffer_Desc. Please see
* hardware/interfaces/graphics/common/1.2/types.hal for more details.
*/
HardwareBuffer buffer;
/**
* The size of a pixel in the units of bytes
*/
uint32_t pixelSize;
/**
* Opaque value from driver
*/
uint32_t bufferId;
/**
* Unique identifier of the physical camera device that produces this buffer.
*/
string deviceId;
/**
* Time that this buffer is being filled
*/
int64_t timestamp;
/**
* Frame metadata. This is opaque to EVS manager
*/
vec<uint8_t> metadata
};
注意：HardwareBufferDescription 被定义为由 10 个 32 位字组成的数组。建议您将其转换为 AHardwareBuffer_Desc 类型并填充内容。
EvsEventDesc 是类型为 enum EvsEventType 的结构，列出了多个流式传输事件和一个 32 位字载荷，开发者可在其中添加可能需要提供的其他信息。例如，开发者可针对流式传输错误事件添加一个错误代码。
/**
* Types of informative streaming events
*/
enum EvsEventType : uint32_t {
/**
* Video stream is started
*/
STREAM_STARTED = 0,
/**
* Video stream is stopped
*/
STREAM_STOPPED,
/**
* Video frame is dropped
*/
FRAME_DROPPED,
/**
* Timeout happens
*/
TIMEOUT,
/**
* Camera parameter is changed; payload contains a changed parameter ID and
* its value
*/
PARAMETER_CHANGED,
/**
* Master role has become available
*/
MASTER_RELEASED,
};
帧传递
有了新的 BufferDesc，IEvsCameraStream 还引入了新的回调方法，用于从服务实现接收帧和流式传输事件。
/**
* Implemented on client side to receive asynchronous streaming event deliveries.
*/
interface IEvsCameraStream extends @1.0::IEvsCameraStream {
/**
* Receives calls from the HAL each time video frames are ready for inspection.
* Buffer handles received by this method must be returned via calls to
* IEvsCamera::doneWithFrame_1_1(). When the video stream is stopped via a call
* to IEvsCamera::stopVideoStream(), this callback may continue to happen for
* some time as the pipeline drains. Each frame must still be returned.
* When the last frame in the stream has been delivered, STREAM_STOPPED
* event must be delivered. No further frame deliveries may happen
* thereafter.
*
* A camera device delivers the same number of frames as number of
* backing physical camera devices; it means, a physical camera device
* sends always a single frame and a logical camera device sends multiple
* frames as many as the number of backing physical camera devices.
*
* @param buffer Buffer descriptors of delivered image frames.
*/
oneway deliverFrame_1_1(vec<BufferDesc> buffer);
/**
* Receives calls from the HAL each time an event happens.
*
* @param event EVS event with possible event information.
*/
oneway notify(EvsEventDesc event);
};
较新版本的帧回调方法旨在传递多个缓冲区描述符。因此，如果 EVS 相机实现管理着多个源，就可以通过单次调用转发多个帧。
此外，以前通过发送 null 帧来通知数据流结束的协议已弃用并被 STREAM_STOPPED 事件所取代。
图 1. 事件通知序列图
使用事件和帧通知机制
确定由客户端实现的 IEvsCameraStream 的版本
该服务可以通过尝试向下转换来确定由客户端实现的传入 IEvsCameraStream 接口的版本：
using IEvsCameraStream_1_0 =
::android::hardware::automotive::evs::V1_0::IEvsCameraStream;
using IEvsCameraStream_1_1 =
::android::hardware::automotive::evs::V1_1::IEvsCameraStream;
Return<EvsResult> EvsV4lCamera::startVideoStream(
const sp<IEvsCameraStream_1_0>& stream) {
IEvsCameraStream_1_0 aStream = stream;
// Try to downcast. This succeeds if the client implements
// IEvsCameraStream v1.1.
IEvsCameraStream_1_1 aStream_1_1 =
IEvsCameraStream_1_1::castFrom(aStream).withDefault(nullptr);
if (aStream_1_1 == nullptr) {
ALOGI("Start a stream for v1.0 client.");
} else {
ALOGI("Start a stream for v1.1 client.");
}
// Start a video stream
...
}
notify() 回调
EvsEvent 通过 notify() 回调进行传递，然后客户端就可以根据判别器确定其类型，如下所示：
Return<void> StreamHandler::notify(const EvsEvent& event) {
ALOGD("Received an event id: %u", event.aType);
// Handle each received event.
switch(event.aType) {
case EvsEventType::ERROR:
// Do something to handle an error
...
break;
[More cases]
}
return Void();
}
使用 BufferDesc
AHardwareBuffer_Desc 是 Android NDK 的数据类型，用于表示可绑定到 EGL/OpenGL 和 Vulkan 基元的原生硬件缓冲区。它包含以前的 EVS BufferDesc 中的大多数缓冲区元数据，并因此在新的 BufferDesc 定义中将其取而代之。但是，由于此数据类型被定义为 HIDL 接口中的一个数组，因此无法直接将成员变量编入索引。不过，您可以将该数组转换为 AHardwareBuffer_Desc 类型，如下所示：
BufferDesc bufDesc = {};
AHardwareBuffer_Desc* pDesc =
reinterpret_cast<AHardwareBuffer_Desc *>(&bufDesc.buffer.description);
pDesc->width = mVideo.getWidth();
pDesc->height = mVideo.getHeight();
pDesc->layers = 1;
pDesc->format = mFormat;
pDesc->usage = mUsage;
pDesc->stride = mStride;
bufDesc_1_1.buffer.nativeHandle = mBuffers[idx].handle;
bufDesc_1_1.bufferId = idx;