Android渲染-AHardwareBuffer
本文主要从应用的角度介绍android的native层AHardwareBuffer创建纹理以及保存渲染数据。
HardwareBuffer
要介绍native层的AHardwareBuffer，就需要先从Java层的HardwareBuffer说起。Android官方对于HardwareBuffer介绍如下：
HardwareBuffer wraps a native AHardwareBuffer object, which is a low-level object representing a memory buffer accessible by various hardware units. HardwareBuffer allows sharing buffers across different application processes. In particular, HardwareBuffers may be mappable to memory accessibly to various hardware systems, such as the GPU, a sensor or context hub, or other auxiliary processing units. For more information, see the NDK documentation for AHardwareBuffer.
HardwareBuffer 官方介绍为一种底层的内存 buffer 对象，可在不同进程间共享，可映射到不同硬件系统，如 GPU、传感器等，从构造函数可以看出，其可以指定 format 和 usage，用来让底层选择最合适的实现。
从HardwareBuffer的源码中可以了解到，HardwareBuffer只是 GraphicBuffer 的一个包装。在Android早期版本(APIi32Width; h_buffer_desc.height = frameData->i32Height; h_buffer_desc.width = frameData->i32Width; h_buffer_desc.layers = 1; h_buffer_desc.format = 0x11; h_buffer_desc.usage = AHARDWAREBUFFER_USAGE_CPU_WRITE_OFTEN|AHARDWAREBUFFER_USAGE_GPU_SAMPLED_IMAGE; int ret = AHardwareBuffer_allocate(&h_buffer_desc, &inputHWBuffer); EGLint attr[] = {EGL_NONE}; EGLDisplay edp; edp = (EGLDisplay)eglGetCurrentDisplay(); inputEGLImage) = eglCreateImageKHR(edp, EGL_NO_CONTEXT, EGL_NATIVE_BUFFER_ANDROID, eglGetNativeClientBufferANDROID(inputHWBuffer), attr); glGenTextures(1, &textureID); glBindTexture(GL_TEXTURE_EXTERNAL_OES, textureID); glTexParameteri(GL_TEXTURE_EXTERNAL_OES , GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_EXTERNAL_OES , GL_TEXTURE_MAG_FILTER, GL_LINEAR); glEGLImageTargetTexture2DOES(GL_TEXTURE_EXTERNAL_OES , (GLeglImageOES)inputEGLImage); GLUtils::CheckGLError("eglCreateImageKHR"); } AHardwareBuffer_Planes planes_info = {0}; int ret = AHardwareBuffer_lockPlanes(inputHWBuffer, AHARDWAREBUFFER_USAGE_CPU_WRITE_MASK, -1, nullptr, &planes_info); if (ret != 0) { LOGI("Failed to AHardwareBuffer_lockPlanes"); } else { memcpy(planes_info.planes[0].data,frameData->ppu8Plane[0],frameData->i32Width * frameData->i32Height*3/2); ret = AHardwareBuffer_unlock(inputHWBuffer, nullptr); if (ret != 0) { LOGI("Failed to AHardwareBuffer_unlock"); } } glBindTexture(GL_TEXTURE_EXTERNAL_OES, textureID); FUN_END_TIME("RenderContext::CreateOESTexture")
AHardwareBuffer读取纹理图像数据
读取纹理图像数据的方式和创建纹理的方式类似，通过上述创建纹理的方式，我们实现了AHardwareBuffer 和 EGLImageKHR的绑定，因此，我们可以通过反向思维，将纹理读取出来，代码如下：
FUN_BEGIN_TIME("RenderContext::ReadOESTexture")
unsigned char *ptrReader = nullptr;
ret = AHardwareBuffer_lock(inputHWBuffer, AHARDWAREBUFFER_USAGE_CPU_READ_OFTEN, -1, nullptr, (void **) &ptrReader);
memcpy(dstBuffer, ptrReader, imgWidth * imgHeight * 3 / 2);
ret = AHardwareBuffer_unlock(inputHWBuffer, nullptr);
FUN_END_TIME("RenderContext::ReadOESTexture")
至此，我们可以将dstBuffer通过字节，或者其他形式，保存为图像数据。
总结
针对Android侧，我们需要理清GraphicBuffer、AHardwareBuffer、ANativeWindowBuffer之间的关系。从联系上，GraphicBuffer 继承了ANativeWindowBuffer，所以可以直接通过static_cast类型转换成ANativeWindowBuffer，不过由于是多继承，所以转完有一个地址偏移（static_cast 自动完成）。而AHardwareBuffer只是一个抽象的概念，没有具体类型，与GraphicBuffer 没有任何继承关系，也没有具体的类型，是个空结构体，类似于void 类型。从源码可以看到，aosp封装的那些AHardwareBuffer_xxx接口，本质上底层都是通过AHardwareBuffer_to_GraphicBuffer,转成GraphicBuffer，依旧用GraphicBuffer的形式做的后续处理。
