Prototype the Stop Motion Recorder Application

Just as you’ve done in the section Prototype the Recorder, start by creating a rough sketch of your QTKit stop motion recorder application. Think, again, of what design elements you want to incorporate into the application.

In this design prototype, you start with three simple objects: a capture view, a QuickTime movie view, and a single button to add frames. These will be the building blocks for your application. You can add more complexity to the design later on. After you’ve sketched out your prototype, think how to hook up the objects in Interface Builder and what code you need in your Xcode project to make this happen. Note that you need to add a movie controller to the QTMovieView object in the illustration above.

Create the Project Using Xcode 3.2

To create the project, follow these steps, as in previous chapters:

1. Launch Xcode 3.2 and choose File > New Project.

2. When the new project window appears, select Cocoa Document-based Application and click Choose.

3. Name the project StopMotion and navigate to the location where you want the Xcode application to create the project folder.

   • The Xcode project window appears.

   

4. From the Action menu in your Xcode project, choose Add > Add to Existing Frameworks.

   Add the QTKit framework to your StopMotion project, which resides in the /System/Library/Frameworks directory.

5. Now add the Quartz Core framework to your project, which also resides in the /System/Library/Frameworks directory.

   • Select QuartzCore.framework, and click Add when the Add To Targets window appears.

   This completes the first sequence of steps in your project. In the next sequence, you define actions and outlets in Xcode before working with Interface Builder.

   Because you’ve already prototyped your QTKit stop motion recorder application, at least in rough form with a clearly defined data model, you can now determine which actions and outlets need to be implemented. In this case, you have a QTCaptureView object, which is a subclass of NSView, a QTMovieView object to display your captured frames and one button to record your captured media content and add each single frame to your QuickTime movie output.

Create the User Interface

In the next phase of your project you construct the user interface for your project.

1. Open Interface Builder 3.2 and click the MyDocument.xib file in your Xcode project window.

2. In Interface Builder 3.2, select Tools > Library to open a library of objects.

   • Scroll down to the QuickTime capture view object.

   

   The QTCaptureView object provides you with an instance of a view subclass to display a preview of the video output that is captured by a capture session.

   • Drag the QTCaptureView object into your window and resize the object to fit the window, allowing room at the bottom for your Add Frame button (already shown in the illustration below) and to the right for your QTMovieView object in your QTKit stop motion recorder application.

   • Choose Tools > Inspector. In the Identity panel, select the information (“i”) icon. Click in the field Class and your QTCaptureView object appears.

     

   • Set the autosizing for the object in the Capture View Size panel.

     

3. Now repeat the same sequence described in Step #2 to add your QTMovieView object to your window (already shown above).

   • Scroll to the QTMovieView object.

   • Select the QTMovieView object (symbolized by the blue Q) and drag it into your Window next to the QTCaptureView object, shown below.

   • Choose Tools > Inspector. In the Identity Inspector, select the information (“i”) icon. Click in the Class field and your QTMovieView object appears.

     

   • Set the autosizing for your QTMovieView object, as you did for the QTCaptureView object in the steps above.

4. Now specify the window attributes in your MyDocument.xib file.

   • Select the window object in your nib.

   • Click the attributes icon in the panel.

5. Define the window size you want in your MyDocument.xib by selecting the size icon (symbolized by a yellow ruler) in the Window Size panel.

   

6. Specify the delegate outlet connections of File’s Owner in the Window Connections panel.

   

7. In the Library, select the Push Button control and drag it to the window.

   • Enter the text Add Frame.

   • Set the autosizing for the button at the center and right outside corner, leaving the inside struts untouched, as shown in the illustration.

   

8. Select the MyDocument.xib file and click the Connections Inspector.

   • Now wire up the outlets and received actions.

   • Control-drag each outlet instance variable to the File’s Owner object.

   

9. Select the File’s Owner object in your MyDocument.xib file.

   • Click the Class Identity icon in the Interface Builder Inspector.

   • Note that the green light at the left corner of your StopMotion.xcodeproj is turned on, indicating that Xcode and Interface Builder have synchronized the actions and outlets in your project.

