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Android Camera预览过程数据流浅析

2024-07-03 12:33:02   225人阅读

硬件平台：Atmel SAMA5D3 SoC + OV2640 Camera Sensor
Android版本：4.2.2

mediaserver进程是Camera Service的容器进程，它会动态加载Camera HAL和Gralloc HAL。
视频数据帧首先必须从Camera驱动程序到达Camera硬件抽象层。
在Camera硬件抽象层，视频数据帧被从video capture buffer拷贝到gralloc buffer。

surfaceflinger进程作为显示服务器会动态加载HWComposer HAL和Gralloc HAL。
在HWComposer硬件抽象层，会把数据帧从gralloc buffer拷贝到video output buffer。

经过上述过程，Camera Sensor采集的图像最终通过LCDC HEO显示在显示屏上。


图中红色实线为视频数据帧流向，不带箭头的红线连接的两端为同一块内存。

涉及三块内存，分别如下：
video capture buffer /dev/video1
gralloc buffer       匿名共享内存 mediaserver进程和surfacelinger进程都可以访问这块内存 
video output buffer /dev/video0

进行了两次数据拷贝操作，如下：
media server进程
Camera HAL    video capture buffer -> gralloc buffer

surfaceflinger进程
HWCompser HAL   gralloc buffer -> video output buffer


video capture buffer的分配与内存映射
https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/CameraHardwareSam.cpp
https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/V4L2Camera.cpp

申请video capture buffer

CameraHardwareSam::startPreviewInternal -> V4L2Camera::startPreview -> isi_v4l2_reqbufs

    ret = ioctl(fp, VIDIOC_REQBUFS, &req);

内存映射
CameraHardwareSam::startPreviewInternal

    mPreviewHeap = mGetMemoryCb((int)mV4L2Camera->getCameraFd(),
                                aligned_buffer_size,
                                kBufferCount,
                                0);

获得gralloc buffer以及视频数据帧从video capture buffer到gralloc buffer的拷贝
https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/camera/CameraHardwareSam.cpp


CameraHardwareSam::previewThread
    if (mPreviewWindow && mGrallocHal) {
        buffer_handle_t *buf_handle;
        int stride;
        if (0 != mPreviewWindow->dequeue_buffer(mPreviewWindow, &buf_handle, &stride)) {
            ALOGE("Could not dequeue gralloc buffer!\n");
            goto callbacks;
        }

        void *vaddr;
        if (!mGrallocHal->lock(mGrallocHal,
                               *buf_handle,
                               GRALLOC_USAGE_SW_WRITE_OFTEN,
                               0, 0, width, height, &vaddr)) {
            char *frame = ((char *)mPreviewHeap->data) + offset;

            // the code below assumes YUV, not RGB
            {
                int h;
                char *src = http://www.mamicode.com/frame;>
video output buffer的分配与内存映射
https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/hwcomposer/hwcomposer.cpp
https://github.com/Android4SAM/platform_hardware_atmel/blob/android4sam_v4.0/hwcomposer/v4l2_utils.cpp


申请video ouput buffer
hwc_prepare -> assign_heo_overlay_window -> v4l2_overlay_req_buf
    ret = ioctl(win->fd, VIDIOC_REQBUFS, &reqbuf);

内存映射
hwc_prepare -> assign_heo_overlay_window -> v4l2_overlay_map_buf
    *len = buf.length;
    *start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED,
            fd, buf.m.offset);

视频数据帧从gralloc buffer到video output buffer的拷贝

hwc_set ->  copy_heo_src_content

    for (unsigned int i = 0; i < cur_layer->visibleRegionScreen.numRects; i++) {
        uint8_t *cur_dst_addr = dst_addr;
        uint8_t *cur_src_addr = src_addr;

        for (int j = 0; j < h ; j++) {
            memcpy(cur_dst_addr, cur_src_addr, cpy_size);
            cur_dst_addr = &cur_dst_addr[cpy_size];
            cur_src_addr = &cur_src_addr[(cur_layer->displayFrame.right - cur_layer->displayFrame.left) * (prev_handle->uiBpp / 8)];
        }
        cur_rect++;
    }




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