Question

I’ve been following apples example, QA1702, on how to capture images using the AVFoundation. I won’t cite the code here because of space concern. A brief description of what I’m trying to achieve:
Use the iPhone camera to pass a “video” (actually a sequence of images) to a web server, and I know this is possible. However in order to be able to pass the an image using the HTTP POST as in this example, I have to save the image. not necessarily in the photos album but I wan’t to be able to view the pictures there as well in debug purposes.

The apple QA1702 contains 3 methods:

- (void)setupCaptureSession 

- (void)captureOutput:(AVCaptureOutput *)captureOutput 
    didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer 
    fromConnection:(AVCaptureConnection *)connection

//this is modified to be void as you might see, will get back to this
- (void) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer

in setupCaptureSession I start the session as in the example. the captureOutput is only running the imageFromSampleBuffer, and that’s where I’ve added some changes:

// Create a Quartz image from the pixel data in the bitmap graphics context
CGImageRef quartzImage = CGBitmapContextCreateImage(context); 
// Unlock the pixel buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);

// Free up the context and color space
CGContextRelease(context); 
CGColorSpaceRelease(colorSpace);

//library is declared in .h and is a ALAssetsLibrary
[library writeImageToSavedPhotosAlbum:quartzImage orientation:ALAssetOrientationDown completionBlock:nil];

// Release the Quartz image
CGImageRelease(quartzImage); 

I’ve removed the creation of the UIImage and changed it to void typ since I do the writeImageToSavedPhotosAlbum: with the CGImageRef here instead.

The problem as I see it is that during the 10sec that I capture images ~150 calls to captureOutput are made, and therefor the same amount to writeImageToSavedPhotos but only ~5-10 pictures are saved. I’m aware of the memory abuse this is but since I’m not getting any warnings I can’t figure out why not more images are created. and what can I do about it? Is it because, and I’m only guessing now, writeImageToSavedPhotos starts new threads and the iPhone can’t handle more than a certain amount of threads. I’ve read something about NSOperationQueue, should I look into it?

On a side note, I use a NSTimer in setupCaptureSession:

    [NSTimer scheduledTimerWithTimeInterval: 10.0 target:self selector:@selector(timerFireMethod:) userInfo:nil repeats: NO];

however I want to start it in first call to captureOutput in order to avoid time elapsing during the startup of video camera. but if I move this code line to captureOutput then timerFireMethod: is never called? any ideas?

Answer 1

This is solvable with NSOperationQueue, but is no longer interesting to me since writing to file is way to ineffective for most applications.