10. Save your Interface Builder file.

Prepare to Capture Single-Frame Video

In the last phase of your project, after adding and completing the implementation code, you’re ready to capture single-frame video, using your stop motion recorder application, as shown below.

Complete the Implementation of the MyDocument Class in Xcode

To complete the implementation of the MyDocument class, you define the instance variables that point to the capture session, as well as to the device input and decompressed video output objects.

1. In your Xcode project, add the instance variables to the interface declaration.

   Add these lines of code in your MyDocument.h declaration file:

   @interface MyDocument : NSDocument

   {

   QTMovie *mMovie;

   QTCaptureSession *mCaptureSession;

   QTCaptureDeviceInput *mCaptureDeviceInput;

   QTCaptureDecompressedVideoOutput *mCaptureDecompressedVideoOutput;

   }

   The mMovie instance variable points to the QTMovie object, and the mCaptureSession instance variable points to the QTCaptureSession object. Likewise, the *mCaptureDeviceInput instance variable points to the QTCaptureDeviceInput object, and the next line declares that the mCaptureDecompressedVideoOutput instance variable points to the QTCaptureDecompressedVideoOutput object.

2. Declare the mCurrentImageBuffer instance variable.

   CVImageBufferRef mCurrentImageBuffer;

   This instance variable stores the most recent frame that you’ve grabbed in a CVImageBufferRef.

   That completes the code you need to add to your MyDocument.h file.

3. Open your MyDocument.m file and follow these steps.

4. Create an empty movie that writes to mutable data in memory, using the initToWritableData: method.

   - (void)windowControllerDidLoadNib:(NSWindowController *) aController

   {

   NSError *error = nil;

   [super windowControllerDidLoadNib:aController];

   [[aController window] setDelegate:self];

   if (!mMovie) {

   mMovie = [[QTMovie alloc] initToWritableData:[NSMutableData data] error:&error];

   if (!mMovie) {

   [[NSAlert alertWithError:error] runModal];

   return;

   }

   }

5. Set up a capture session that outputs the raw frames you want to grab.

   [mMovieView setMovie:mMovie];

   if (!mCaptureSession) {

   BOOL success;

   mCaptureSession = [[QTCaptureSession alloc] init];

6. Find a video device and add a device input for that device to the capture session.

   QTCaptureDevice *device = [QTCaptureDevice defaultInputDeviceWithMediaType:QTMediaTypeVideo];

   success = [device open:&error];

   if (!success) {

   [[NSAlert alertWithError:error] runModal];

   return;

   }

   mCaptureDeviceInput = [[QTCaptureDeviceInput alloc] initWithDevice:device];

   success = [mCaptureSession addInput:mCaptureDeviceInput error:&error];

   if (!success) {

   [[NSAlert alertWithError:error] runModal];

   return;

   }

7. Add a decompressed video output that returns the raw frames you’ve grabbed to the session and then previews the video from the session in the document window.

   mCaptureDecompressedVideoOutput = [[QTCaptureDecompressedVideoOutput alloc] init];

   [mCaptureDecompressedVideoOutput setDelegate:self];

   success = [mCaptureSession addOutput:mCaptureDecompressedVideoOutput error:&error];

   if (!success) {

   [[NSAlert alertWithError:error] runModal];

   return;

   }

8. Preview the video from the session in the document window.

   [mCaptureView setCaptureSession:mCaptureSession];

9. Start the session, using the startRunning method you’ve used previously in the MyRecorder sample code.

   [mCaptureSession startRunning];

   }

   }

10. Implement a delegate method that QTCaptureDecompressedVideoOutput calls whenever it receives a frame.

    - (void)captureOutput:(QTCaptureOutput *)captureOutput didOutputVideoFrame:(CVImageBufferRef)videoFrame withSampleBuffer:(QTSampleBuffer *)sampleBuffer fromConnection:(QTCaptureConnection *)connection

11. Store the latest frame. Do this in a @synchronized block because the delegate method is not called on the main thread.

