Capturing Stills, Sounds, and Scenes with AV Foundation

Capturing Stills, Sounds,
and Scenes with AV
Foundation
Chris Adamson • @invalidname
Voices That Matter: iOS Developer Conference
Nov. 12, 2011 • Boston, MA

Tuesday, November 15, 11

Road Map

• Media capture technologies in iOS

• AV Foundation capture concepts

• Device-speciﬁc concerns

• Doing stuff with captured media


Capture?
• Digital media encoding of some real-world source,
such as still images, moving images, and/or sound

• Contrast with synthetic media: musical
synthesizers, CG animation

• Not the same as "recording", which implies storage

• Capture devices include cameras and
microphones


iOS Capture Devices


Accessing Capture
Devices
• Simple shoot-and-save -
UIImagePickerController

• Core Audio - low level capture and real-time
processing

• More info in my talk tomorrow

• AV Foundation


AV Foundation
• Introduced in iPhone OS 2.3 as Obj-C
wrapper for Core Audio playback, added
capture in 3.0

• Repurposed in iOS 4 as audio/video capture,
editing, export, and playback framework

• Ported to OS X in Lion, heir apparent to
QuickTime


#import this!
• AVFoundation.framework

• CoreMedia.framework

• Possibly also:

• CoreVideo, CoreImage, CoreGraphics

• AudioToolbox, AudioUnits


Core Media
• C-based helper framework for AVF

• Structures to represent media buffers and
queues of buffers, media times and time
ranges

• Low-level conversion and calculation functions

• Does not provide capture, editing, or
playback functionality


AV Foundation
• Editing / Playback classes

• Assets, compositions, and tracks. Player
and player layer. Asset readers and writers

• Capture classes

• Devices, inputs, outputs, and the session


How it ﬁts together…


AVCaptureSession


AVCaptureDevice

AVCaptureSession


AVCaptureDevice

AVCaptureInput

AVCaptureSession


AVCaptureDevice AVCaptureDevice

AVCaptureInput

AVCaptureSession



AVCaptureInput AVCaptureInput

AVCaptureSession




AVCaptureVideo
AVCaptureSession
PreviewLayer




AVCaptureVideo
AVCaptureSession
PreviewLayer

AVCaptureOutput




AVCaptureVideo
AVCaptureSession
PreviewLayer

AVCaptureOutput AVCaptureOutput


AVCaptureSession

• Coordinates the ﬂow of capture from inputs to
outputs

• Create, add inputs and outputs, start running

captureSession = [[AVCaptureSession alloc] init];


AVCaptureDevice
• Represents a device that can perform media
capture (cameras, microphones)

• Could be connected as external accessory or
Bluetooth

• You cannot make assumptions based on
device model


Discovering Devices

• AVCaptureDevice class methods devices,
deviceWithUniqueID, devicesWithMediaType,
defaultDeviceWithMediaType

• Media types include audio, video, muxed
(audio and video in one stream), plus some
outliers (timecode, etc.)


Inspecting Devices

• position (property): is the camera on the front
or the back?

• supportsAVCaptureSessionPreset: allows you
to inspect whether it can copy at one of
several predeﬁned image resolutions


Photo traits
• Focus & exposure

• isFocusModeSupported:, focusMode,
focusPointOfInterestSupported, focusPointOfInterest,
focusAdjusting

• isExposureModeSupported:, exposureMode,
exposurePointOfInterestSupported, etc.

• White balance

• isWhiteBalanceModeSupported:, whiteBalanceMode,
whiteBalanceModeAdjusting


Light up

• Flash and Torch

• hasFlash, isFlashModeSupported:
flashMode, flashActive, flashAvailable

• hasTorch, isTorchModeSupported:,
torchMode, torchLevel, torchAvailable


AVCaptureSession
sessionPreset
• Constants for video capture quality. Allows
you to inspect capabilities, trade
performance/framerate for resolution

• Default is AVCaptureSessionPresetHigh

• For still photos:
AVCaptureSessionPresetPhoto


iFrame
• Session presets for use when capturing video
intended for subsequent editing

• AVCaptureSessionPresetiFrame960x540,
AVCaptureSessionPresetiFrame1280x720

• No P- or B-frames; ﬁles are much larger than
typical H.264.

