Q: Does DV compression quality affect playback performance? Currently my application plays back two DV streams which were compressed using the "High Quality" setting and I'd like to make sure I'm getting the best playback performance possible.
A: There is an important distinction between "Compression" quality and "Decompression" quality (Decompression Quality is also known as Decompression Accuracy). The quality settings used when "Compressing" DV will affect image fidelity, but will not affect decompression performance. However, the "Decompression" quality settings will dramatically affect both displayed quality and performance.
The DV decompressor supports 4 modes. In increasing CPU load order they are: Low Quality, High Quality Single-Field, High Quality and High Quality Deinterlaced. (Deinterlacing is only available with QuickTime 6.5 or greater). These modes can be selected by using SetMoviePlayHints with the appropriate flags (hintsHighQuality, hintsSingleField and hintsDeinterlaceFields) or by how you call the Image Compression Manager.
- Low Quality (Default Setting) - Not all coefficients are necessarily decoded. Only one field is decompressed, at half width (360x240 for DV-NTSC). The image is then scaled to the desired size.
- High Quality, Single-Field - All coefficients are decoded, but only one field is decompressed (720x240 for DV-NTSC). The image is then scaled to the desired size.
- High Quality - All coefficients are decoded, and the image is decompressed at full resolution (720x480 for DV-NTSC).
- High Quality, Deinterlaced - All coefficients are decoded, the image is decompressed at full resolution (720x480 for DV-NTSC) then filtered to reduce interlacing artifacts.
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Note:
Turning off the deinterlacing flag is recommended when recompressing in a field-based format -- for example, burning some text on top of a DV clip to create another DV clip. When recompressing in a non-field-based format however -- large-size MPEG-4 for example, deinterlacing may reduce field artifacts at the expense of some blurring.
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Low Quality mode is likely to be the right choice for real-time playback on the widest variety of hardware. When YUV hardware acceleration is available, low quality mode involves a much smaller bandwidth of data transfer to video memory, an important bottleneck on some computers.
| Table 1. Hint flag interaction
No flags = Low Quality
hintsHighQuality = High Quality
hintsHighQuality | hintsSingleField = High Quality, Single-Field
hintsHighQuality | hintsDeinterlaceFields = High Quality, Deinterlaced
Note: hintsSingleField is ignored if hintsHighQuality is not set. hintsDeinterlaceFields is ignored if hintsHighQuality is not set, or if hintsSingleField is set.
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Setting
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Low Quality (1)
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Low Quality (1)
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Low Quality (1)
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Low Quality (1)
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X
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High Quality (3)
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X
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High Quality, Deinterlaced (4)
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X
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High Quality, Single-Field (2)
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X
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High Quality, Single-Field (2)
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References:
[Jan 20, 2004] |