ofVideoGrabber frame lag?

beginners
#1

Hi All!

I’ve been having a problem with what appears to be a lag or maybe buffering in ofVideoGrabber and would really appreciate a bit of help.

THE BACKGROUND:
we have an app where the relationship between whats shown on screen and what the cam records at any one moment is really important. I need to know that when I am showing frame x to the screen at time t, the grab at time t shows exactly frame x. as such the ofqtvideosaver isn’t much use because it occasionally drops frames.

HOW WE ARE GRABBING
We are trying to grab quickly while keeping that relationship between screen and camera intact so we push into a vector of unsigned chars each frame and then write those to a file at the end. Like this:

//show things to the screen here

unsigned char * someLocalPixels = new unsigned char[camWidth*camHeight*3];
memcpy(someLocalPixels, pixels, (camWidth*camHeight*3));
imgPixels.push_back(someLocalPixels);

for (i = 0; i < imgPixels.size(); i++)
{
//cout<<i<<"< i in COLOR_SINGLE ** frames being written to images \n";
//cout << i/framesPerGreyValue << " frameCounter/framesPerGreyValue \n";
//cout << i%framesPerGreyValue << " frameCounter%framesPerGreyValue \n";
cameraCapture.setFromPixels(imgPixels_, camWidth, camHeight, OF_IMAGE_COLOR, true);
cameraCapture.saveImage(whichAnalysis+"_"+ofToString(i-latency)+"_"+ofToString(i)+".jpg");
}

[i][i]THE PROBLEM

The problem is that we when we look at the images saved out there are about 3 frames at the beginning which have clearly been recorded BEFORE we started showing things to the screen.

HELP? :’(

Can anyone tell me if the videoGrabber buffers a few frames? Ie when we call getPixels() on the grabber object is it definitely the pixels from the camera image which was captured this frame or a couple of frames previously?

I really hope that’s clear enough to understand.

we are using oF007 and running OSX 10.7 lion on a macbookpro i7 2.2

thanks everyone.
_[/i]

#2

Are you calling isFrameNew() on the grabber instance before copying the pixels over? There’s no guarantee that each OF application frame corresponds to a video grabber frame, since the device might not report new data each time you’re calling update(). I could probably offer some better advice if I could see some more of your code, the update() and where you’re removing/deleting the unsigned char* from your vector.

Not necessary at all, but if you’re doing lots of vector manipulation, I would recommend using ofPixels instead of an unsigned char* as it has proper copy constructors and destructors.

#3

Hey Joshua!

thanks for your quick reply (Jamie Allen says thank you too - he’s sitting next to me). We stuck an isFrameNew() around everything but still same problem. We are looking at using ofPixels now - that deconstructor might really help. Cheers fella will come back with some more code if we can’t sort it out.

#4

Nice. Ok, well just noticing that either this:

  
 cameraCapture.setFromPixels(imgPixels, camWidth, camHeight, OF_IMAGE_COLOR, true);

is wrong or this:

  
 imgPixels.push_back(someLocalPixels);

is wrong. Also, if you’re just saving them out once you don’t really need to store them in a vector I think. Doubt that’s the root of your problems but it’s just a thought.