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Sunday, February 24, 2008

Balancing Act: Keeping Your Screen Movies Small and Beautiful

Screen recordings are a valuable tool for enhancing training, tutorials, manuals and websites. Companies use this technique to produce streaming and downloadable content. The recording tools are readily available and affordable.

In this article, we explore some techniques, tips and tricks for recording sound, mouse movement and happenings from your screen to an AVI file.

File Size vs. Movie Quality

One key to successful screen movies is keeping the files small. Larger files mean longer processing times for you and longer download times for your users. Especially large screen movies may not play at all for some users with outmoded PCs. Also, large files do not stream well over the Internet.

Unfortunately, larger files result in better quality movies. More colors, smoother action, and higher quality sound are all benefits of larger file sizes. Therefore, your most important - and most difficult -- decisions will balance file size against quality. This article presents technique for keeping file sizes small while retaining the quality you need in your screen movies.

Tip 1: Select Your Color Settings

Before you record a screen movie, you need to configure your display for color depth. We recommend setting your color depth to 256 colors, or 8 bits. In Windows, you do this by selecting Start | Settings | Control Panel and then double-clicking the Display icon.

Today, many computers work in True Color. True Color uses 24 or 32 bits per pixel, and can render millions of colors. Let's assume you're capturing an area that is 320 by 240 pixels. That's 76,800 pixels for every complete frame captured. How much storage space would each complete frame in this movie occupy?

Bit Depth Colors Storage Space per Complete Frame (320 by 240 pixels)
8 256 76,800 Bytes (76.8kB)
16 64K 153,600 Bytes (153.6kB)
24 16.7 million 230,400 Bytes (230.4kB)

As you can see from the table above, each frame in True color mode contains a lot of information to capture from the screen memory. This information must then be compressed and written to the AVI file. That process requires a lot of time, processor power, and disk space.

In many cases, the programs that you capture will look as good in 8 bit color mode as in True Color. The capture programs optimize the color map to make the best use of them, so you do not lose much quality unless your subject demands higher color depth. In 8-bit color mode the amount of information to capture and compress is a fraction of that in the other modes. These pictures compress the fastest, and produce the smallest AVI files.

If you must record in higher than 256 color mode, consider using 16 bits, and use Intel Indeo codec, configured to the "Quick Compress" option. If you don't have Indeo installed on your machine, you can download it for free from Intel's web site, On HyperCam, for example, this will compress about 10-20% faster than with the default codec of MS Video 1.

A final word of advice on setting colors: You may be tempted to record in True Color and then convert the file to 8 bit color depth. There are two reasons to avoid this. First, the software you are recording will probably do a better job of picking which 256 colors it displays best in, than the screen recording software. So, set your display to 256 colors and let the application you are recording pick its palette. Second, saving and processing a file in True Color takes more time than in 256-color mode. This extra time is wasted if you're just going to convert to 256 colors.

Tip 2: Select a Frame Rate. How Low Can You Go?

The frame rate is the number of frames per second that you record and play back. Television uses about 30 frames per second, and movies about 60 frames per second. This results in very smooth action. However, frame rates this high create very large computer files.

The frame rate needed for smooth motion depends on how fast objects move across the screen, and on the size of the objects. Small, fast moving objects tend to blink as they move. This occurs when the image of a moving object is present in one frame, but not in the next. For example, a cursor moving quickly across the screen will tend to blink. If your screen movie must include such objects, you'll need a high frame rate: 15 to 60 frames per second. If you keep the action on the screen slow, you can obtain good results with a frame rate as low as 2 to 5 frames per second.

Use these recommendations as a guideline, and experiment with a few settings to see how low you can go with the frame rate. The only way to determine how low you can go while maintaining quality is to record and play back a few samples.

HyperCam, like most screen recording software, enables you to select the frame rate for your recording.

Tip 3: Set Key Frames

Your screen capture software does not store information for every single pixel in a frame. Instead, it stores the information for which pixels have changed since the previous frame. For example, assume two frames are identical except that the cursor has changed position. The capture software will store information for only the pixels that have changed because of the cursor's movement. The majority of the screen stayed the same from the first frame to the second. Therefore, there is no need to repeat this information in the second frame. This storage method saves a lot of disk space and results in faster playback.

However, the longer a movie plays, the greater the errors introduced by this storage method. To correct any errors, your recording software inserts key frames. A key frame is a completely recorded frame, with all of its information intact. Then, beginning with the key frame, the software once again records only the changes from frame to frame. When it hits another key frame, it records the entire frame, begins recording only the changes... and so on, again and again.

Because key frames take more storage space than normal frames, the more key frames in your screen movie the larger the file. In our example of recording a 320 by 240 movie at 256 colors, each key frame occupies 76.8kB.

Most programs will automatically choose every tenth frame as a key frame. Most will also enable you to choose how often to insert a key frame.

If your screen movie contains a lot of zooming, panning, and other movement, you'll need more key frames to keep the quality high (every tenth or even fifth frame). If changes from frame to frame are small, you can select fewer key frames and still retain high quality (every fifteenth to thirtieth frame).

As in choosing the frame rate, the only way to determine the minimum number of key frames for the quality you need is to take a few test recordings.

Tip 4: Select the Recording Region

Some screen recording software enables you to select a specific region or Window to record. You can usually select the recording region in three ways:

  • Select an area of you display by dragging a selection rectangle. Everything inside the rectangle will be recorded.
  • Select a specific window.
  • Specify the X and Y coordinates of the recording area on your display.

The smaller the recording area, the smaller your file size.

The best method for determining the minimum size recording area is practice. Run through the sequence you need to record several times, and determine the minimum size window that the sequence requires.

Also, instead of choosing to record the entire window in which the program is running, consider selecting only the interior of the window. If the window's title bar, scroll bar, and status bar do not add useful information, do not record them.

Some programs enable you to move the recording area around the screen, so that you can pan from one area to another. The resulting movie is like watching a large screen through a small, moving window. This is a powerful technique for showing large areas with a small movie. However, it can be disorienting. Keep the action slow and use narration to clearly explain when and where you are panning.

Tip 5: Select the Right Audio Settings

Most of the space your screen movie occupies is comprised of video information. That is why we have focused on techniques for minimizing the size of the video information while still retaining quality. However, selecting the right audio settings can also minimize file size.

First, determine if your screen movie software enables you to select mono or stereo sound. Unless you have a compelling reason to record in stereo, select mono.

Second, select the proper number of bits rate for the quality of sound you want. Most software enables you to choose between 8 bit and 16 bit samples. Think of a sample as a pixel of sound. The more bits you use to store a screen pixel, the greater the number of colors that pixel can take. The more bits you use to store a sound sample, the greater the frequency response of the sample captured. Try your audio setting at 16 bits per sample, before going down to 8 bits. The difference in sound quality between 8 and 16 bits is usually very noticeable, so this is a good place to spend some file size.

Third, select the sampling rate. The sampling rate is how many times per second the capture software will record sound. For example, a sampling rate of 8000 means that the software is capturing 8000 slices of sound every second. For most screen movies with voice narration, a sampling rate of 11025 gives good quality with minimum file size. This combination of bits per sample and sampling rate is approximately equal in quality to an FM radio.

Screen recordings can be valuable tools for demos, tutorials, training videos and various technical applications. The latest commercial and open source tools provide this capability in easy-to-use and affordable packages - take advantage of this great technology!

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