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Media Capture Using the Sequence Grabber


[IMAGE 085-088_QuickTime_col._h1.GIF]

A very important and often overlooked feature of QuickTime is the standardization of media capture. Since its initial release, QuickTime has defined an API for capturing different types of media, including video and sound. This API, known as the sequence grabber, makes it possible to easily add media capture to any application.

Not only are applications that use the sequence grabber able to automatically support any QuickTime-compatible media capture hardware, but they also perform flawlessly and efficiently regardless of system configuration. This is not an easy task considering all the variations in hardware features and system configurations. In fact, we're even hesitant to say that the sequence grabber "supports video and sound capture" because the sequence grabber API also insulates the programmer from the actual media type being captured. The sequence grabber supports any media type, and, with the release of QuickTime 2.0, users can automatically capture the new music media type in addition to sound and video.

To demonstrate the proper use of the sequence grabber, we've included on this issue's CD a simple, but complete, sequence grabber application -- all in about 10K of compiled C code! If you're looking for a general all-purpose capture application that's efficient, reliable, and best of all, customizable, look no further. After a brief introduction to the sequence grabber, we'll discuss the sample code, and then end with some special considerations for media capture on Macintosh AV models.

The sequence grabber is actually a component of type 'barg' (read it backwards). Although the specification for the component is completely defined in Chapter 5 ofInside Macintosh: QuickTime Components , it's very unlikely that you'll ever want to implement your own 'barg' component. Instead, you'll be using this component specification as the API definition for the standard sequence grabber.

The sequence grabber component implements the basic functionality of media capture. For handling specific media-related functions, the sequence grabber calls on various sequence grabber channel components (as defined in Chapter 6 ofInside Macintosh: QuickTime Components ); there's one such component for each media type. Before QuickTime 2.0, the two standard channel components available were the video and sound sequence grabber channels, enabling the sequence grabber to capture video and sound media. QuickTime 2.0 includes the new music sequence grabber channel, allowing real-time capture of music from MIDI instruments.

Sequence grabber panel components (described in Chapter 7 ofInside Macintosh: QuickTime Components ) manage items in a settings dialog box that allows the sequence grabber to obtain configuration information from a user. Applications typically don't use sequence grabber panel components directly; instead, the sequence grabber automatically uses them for relevant sequence grabber component calls.

Using the sequence grabber is as simple as opening the sequence grabber component and calling SGInitialize (complete error checking can be found in the sample code on the CD):

theSG = OpenDefaultComponent(SeqGrabComponentType, 0);

It's also important to call SGSetGWorld to set the window used for displaying any visual data.

SGSetGWorld((**myWindowInfo).theSG, (CGrafPtr) myWindow, nil);

Opening the channel components. Now it's a matter of calling SGNewChannel to open a sequence grabber channel component to access a particular channel media type. However, rather than hard- coding the media types into the sample application, as in the call

SGNewChannel(theSG, VideoMediaType, &videoChannel);

it's better to use the Component Manager to search for all the different sequence grabber channel components and open a connection to each one. This guarantees that the capture application can automatically support new media types such as the music media type in QuickTime 2.0.

For example, the following code compiles a list of sequence grabber channel components:

cd.componentType = SeqGrabChannelType;
cd.componentSubType = 0;
cd.componentManufacturer = 0;
cd.componentFlags = 0;
cd.componentFlagsMask = 0;
aComponent = 0;

for (i=0, done=false; i<kMAXCHANNELS && !done; i++) {
    aComp = FindNextComponent(aComp, &cd);
    if (aComp != 0) {
        // Get the channel name and type.
        gSGInfo.channelName[i] = NewHandle(4);
        GetComponentInfo(aComp, &theCD, 
            gSGInfo.channelName[i], nil, nil);
        gSGInfo.channelType[i] = theCD.componentSubType;
    } else
        done = true;

This list of component types can then be used to open a connection to each of the media types with SGNewChannel, or SGNewChannelFromComponent if the channel component is already open.

Saving and restoring settings. We want the sample application to start up each time with the same channel settings and video compression settings as when the application was last used. To implement this, we use a preferences file to store these settings. The compression settings are restored with two sequence grabber calls:

    gSGInfo.cInfo.depth, nil,

The channel settings are restored by a simple call to SGSetChannelSettings with the settings retrieved from the preferences file:

SGSetChannelSettings(theSG, channel[i],
    channelSettings[i], 0);

Previewing. We're almost ready to begin previewing. But note that some sequence grabber channel components require additional calls before they can be used. For instance, spatial channels such as video require a call to SGSetChannelBounds to set the channel's display boundary rectangle. So, once the channels are created and the previous settings are restored as discussed above, we make a call to SGSetChannelBounds for the video media to set the video capture to encompass the entire window. We also call SGSetChannelUsage for all sequence grabber channels, which tells the sequence grabber that we want to preview and record every channel.

To start previewing, we simply call SGStartPreview. However, while we're previewing, any changes to the system must be handled with care. First, we'll pause the preview whenever an event that requires updating of the channel information occurs. For example, if the capture window is dragged, we'll pause the video, move the window, and then unpause the video. Likewise, if we resize the window, we'll want to pause the preview, resize the window, and then unpause the preview:

// Pause the sequence grabber before resizing.
SGPause((**myWindowInfo).theSG, true);

// Resize and then update the video channel.
SizeWindow(theWindow, width, height, false);

// OK. We can restart again.
SGPause((**myWindowInfo).theSG, false);

Notice the call to MyUpdateChannels. This is a routine in the sample application that updates the video bounds and channel usage by calling SGSetChannelBounds and SGSetChannelUsage.

