qamlib Internals

Buffering and V4L2 buffers

To allow buffering and still be able drop the oldest frames if we run out of buffer space, qamlib implements a ring buffer in userspace where we hold the latest V4L2 buffers untill they are wanted or dropped/skipped. This means that we create more V4L2 buffers than strictly necessary, so that we can fill the userspace ringbuffer while still having buffers for the driver.

The user is able to specify the amount of buffering they want by setting the buffers argument for the Camera constructor, if set, starting the stream will result in an exception if we are unable to allocate enough V4L2 buffers to reach the desired amount of buffers. If the user does not request a specific amount of buffers then qamlib will default to a ringbuffer of size 10 but it may be smaller if we are not able to allocate enough V4L2 buffers to reach that size, but if we can’t allocate more than 2 V4L2 buffers, then it will also result in an exception.

When streaming, to use the buffered call get_frame(buffered=True), this will get the next frame in the buffer as opposed to the newest frame. When calling get_frame(buffered=True) if qamlib detects that one or more frames have been dropped from the ringbuffer then a DroppedFrameException will be thrown, it is possible to suppress throwing this exception.