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Achieving High Quality Video Captures
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Compression options can lower capture quality
The compressor/decompressor ("codec") settings you choose when capturing video or still images on your computer affects the resulting image quality. Many codecs are termed "lossy" -- they actually throw away data that they deem unimportant. Typically, it is the fine details of a picture that go first. Most codecs have variable quality settings that allow you to decide the best balance between file-size and image fidelity.
On the other hand, when you choose to save video in uncompressed format, it creates a huge file that doesn't play back smoothly on anything less than a top-of-the-line powerhouse PC or Mac. For this reason, most people capture video in a compressed format. Common formats include the DV standard, MPEG-2, MPEG-4 and, when lower quality is acceptable, MPEG-1, VideoCD or various streaming formats. Our feature on Net Video Trends describes how to achieve the the best results from codecs on PC and Mac, respectively.
Colour
Another issue to be decided when saving images is how many colours will be captured. The more colours captured, the more faithfully your source image can be described, but, of course, the larger the file. Generally, 16.8 million colours (sometimes referred to as "24-bit" video) describes as many colours as the human eye is capable of perceiving. Any additional colours captured or scanned are useful primarily as "headroom" to allow for adjustments to hue or saturation without significantly reducing the dynamic range of the image. You can usually squeeze a few more frames per second out of your hardware during playback by setting the number of colours on-screen to thousands instead of millions.
Resolution
The highest quality NTSC video captures possible from a PC currently come from a Windows-compatible product called Snappy. Snappy 4.0 produces high-resolution images from video -- up to 1500 x 1125 with 16.8 million colors. (details at www.play.com) Because this is higher than the resolution of the source signal, where 500 lines of resolution is considered above average, it is clear that the software is doing some tricks. Indeed, it uses a process called interpolation to maximize the information derived from the onscreen display, essentially "averaging" the pixels to derive in-between values to help smooth out the image. Most scanners use a similar trick to provide scanning resolutions higher than the scanner's optical sensors are capable of capturing. And it really does improve the quality.
However, multifunction capture cards such as the ATI All in Wonder, or the ATI Xclaim VR for the Mac are capable of fairly high-res captures, too. The ATI All in Wonder 128 card's software, for example, captures at a maximum of 720480. You can set this in the setup page of the capture ("Television") program (click the "Set" button).
As you might suspect, the quality of your video input signal helps determine the maximum quality you can get out of your video capture card. An article at Tom's Hardware on All-in-One multitalented boards reveals that not all capture cards respect the copy-protection restrictions of DVDs and other video sources, as this excerpt reveals:
All the other products surprised us. Copy protection or not, recording was possible with all the other test candidates.
Read the article (and check out the other reviews on our own Video Buyer's Guide hardware page) for an interesting look at the features and foibles of the best video capture cards on the market.
Resolution
For many video and photography enthusiasts, the final step in the digital imaging process is to print out the image. Here, too, there are some complicating factors. Most notable is the fact that, if the image is destined for offset press reproduction, you do not need to produce an image with a resolution higher than twice that of the final printout's lines-per-inch value. In other words, if you plan to produce a 5-by-7 image for reproduction in a brochure that will be printed with a line screen of 133 lines per inch (a typical value for glossy magazines), you only need a 5-by-7 source image at a resolution of 266 DPI. Having a higher value does you absolutely no good and simply hogs hard disk space and slows down printing. This optimal size calculation, in fact, is performed automatically by Photoshop's AUTOSIZE "high quality" command.
Remember, also, that a term such as "dots per inch" (DPI) is a meaningless term without knowing how many inches wide and tall the image is.
Thus, a "one DPI image" that is 300 inches square would be just as high a resolution image as a 300 DPI image that is one inch square.
After capturing, you can resize images in Photoshop or a similar graphics program to correct the DPI values, with or without resampling them, by using the Image Size option. In fact, a growing number of digital cameras, scanners and other capture devices can scan directly to Photoshop, via a so-called TWAIN plug-in, which, believe it or not, stands for "technology without an interesting name."
For Further Reading
- Play's Snappy 4.0
- Net Video Trends
- Optimizing Bitmapped Graphics for use on the Web.
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