The user configuration dialog. Another feature that needs to be handled in a capture application is the user configuration dialog for each of the different capture medias. This is actually one of the simplest things to deal with because the sequence grabber component handles everything. It even stores the settings internally. To retrieve the settings, we can simply call SGGetChannelSettings at a later time. In the sample application, we get the channel settings before we close the connection to the sequence grabber. Then we save the settings in the preferences file.

This is all the code necessary to display and handle the user configuration dialog:

SGSettingsDialog(theSG, theChannel, 0, nil, 0, nil, 0);

Recording. The last important part of the sample code is sequence grabber recording. Before recording can begin, we need to specify an output file with SGSetDataOutput so that the sequence grabber knows where to save the captured media data:

StandardPutFile("\PName of new movie:", "\pMovie", &reply);
if (!reply.sfGood)
SGSetDataOutput(mySG, &reply.sfFile, seqGrabToDisk);

Then we start recording by simply calling


We loop and call SGIdle until the mouse button is pressed to stop recording. This is the most efficient way to record: we don't want to call WaitNextEvent, since that would give other processes time. Instead, we want to hog the CPU time until the recording process is done.

while (!Button() && !err) {
    err = SGIdle(theSG);

We stop recording and start previewing again as follows:


And, of course, just to be nice, we flush the mouse-down events so that no application switching takes place after the mouse button is pressed:

FlushEvents(mDownMask, 0);

That's really all there is to the sequence grabber sample application.

As mentioned earlier, one of the key features of the sequence grabber is its ability to work with all hardware and system configurations. This is not an easy task considering all the different types of video capture boards. For example, there are boards that are simply frame grabbers, and there are those that support on-board hardware video compression. To make every configuration work, the sequence grabber has to handle every case. Here we'll discuss the unique features of the Macintosh AV models and some steps you can take to improve their capture rate.

The video-in circuitry allows the AV models to display 16-bit color and 8-bit grayscale. And, although the hardware can't display video-in at 24 bits per pixel, you can capture video using YUV 4:2:2 compression and achieve an effective 24 bits per pixel. To capture in YUV, you must use the AV's video digitizer hardware compression feature, which you can do simply by selecting "Component Video - YUV" from the list of compressors in the Compression panel of the video settings dialog. You should also make sure that you haven't checked a "Post Compress Video" or similar checkbox in a movie-grabbing application. Selecting this checkbox would bypass the hardware compression, and the sequence grabber would grab the data in raw RGB format.

The AV circuitry can't display video when it's capturing the compressed data. The sequence grabber realizes that it needs to decompress the data into the capture window in order to give the visual feedback that's normally expected. This is fine and dandy, but since there's no hardware decompression in the system, the image decompression is completed in software. This degrades the capture rate.

Knowing that the decompression during recording is what's hurting the capture rate, you can easily rectify the problem by turning off preview during recording so that decompression into the capture window won't take place. To do this, you just call SGSetChannelUsage for the video channel with the seqGrabPlayDuringRecord flag set to 0. In the sample code, a menu selection allows you to turn off video playthrough during recording.

The downside of using YUV compression is that playback without hardware decompression isn't very smooth because of the high data rate and raw processing power needed to decompress each pixel. After capturing, you should recompress the movie using a compressor such as Cinepak or Video that provides a better playback rate.

The sequence grabber obviously makes the job of media capture simpler. But there are many other factors that can play a part. Hard drive transfer rate, disk fragmentation, SCSI bandwith, sound settings, and AppleTalk activity all play an important part in limiting the maximum capture rate. You can also maximize the capture rate by rebooting with no AppleTalk connections. You should also experiment with the different sound sample rates, as these also affect the capture rate.

New additions to the sequence grabber in QuickTime 2.0 also help. Instead of capturing to a single movie file, it's now possible to specify a different file for each channel. For example, you can record video to a large and fast external hard drive and record audio to the internal hard drive. This optimization allows for better allocation of resources and better efficiency because each channel has higher bandwidth. Using the sample code, if QuickTime 2.0 is installed, you can select recording to separate files.

There are, of course, other optimizations that can be explored. With a bit of creativity and testing, you can achieve the optimal capture rates.


  • Inside Macintosh: QuickTime Components , Chapters 5-7, and Inside Macintosh: More Macintosh Toolbox , Chapter 6, "Component Manager" (Addison-Wesley, 1993).
  • "Video Digitizing Under QuickTime" by Casey King and Gary Woodcock, develop Issue 14. About the sequence grabber and video capture.

JOHN WANG (AppleLink WANG.JY) While writing the sequence grabber sample code for this column, John watched the movie Top Gun so many times that he can now duplicate each and every air combat scene with his favorite flight simulator, FA/18 Hornet. John once aspired to become a private pilot, but that idea was quickly quelled once his significant others found out. As Skate so succinctly put it, "Woof woof wooof?" Translation: "Who's going to feed me if you kill yourself?"*

FERNANDO ("NANO") URBINA (AppleLink NANO) uses his Macintosh AV to capture the views of the Rockies from his home office in Colorado Springs. He still doesn't understand how it can thunder and snow at the same time, but thinks he'll be able to figure this out once he adjusts to the lack of oxygen. Nano suffers severe withdrawal from his favorite coffee shop near the Apple campus in Cupertino, but manages to get a fix about once a month when he returns there. He worked on the original AV models and is now a member of the second-generation AV team. *

Thanks to Peter Hoddie and Don Johnson for reviewing this column. *


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