http://en.wikipedia.org/wiki/Video_compression_picture_types

Capture inputs

• Connect a device to the capture session

• Instances of AVCaptureDeviceInput

• create with -initWithDevice:error: or
deviceInputForDevice:error


AVCaptureDevice *videoDevice =
! [AVCaptureDevice defaultDeviceWithMediaType:
! AVMediaTypeVideo];

if (videoDevice) {
! AVCaptureDeviceInput *videoInput =
! [AVCaptureDeviceInput
! ! deviceInputWithDevice:videoDevice
! ! ! ! ! ! ! ! error:&setUpError];
! if (videoInput) {
! ! [captureSession addInput: videoInput];
! }
}


Capture preview
• AVCapturePreviewLayer: A CALayer that
shows what's currently being captured from
video input

• Remember: CALayer, not UIView

• videoGravity property determines how it will
deal with preview that doesn't match bounds:
aspect, ﬁll, or resize


AVCaptureVideoPreviewLayer *previewLayer =
! [AVCaptureVideoPreviewLayer
! ! layerWithSession:captureSession];
previewLayer.frame = captureView.layer.bounds;
previewLayer.videoGravity =
! AVLayerVideoGravityResizeAspect;
[captureView.layer addSublayer:previewLayer];


Capture Outputs
• File output: AVCaptureMovieFileOutput and
AVCaptureAudioFileOutput

• Photo output: AVCaptureStillImageOutput

• Image processing: AVCaptureDataOutput

• More on this one later…


AVCaptureFileOutput
• startRecordingToOutputURL:recordingDelegate:

• The delegate must be set and must implement two
callbacks:

• captureOutput:didStartRecordingToOutputFileAt
URL:fromConnections:

• captureOutput:didFinishRecordingToOutputFileAt
URL:fromConnections:

• Then connect to capture session


captureMovieOutput =
! [[AVCaptureMovieFileOutput alloc] init];

if (! captureMovieURL) {
! captureMoviePath = [getCaptureMoviePath() retain];
! captureMovieURL = [[NSURL alloc]
! ! ! initFileURLWithPath:captureMoviePath];
}

NSLog (@"recording to %@", captureMovieURL);
[captureSession addOutput:captureMovieOutput];


Cranking it up
• -[AVCaptureSession startRunning] starts
capturing from all connected inputs

• If you have a preview layer, it will start
getting updated

• File outputs do not start writing to ﬁlesystem
until you call startRecording on them


Demo
AVRecPlay

http://dl.dropbox.com/u/12216224/conferences/vtm10/mastering-media-with-av-
foundation/VTM_AVRecPlay.zip


Orientation issues
• Default orientation of an iOS device is portrait

• The AVCaptureConnections between the
device inputs and the session have a read-
write videoOrientation property.

• Capture layer's orientation property should
match


Capture Processing
• Analyzing or manipulating capture data as it
comes in

• Audio: real-time effects ("I Am T-Pain"),
oscilloscopes, etc.

• May make more sense to use Audio Units

• Video: bar code readers, face-ﬁnders, etc.


Data Outputs
• Connects your code to the capture session
via a delegate callback

• Delegate callback occurs on a serial GCD
queue that you provide (can be
dispatch_get_main_queue(), should not be
dispatch_get_current_queue(), must not be
NULL).


Creating the data
output

AVCaptureVideoDataOutput *captureOutput =
! [[AVCaptureVideoDataOutput alloc] init];
captureOutput.alwaysDiscardsLateVideoFrames =
! YES;
[captureOutput setSampleBufferDelegate:self
! ! ! ! queue:dispatch_get_main_queue()];


Conﬁguring the data
output
NSString* key =
! (NSString*)kCVPixelBufferPixelFormatTypeKey;
NSNumber* value =
! [NSNumber numberWithUnsignedInt:
! ! kCVPixelFormatType_32BGRA];
NSDictionary* videoSettings = [NSDictionary
! dictionaryWithObject:value forKey:key];
[captureOutput setVideoSettings:videoSettings];


Analyzing the data
• You get the callback captureOutput:
didOutputSampleBuffer:fromConnection:

• Second parameter is a CMSampleBufferRef,
Core Media's opaque type for sample buffers

• Could be video… could be audio… (but you
can tell from the connection and its input
and output ports)