    CVImageBufferRef imageBufferToRelease;

    CVBufferRetain(videoFrame);

    @synchronized (self) {

    imageBufferToRelease = mCurrentImageBuffer;

    mCurrentImageBuffer = videoFrame;

    }

    CVBufferRelease(imageBufferToRelease);

    }

12. Handle window closing notifications for your device input and stop the capture session.

    - (void)windowWillClose:(NSNotification *)notification

    {

    [mCaptureSession stopRunning];

    QTCaptureDevice *device = [mCaptureDeviceInput device];

    if ([device isOpen])

    [device close];

    }

13. Deallocate memory for your capture objects.

    - (void)dealloc

    {

    [mMovie release];

    [mCaptureSession release];

    [mCaptureDeviceInput release];

    [mCaptureDecompressedVideoOutput release];

    [super dealloc];

    }

14. Specify the output destination for your recorded media, in this case an editable QuickTime movie.

    - (BOOL)readFromURL:(NSURL *)absoluteURL ofType:(NSString *)typeName error:(NSError **)outError

    {

    QTMovie *newMovie = [[QTMovie alloc] initWithURL:absoluteURL error:outError];

    if (newMovie) {

    [newMovie setAttribute:[NSNumber numberWithBool:YES] forKey:QTMovieEditableAttribute];

    [mMovie release];

    mMovie = newMovie;

    }

    return (newMovie != nil);

    }

    - (BOOL)writeToURL:(NSURL *)absoluteURL ofType:(NSString *)typeName error:(NSError **)outError

    {

    return [mMovie writeToFile:[absoluteURL path] withAttributes:[NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:QTMovieFlatten] error:outError];

    }

15. Add the addFrame: action method that you specified previously in your implementation file. This enables you to get the most recent frame that you’ve grabbed. Do this in a @synchronized block because the delegate method that sets the most recent frame is not called on the main thread. Note that you’re wrapping a CVImageBufferRef object into an NSImage. After you create an NSImage, you can then add it to the movie.

    - (IBAction)addFrame:(id)sender

    {

    CVImageBufferRef imageBuffer;

    @synchronized (self) {

    imageBuffer = CVBufferRetain(mCurrentImageBuffer);

    }

    if (imageBuffer) {

    NSCIImageRep *imageRep = [NSCIImageRep imageRepWithCIImage:[CIImage imageWithCVImageBuffer:imageBuffer]];

    NSImage *image = [[[NSImage alloc] initWithSize:[imageRep size]] autorelease];

    [image addRepresentation:imageRep];

    CVBufferRelease(imageBuffer);

    [mMovie addImage:image forDuration:QTMakeTime(1, 10) withAttributes:[NSDictionary dictionaryWithObjectsAndKeys:

    @"jpeg", QTAddImageCodecType, nil]];

    [mMovie setCurrentTime:[mMovie duration]];

    [mMovieView setNeedsDisplay:YES];

    [self updateChangeCount:NSChangeDone];

    }

    }

Build and Compile Your Stop Motion Recorder Application

After you’ve saved your project, click Build and Go. After compiling, click the Add Frame button to record each captured frame and output that frame to a QuickTime movie. The output of your captured session is saved as a QuickTime movie.

Now you can begin capturing and recording with your QTKit stop motion recorder application. Typically, you can record any number of frames illustrating movement or action, using objects of clay or stick figures, for example, which, when combined and recorded, will create the illusion of motion in a movie or animated sequence. The technique is common in working with various inanimate objects that can be assembled into a particular story or narrative.

Summary

In this chapter, you focused on building a stop motion recorder application using three capture objects: QTCaptureSession, QTCaptureDecompressedVideoOutput and QTCaptureDeviceInput. These were the essential building blocks for your project. You learned how to:

• Prototype and design the data model and control for your project in a rough sketch before constructing the actual user interface.

• Define a specific IB action as a button to add frames to your recording.

• Create the user interface using the QTCaptureView plug-in from the Interface Builder library of controls.

• Wire up the Add Frame control button for your user interface.

• Define the instance variables that point to the capture session, as well as to the device input and decompressed video output objects.

• Build and run the stop motion recorder application, using the iSight camera as your input device.

• Output the single captured frames to a QuickTime movie for animated effects.