C / C++ / MFC

Kiran Satish28-Jun-08 12:52

28-Jun-08 12:52

The source of timestamps is a small custom defined class with calls to QueryPerformanceCounter/frequency. Yes, those critical sections have no real use as far as I tested and we dont use in our application. Actually another lab took our code and modified it for their application and their programmer left without actually resolving these issues and they want me to check it. The timing is pretty much accurate for 'frame write' around ~20ms, its the 'frame read' that are not what we expect.

PKNT

Mark Salsbery28-Jun-08 12:59

28-Jun-08 12:59

Interesting.

The timestamps look like something somewhere is relying on a low-res timer...
suspiciously with an accuracy very close to the tickcount in Windows Smile | :)

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish28-Jun-08 13:24

28-Jun-08 13:24

But its not either ~15ms or ~31ms all the time, when he displays some real-time graph from analysis of each frame, the time diff between two successive frame reads is all over the place from ~2ms to ~42ms. Maybe I have to provide with more clear code to understand. But anyways the basic idea of their application is, camera frame grabbing is always ~20ms so, as soon as a frame is grabbed and while we are doing analysis of the current frame, we have the camera to capture the next frame which will take another 20ms and since we know analysis doesnt take more than 10ms, theoretically the loop has to wait for the camera to get the next frame for the rest of 10ms while the camera is capturing next frame. What would be the best way to accomplish this, I have some ideas that I would like to try on Monday, but meanwhile if you come up with something, please lemme know.

PKNT

Mark Salsbery28-Jun-08 15:31

28-Jun-08 15:31

Kiran Satish wrote:
I have to say that the loop function runs on a TIMER which has 5msec timeout value

What kind of timer?

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish28-Jun-08 15:37

28-Jun-08 15:37

Its the default MFC Timer that we initialize using CWnd::SetTimer function, but the camera grabbing runs in a different thread.

PKNT

Mark Salsbery28-Jun-08 16:16

28-Jun-08 16:16

Kiran Satish wrote:
ts the default MFC Timer that we initialize using CWnd::SetTimer function

Not good. I recommend multimedia timers (after all, this is what they're for!).
IME, on XP+ they are pretty precise to 1ms Smile | :)

Kiran Satish wrote:
but the camera grabbing runs in a different thread

Are you responsible for grabbing frames at the interval you want or does the
hardware/driver signal you somehow when a frame is available?

IME, the ideal situation is when the driver can signal an event that wakes a thread for each frame.
If that's not possible and you have to grab frames at a given interval, the next best performance is
with multimedia timers.

I personally wouldn't use Windows timers except for situations where timer resolution doesn't matter at all.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish28-Jun-08 16:21

28-Jun-08 16:21

I never used those multimedia timers, maybe its time for me to use them Smile | :)

. Coming to camera grabbing, we use a command from the frame grabber's library and whenever you call that command, it grabs a frame with a set predefined exposure time which, in our case is 20ms. So unless you call that command it wont capture a frame.

PKNT

Mark Salsbery29-Jun-08 7:03

29-Jun-08 7:03

In that case, it seems like you don't need a timer.

The grabber thread can loop grabbing frames - the exposure time will
throttle the loop to 50 fps.
After each frame is grabbed, set an event that wakes the processing thread.

The processing thread can loop waiting on the grabber event. When the event
is signaled, it can process the frame and wait again.
If your processing time is ~10ms your frame-data critical section should never
have to wait/block.

Ideally, you'd want to eliminate the need for the critical section altogether
if possible, even if you have to copy the frame data to another buffer on the
processing thread.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Mark Salsbery29-Jun-08 7:26

29-Jun-08 7:26

Here's what I meant in the last post:

// Camera Thread
While capturing
{
   Capture frame (20ms exposure)
   Add frame data to shared buffer
   Set frame-grabbed event
}

// Processing Thread
While processing
{
   Wait on frame-grabbed event
   Copy data from shared buffer
   Process the data
}

You still need to synchronize access to the shared buffer. Only lock
long enough to copy data to and from the buffer, then release the lock
immediately.

