Android FFmpeg专题结构

利用Android 写视频显示应用时，经常会用到SurfaceView等控件来显示视频。
在前面的文章Android基础--SurfaceView, Surface, SurfaceHolder中，也简单得提了一下SurfaceView和Surface的关系：SurfaceView提供了一个专门用于绘制的surface。java层实际上是利用SurfaceView将视频数据渲染到Surface上。

而Native层要渲染视频可以通过ANativeWindow来渲染。

Surface, ANativeWindow

先来看下官网的解释

The public surface class is implemented in the Java programming language. The equivalent in C/C++ is the ANativeWindow class, semi-exposed by the Android NDK. You can get the ANativeWindow from a surface with the ANativeWindow_fromSurface () call. Just like its Java-language cousin, you can lock it, render in software, and unlock-and-post.

可以看到 native层的ANativeWindow 就等价于Java层的Surface，可以通过接口ANativeWindow_fromSurface ()从surface对象中获取到，可以通过软件对其进行lock, render, unlock-and-post等操作

ANativeWindow的用法

看一下大体的流程图

ANativeWindow.png

主要流程是下面几点

• java层将Surface传递给native层
• 获取ANativeWindow对象
• 将显示数据写到ANativeWindow的buffer中，注意需要将显示的数据格式转换成ANativeWindow设置的数据格式
• 释放ANativeWindow

涉及到的类和接口：
https://developer.android.google.cn/ndk/reference/group/a-native-window?hl=zh-cn

Typedefs
ANativeWindow	typedef struct ANativeWindow Opaque type that provides access to a native window.
ANativeWindow_Buffer	typedef struct ANativeWindow_Buffer Struct that represents a windows buffer.

• 获取ANativeWindow

ANativeWindow* ANativeWindow_fromSurface(JNIEnv* env, jobject surface)

• 释放ANativeWindow引用

void ANativeWindow_release(ANativeWindow* window);

• 设置窗口buffer的格式和大小

int32_t ANativeWindow_setBuffersGeometry(ANativeWindow* window,
        int32_t width, int32_t height, int32_t format);

• 锁定窗口的写surface并获取下一个可写的显示缓冲区

int32_t ANativeWindow_lock(ANativeWindow* window, ANativeWindow_Buffer* outBuffer,
        ARect* inOutDirtyBounds);

• 释放窗口的写surface，并将新的缓冲buffer发送给显示模块

int32_t ANativeWindow_unlockAndPost(ANativeWindow* window)

代码节选

static int com_iview_camera_native_setSurface(JNIEnv *env, jobject thiz, jobject surface) {
    if (v4l2Camera == 0) {
        return ERROR_CAPABILITY_UNSUPPORT;
    }

    ANativeWindow *window = ANativeWindow_fromSurface(env, surface);

    v4l2Camera->setSurface(window);

    return 0;
}

void V4L2Camera::setSurface(ANativeWindow *window) {
    std::lock_guard<std::mutex> lock(windowLock);
    if (this->window != 0) {
        ANativeWindow_release(this->window);
    }

    this->window = window;
}

void V4L2Camera::renderVideo(unsigned char *preview) {
    std::lock_guard<std::mutex> lock(windowLock);
    if (window == 0) {
        return;
    }
    ALOGE("RenderVideoElement width:%d, height:%d", width,height);
    ANativeWindow_setBuffersGeometry(window, width,
                                     height,
                                     WINDOW_FORMAT_RGBA_8888);
    ANativeWindow_Buffer window_buffer;
    if (ANativeWindow_lock(window, &window_buffer, 0)) {
        ANativeWindow_release(window);
        window = 0;
        return;
    }
    //把buffer中的数据进行赋值（修改）
    uint8_t *dst_data = static_cast<uint8_t *>(window_buffer.bits);
    memcpy(dst_data, preview, width*height*4);

    ANativeWindow_unlockAndPost(window);

}

完整的代码可以去参考：
利用ffmpeg实现的Android播放器
https://github.com/yizhongliu/FFMediaPlayer

利用V4L2接口操作Android usb摄像头
https://github.com/yizhongliu/AnV4L2Camera