}
现在让我们看看Preview功能的实现,也许可以给我们启发,我们不难注意到previewThread方法,其中mPreviewHeap存储着n个帧的缓冲,这块区域被分割为n个mBuffers。buffer为当前帧的引用,通过mDataCb(CAMERA_MSG_PREVIEW_FRAME, buffer, mCallbackCookie)就可以将buffer输出到屏幕。那每一个帧是怎么存到mPreviewHeap上的呢?关键的一句就是Ov965xCamera->getNextFrameAsRgb565((uint16_t *)frame) ,通过看它的实现可以知道(在Ov96xCamera.app中),一个帧的数据以16位的格式写入frame中,这里的frame即是对mPreviewHeap上某个mBuffer的引用:
int CameraHardware::previewThread()
{
mLock.lock();
// the attributes below can change under our feet...
int previewFrameRate = mParameters.getPreviewFrameRate();
// Find the offset within the heap of the current buffer.
ssize_t offset = mCurrentPreviewFrame * mPreviewFrameSize;
sp<MemoryHeapBase> heap = mPreviewHeap;
// this assumes the internal state of fake camera doesn't change
// (or is thread safe)
Ov965xCamera* Ov965xCamera = mOv965xCamera;
USBCamera* USBCamera = mUSBCamera;
sp<MemoryBase> buffer = mBuffers[mCurrentPreviewFrame];
mLock.unlock();
// TODO: here check all the conditions that could go wrong
if (buffer != 0) {
// Calculate how long to wait between frames.
int delay = (int)(1000000.0f / float(previewFrameRate));
// This is always valid, even if the client died -- the memory
// is still mapped in our process.
void *base = heap->base();
// Fill the current frame with the fake camera.
uint8_t *frame = ((uint8_t *)base) + offset;
//Ov965xCamera->getNextFrameAsYuv420(frame);