That's all based on ideal situation where processing time is always
~10ms.

In reality, your threads can (and will be) interrupted by other threads
in the system. If it's critical to process every frame, then change the shared
buffer to a FIFO queue of some kind. The grabber thread can queue each frame
and the processing thread can dequeue frames. If the queue holds pointers to
frame buffers, then accessing the queue is fast, which makes lock time shorter.
That's pretty much how I handle audio capture, where I want every sample regardless
of thread timing fluctuations.

Does that make sense? Smile | :)

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish29-Jun-08 8:33

29-Jun-08 8:33

Yeh, it makes sense, thanks a lot. Actually thats what I like to implement , but I am not thinking of implementing locking of shared buffer earlier. But before implementing this, I would like to time exactly how much time each function in the processing thread is taking in worst case. Based on those timings, I would implement queuing on the captured frames. I have to change a lot of code thats currently been implemented. Since, this is not our lab's work, I might be working on and off on this. So, will keep here posted on how it goes Smile | :)

PKNT

Mark Salsbery29-Jun-08 8:48

29-Jun-08 8:48

Cool good luck!

Keep in mind: Locking is bad - eliminate it if possible - only lock for as long as necessary,
and there's lots of other threads running in the system to mess with your timing Smile | :)

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish30-Jun-08 10:36

30-Jun-08 10:36

I have a question, I tried to implement the same thing in my application (with some difference, in our application we can not grab frame while processing the current image as we have to update other hardware before grabbing next frame). Here is what I tried to do, I have a camera thread and my loop thread. The loop thread waits for the Camera thread to get a new frame and once the camera gets new frame, it waits for the loop thread until it process and signals it to get next frame. Thats the basic idea. After doing the current image analysis and changing the hardware status based upon current image analysis results, I spend around 6-8ms to update some displays and do some logs, so I thought I can signal the camera meanwhile to get new frame. But I am stuck with deadlocks with signal flags between threads.

Loop thread
do{
if (newframe){
do analysis on the frame
signal = true;//signal the camera to get new frame
update displays and do logs for the current frame
}
}while(looprunning);

camera thread
do{
if (signal){
newframe = false;
get new frame - takes 20ms
newframe = true;
signal = false;
}
}while(camrunning);

I can see an obvious deadlock, how can I get around this problem Confused | :confused:

?

-thanks

PKNT

Mark Salsbery30-Jun-08 10:53

30-Jun-08 10:53

Flags? no no no Smile | :)

There's handy synchronization objects for that, something like:

autoreset events:
   newframeevent
   signalevent

Loop thread
do
{
   waitForSingleObject(newframeevent)
   do analysis on the frame
   SetEvent(signalevent)  //signal the camera to get new frame
   update displays and do logs for the current frame
}
while(looprunning);


camera thread
do
{
   waitForSingleObject(signalevent)
   get new frame - takes 20ms
   SetEvent(newframeevent)
}
while(camrunning);

The worst thing I see in the flags implementation is two threads modifying the signal flag.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish30-Jun-08 11:00

30-Jun-08 11:00

Thanks, I feel like a complete dumbo. I always use CEvent::WaitForSingleObject(,) in many threads so that they wont lock the CPU and never thought of using them here. Maybe I need to start thinking slowly Poke tongue | ;-P

PKNT

Mark Salsbery30-Jun-08 11:03

30-Jun-08 11:03

Kiran Satish wrote:
I always use CEvent::WaitForSingleObject(,) in many threads so that they wont lock the CPU

Yes!. It also throttles your thread loops so they don't sit there spinning
while waiting for the flag to be set, eating all your CPU for 1 processor
(each).

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Mark Salsbery30-Jun-08 11:05

30-Jun-08 11:05

plus...you can add a terminate event if needed so the threads can be shutdown gracefully.

