Android Camera预览过程数据流浅析

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。 Android Camera预览过程数据流浅析

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喎�"/kf/ware/vc/" target="_blank" class="keylink">vcGxhdGZvcm1faGFyZHdhcmVfYXRtZWwvYmxvYi9hbmRyb2lkNHNhbV92NC4wL2NhbWVyYS9WNEwyQ2FtZXJhLmNwcAo8YnI+CgoKyerH63ZpZGVvIGNhcHR1cmUgYnVmZmVyCjxicj4KCkNhbWVyYUhhcmR3YXJlU2FtPHN0cm9uZz46Ojwvc3Ryb25nPnN0YXJ0UHJldmlld0ludGVybmFsCiAtPiBWNEwyQ2FtZXJhPHN0cm9uZz46Ojwvc3Ryb25nPnN0YXJ0UHJldmlldwogLT4gaXNpX3Y0bDJfcmVxYnVmcwo8YnI+Cgo8cHJlIGNsYXNzPQ=="brush:java;"> ret = ioctl(fp, VIDIOC_REQBUFS, &req);
内存映射 CameraHardwareSam::startPreviewInternal

1

2

3

4

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 = frame;

            char

*ptr = (char

*)vaddr;

            memcpy(ptr,
src, frame_size);

            //YUY2toYV12(frame,
vaddr, width, height);

        }

        mGrallocHal->unlock(mGrallocHal,
*buf_handle);

    }

    else

        ALOGE("%s:
could not obtain gralloc buffer",
__func__);

    if

(0

!= mPreviewWindow->enqueue_buffer(mPreviewWindow, buf_handle)) {

        ALOGE("Could
not enqueue gralloc buffer!\n");

        goto

callbacks;

    }

}

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

1	`ret = ioctl(win->fd, VIDIOC_REQBUFS, &reqbuf);`

内存映射 hwc_prepare -> assign_heo_overlay_window -> v4l2_overlay_map_buf

1

2

3

*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++;

}

Android Camera预览过程数据流浅析

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