2

I am writing a simple video messenger-like application, and therefore i need to get frames of some compromise size to be able to fit into the available bandwidth, and still to have the captured image not distorted.

To retrieve frames I am using QTCaptureVideoPreviewOutput class, and i am successfully getting frames in the didOutputVideoFrame callback. (i need raw frames - mostly because i am using a custom encoder, so i just would like to get "raw bitmaps").

The problem is that for these new iSight cameras i am getting literally huge frames.

Luckily, these classes for capturing raw frames (QTCaptureVideoPreviewOutput) provide method setPixelBufferAttributes that allows to specify what kind of frames would i like to get. If i am lucky enough to guess some frame size that camera supports, i can specify it and QTKit will switch the camera into this specified mode. If i am unlucky - i get a blurred image (because it was stretched/shrinked), and, most likely, non-proportional.

I have been searching trough lists.apple.com, and stackoverflow.com, the answer is "Apple currently does not provide functionality to retrieve camera's native frame sizes". Well, nothing i can do about that.

Maybe i should provide in settings the most common frame sizes, and the user has to try them to see what works for him? But what are these common frame sizes? Where could i get a list of the frame dimensions that UVC cameras generate usually?

For testing my application i am using a UVC compliant camera, but not an iSight. I assume not every user is using iSight either, and i am sure even between different models iSight cameras have different frame dimensions.

Or, maybe, i should switch the camera to the default mode, generate a few frames, see what sizes it generates, and at least i will have some proportions? This looks like a real hack, and doesn't seem to be natural. And the image is most likely going to be blurred again.

Could you please help me, how have your dealt with this issue? I am sure i am not the first one who is faced with it. What would be the approach you would choose? Thank you, James

James
  • 81
  • 2

1 Answers1

0

You are right, iSight camera produces huge frames. However, I doubt you can switch the camera to a different mode by setting pixel buffer attributes. More likely you set the mode of processing the frames in the QTCaptureVideoPreviewOutput. Take a look at QTCaptureDecompressedVideoOutput if you have not done it yet. We also use the sample buffer to get the frame size. So, I would not say it's a hack. A more natural way would be to make your own Quicktime Component that implements your custom encoding algorithm. In this case Quicktime would be able to use inside QTCaptureMovieFileOutput during the capture session. It would be a proper, but also a hard way.

Davyd Geyl
  • 4,578
  • 1
  • 28
  • 35
  • I've tried both QTCaptureVideoPreviewOutput and QTCaptureDecompressedVideoOutput. QTKit 'wisely' picks the camera's mode, at least that's what quick time developers say on their mailing list: it is smart enough to set that camera mode, which takes the least cpu power to convert it into requested pixel format. Could you please explain this 'sample buffer' idea that you are using? How do you do it exactly? Thanks. – James Aug 28 '11 at 10:58
  • 1
    You can get access to the single frames of video or to the packets of audio data. Look at the documentation of [QTCaptureFileOutput captureOutput:didOutputSampleBuffer:fromConnection:]. We use it primarily to cut the captured video to chunks. – Davyd Geyl Sep 01 '11 at 13:22