On 31-May-01 Gerd Knorr wrote:
>> Fair enough. Hmm, by reading at the v4l2 API... The v4l2_capability.flag
>> has
>> defines for V4L2_FLAG_READ, and V4L2_FLAG_WRITE, but theres nothing
>> about
>> MMAP. Unless thats the _STREAMING flag, which I find confusing...
>
> Yes, it is the _STREAMING flag. You have to use the STREAMON/STREAMOFF
> ioctls to start/stop capture via mmap()'ed buffers, so the name choice
> isn't that bad I think...
Yes, and no. READ/WRITE indicate something about the method through which
the data is moved in or out of the application (by using read() or write()),
while STREAMING implies something different. I would never assiociate that
with mmap(). In stead I would think that if the STREAMING flag is off, I
can´t get a video stream at all, but only static snapshots, or something
like that. Renaming to _MMAP might be a lot clearer to video app writers,
since as far as I know all video devices stream data anyway.
I mean, even with read() I can ´stream´ by reading continuously (except it´s
not done automatically, like with VIDIOC_QBUF).
I´ve looked at the V4L2 API description about this capturing; looks good,
but I´m missing one detail: what happens when program can´t dequeue fast
enough (or deliberately throttles the framerate). More specifically: when
the queue is full, do you drop the last frame captured, or the oldest? I ask
because originally in my webcam driver I simply dropped the last frame;
however, this quickly leads to stale data in the buffer (I used a few
internal buffers to queue data); switching to dropping the oldest frame was
a bit more difficult to implement, but leaded to much better response time.
Of course, this is ultimately up to the developer of a driver, but it would
be nice to write a few words about it. Or turn it into an option. Sometimes,
all an application is interested in is the latest frame, and don´t care
about queueing, or dropping a few frames (maybe using only one buffer
will do the trick?). Others, like recording tools would be happy to use a
few buffers.
- Nemosoft
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