Then you'd wait on multiple objects.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish30-Jun-08 13:12

30-Jun-08 13:12

I tried to implement this in simple way to test and rather than waiting for the event to occur, it runs continuously even I define the timeout value to be INIFINITE

getsnap()
{
start camera thread if not running
PulseEvent(getnewframe)
waitForSingleObject(newframeevent, INFINITE)
return;
}

camera thread
do
{
   waitForSingleObject(getnewframe, INFINITE)
   get new frame - takes 20ms
   PulseEvent(newframeevent)
}while(camrunning);

Here the camera thread continues running even after taking a frame. I am sure 'm missing something here. Sorry for bothering Smile | :)

.

-thanks

PKNT

Mark Salsbery30-Jun-08 19:08

30-Jun-08 19:08

I would use (and recommend) SetEvent() on auto-reset events and not PulseEvent():

getnewframe = ::CreateEvent(NULL, FALSE, FALSE, NULL);
newframeevent = ::CreateEvent(NULL, FALSE, FALSE, NULL);

I can't remember why but something about PulseEvent() is flaky/unreliable if I
recall correctly (besides the fact that a thread has to be already waiting on it
for it to work, which is bad for this case)....I just know I never use it.

Compare that to SetEvent() - "The state of an auto-reset event object remains
signaled until a single waiting thread is released". That's the behavior you'd want.

Mark

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish1-Jul-08 6:04

1-Jul-08 6:04

I tired to implement using SetEvent, but it still deadlocks, as I dont see any processor activity at all and just a busy mouse icon on the application interface. Here is how I did it -

I defined the event handles at the start of the application in your way rather than using CEvent objects like I did before

getnewframe = ::CreateEvent(NULL, FALSE, FALSE, NULL);
newframeevent = ::CreateEvent(NULL, FALSE, FALSE, NULL);

when the user clicks snapshot, I call a function (snapshot)

snapshot(exposure)
{
set camera exposure
start snapshotthread if not running
SetEvent(getnewframe)
::WaitForSingleObject(waitfornewframe, INFINITE);
display the frame
}
.
.
.
camthread
{
do
{
::WaitForSingleObject(getnewframe, INFINITE);
take a 20ms snapshot
SetEvent(waitfornewframe);
}while(camrunning)
}

I still dont get where I am doing wrong Confused | :confused:

. I even tried using a while loop in snapshot function and using a 1ms timeout value for WaitForSingleObject and checking for WAIT_OBJECT_0.

PKNT

Mark Salsbery1-Jul-08 6:39

1-Jul-08 6:39

I'm not seeing a deadlock situation in the pseudo code...

If you run it in the debugger and break, are both threads sitting in the Wait function?
If so, why?

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish1-Jul-08 6:53

1-Jul-08 6:53

Here is what happening in debug more

snapshot(exposure)
{
set camera exposure
start snapshotthread if not running
SetEvent(getnewframe) 
::WaitForSingleObject(waitfornewframe, INFINITE); -- set breakpoint(1)
display the frame
}
.
.
.
camthread
{
do
{
dummy variable -- set breakpoint(2)
::WaitForSingleObject(getnewframe, INFINITE);
take a 20ms snapshot
SetEvent(waitfornewframe);
}while(camrunning)
}

when I hit snapshot button, the program stops at breakpoint(1) and then goes to breakpoint(2) and thats it from there it does nothing while the C++ interface shows as the program is in [run] mode. From the definitions of SetEvent and WaitForSingleObject I too dont see any thing wrong here.

Do they need to be in actual two different threads, I dont think thats the case, interesting!

PKNT

Mark Salsbery1-Jul-08 6:57

1-Jul-08 6:57

hmm the only reason I see that could happen is if
"take a 20ms snapshot" never returns so the waitfornewframe
event gets set.

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

Kiran Satish1-Jul-08 7:01