face_detector.py strange behavior

mikael

@pavlinb, if this was originally run on iPhone 6, I bet we can do better than 5 fps on the face detection today. Have you studied enough to know whether we really need to initialize the face detector with options every time it is used?

pavlinb

@mikael My goal is not face detection. I need the method to obtain frames from video camera and use them continuously.

pavlinb

@mikael Correct duration should be 80-90 passes of face detector.

pavlinb

When phone wakes up after auto lock, script makes another 80-90 passes and hangs again.

JonB

I wonder if you need an autorelease pool. There was a thread on this recently, there is a context manager in objc_util.

JonB

Also, get rid of wait_modal, but instead stop everything in a custom view will_close method. Wait_modal keeps your background thread from doing anything else.

pavlinb

@JonB Tried, but same behaviour.

pavlinb

Other way of obtaining video camera frames continuously is welcome.

JonB

For data processing or just displaying as a preview?

pavlinb

@JonB For data processing.

JonB

ok, i do see some issues..

first, we need to implement the didDropSampleBuffer delegate method, and display the reason for a late frame.

second, we need to set the minFrameDuration, so that the delegate is never called more than you need it. on older versions, i believe this was in output.minFrameDuration. in newer iOS versions, i believe you set on the connection, output.connections[0].videoMinFrameDuration

third, there is an issue regarding what dispatch queue things are called on. or, the delegate needs to always return as fast as possible, calling heavy lifting on another thread, and dropping data otherwise. i think what is happening is that your delegate gets called on a background thread, and just keeps falling farther and farther behind. it might be better to ensure the delegate is called on the main thread.

ill post a gist later with some improvements.

pavlinb

@JonB I have tested another good example here

In this script is implemented

self.captureOutput.setMinFrameDuration_(CMTimeMake(1, 2), argtypes=[CMTime], restype=None)

I have plaeyd with CMTimeMake(.,.).

Unfotunately this script also hangs.

JonB

I have a version based of Cethric's. I'll clean it up and post later. I've implemented the drop frame callback so you can see what the issues are.

One problem I have found is that the various ways to set minFrameDuration don't work. That means the callback gets called at a high rate.

pavlinb

It seems that setMinFrameDuration is in camera.captureOutput object ('AVCaptureVideoDataOutput').

But there is also setActiveVideoMinFrameDuration in camera.captureDevice ('AVCaptureDevice').

I couldn't make them working too both.

JonB

try this.
https://gist.github.com/424d4fe1a3c0b1ae3fd705d72f665c1e

increase FRAME_PROC_INTERVAL until you stop seeing FrameLate constantly. or set to 1 to go as fast as possible. you will probably see FrameLate, but if you start seeing OutOfBuffers or something like that, it means you are doing some processing and forgetting to release the capture buffer.

there are many hoops to jump through to get the actual min frame interval set, i think i have all the steps correct, though am on an older device. see if it works for you. I have a DESIRED_FPS that you can change to see if you can get <30 fps..

does this one hang? if so, what message do you get on the first frame when it comes back?

pavlinb

This one works: link

from ctypes import *
from objc_util import *

class CMTime(Structure):
  _fields_ = [('value', c_int64), ('timescale', c_int32), ('flags', c_uint32), ('epoch', c_int64)]

AVCaptureDevice = ObjCClass('AVCaptureDevice')

_device = AVCaptureDevice.defaultDeviceWithDeviceType_mediaType_position_(
  'AVCaptureDeviceTypeBuiltInWideAngleCamera', 'vide', 1)

dur = _device.activeVideoMinFrameDuration(restype=CMTime, argtypes=[])
dur.value *= 3
_device.lockForConfiguration_(None)
_device.setActiveVideoMinFrameDuration_(dur, restype=None, argtypes=[CMTime])
_device.unlockForConfiguration()

pavlinb

@JonB 350 passes and still works. I'll play with parameters to see how it will go.

Thanks.