Analyzing frames with
Core Video
CVImageBufferRef imageBuffer =
! CMSampleBufferGetImageBuffer(sampleBuffer);
/*Lock the image buffer*/
CVPixelBufferLockBaseAddress(imageBuffer,0);
/*Get information about the image*/
size_t bytesPerRow =
! CVPixelBufferGetBytesPerRow(imageBuffer);
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);

This example is from the ZXing barcode reader
http://code.google.com/p/zxing/

Demo
ZXing

http://code.google.com/p/zxing/


Audio considerations
• Can process CMSampleBufferRef by using
CMSampleBufferGetAudioStreamPacket
Descriptions() and CMSampleBufferGet
AudioBufferListWithRetainedBlockBuffer()

• Then use Core Audio call that take these types

• May make more sense to just capture in Core Audio
in the ﬁrst place, especially if you're playing
captured data through an audio queue or audio units


Face Finding in
iOS 5

• iOS 5 introduces Core Image, which allows us
to chain effects on images

• Also includes some interesting image
processing classes


CIDetector
• Core Image class to ﬁnd features in a Core
Image buffer

• Only supported detector type in iOS 5 is
CIDetectorTypeFace

• featuresInImage: returns an NSArray of all
detected features in the image


Convert CM to CV to CI
CVPixelBufferRef cvPixelBuffer =
! CMSampleBufferGetImageBuffer(sampleBuffer);
CFDictionaryRef attachmentsDict =
! CMCopyDictionaryOfAttachments(
! ! kCFAllocatorSystemDefault,
! ! sampleBuffer,
! ! kCMAttachmentMode_ShouldPropagate);
CIImage *ciImage = [[CIImage alloc]
! ! initWithCVPixelBuffer:cvPixelBuffer
! ! options:(__bridge NSDictionary*)
! ! ! ! attachmentsDict];


Creating the CIDetector
NSDictionary *faceDetectorDict =
! ! [NSDictionary dictionaryWithObjectsAndKeys:
! ! ! CIDetectorAccuracyHigh,
! ! ! CIDetectorAccuracy,
! ! ! nil];
CIDetector *faceDetector =
[CIDetector detectorOfType:CIDetectorTypeFace
context:nil
options:faceDetectorDict];
NSArray *faces = [faceDetector
! ! ! featuresInImage:ciImage];


Demo
VTMFaceFinder

http://dl.dropbox.com/u/12216224/conferences/vtm11/VTMFaceFinder.zip


Boxing the faces

for (CIFaceFeature *faceFeature in self.facesArray) {
CGRect boxRect = CGRectMake(
faceFeature.bounds.origin.x * self.scaleToApply,
faceFeature.bounds.origin.y * self.scaleToApply,
faceFeature.bounds.size.width * self.scaleToApply,
faceFeature.bounds.size.height * self.scaleToApply);
CGContextSetStrokeColorWithColor(cgContext,
[UIColor yellowColor].CGColor);
CGContextStrokeRect(cgContext, boxRect);
}


CIFaceFeature

• Inherits bounds from CIFeature

• Adds CGPoint properties leftEyePosition,
rightEyePosition, and mouthPosition (with
"has" properties for each of these)


Image Processing on
the ﬂy
• New CVOpenGLESTextureCache makes it
possible to render Core Video buffers in real
time

• These are what you get in the callback

• See ChromaKey example from WWDC 2011
session 419. Requires mad OpenGL ES skillz.


Erica's "Face Pong"


Recap
• Start with an AVCaptureSession

• Discover devices and create inputs

• Create and conﬁgure outputs

• Start the session

• Start recording or wait to start handling
callbacks


Recap: Easy parts
• Basic capture apps (preview-only or record to
ﬁle) will require little or no Core Media or other
C APIs.

• Default devices are usually the one you want
(back megapixel camera on the iPhone, best
available microphone, etc.)

• Capture API is pretty easy to understand and
remember (compare to the editing API)


Recap: Hard parts
• Core Media calls require high comfort level
with C, Core Foundation, functions that take 8
or more parameters, etc.

• Lots of bit-munging when you parse a CV
buffer (pixel formats, strides)

• Callbacks do not have an inﬁnite amount of
time or resources to ﬁnish their work


Resources
• devforums.apple.com

• No mailing list at lists.apple.com

• WWDC session videos and slides (four in
2011, three in 2010)

• Stack Overﬂow


Q&A

Watch my blog for updated sample code:
http://www.subfurther.com/blog
@invalidname


Capturing Stills, Sounds, and Scenes with AV Foundation

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