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Compression

Compression is the magical technology that allows a Video CD to hold 70 minutes of video on a CD. While the topic of compression could be a book by itself, here are some of the common compression issues and workarounds.

JPEG, MPEG, VideoCD, DVD – all use something called lossy compression. It’s compression that makes the file smaller, but at the expense of throwing away parts of the image. The trick is to throw away information, make the file smaller, and fool the eye into thinking that nothing has changed. The tradeoff with compression therefore is size vs. quality. Compress more of the image, and you’ll have a smaller but poorer looking image.

If you’re familiar with JPEG encoding of pictures, you already have some idea of the problems or artifacts with MPEG encoding, as they both use similar concepts. You probably also know that not all encoders are created equal - some are simply better at compressing an image and still having it look good. Just because a product is capable of encoding MPEG-1 (or MPEG-2) doesn’t mean that it’s capable of encoding it well.

Considerations when encoding pictures:

Gibbs Effect: This is an artifact, also sometimes referred to as the "mosquito effect” which appears as a blurring around the edges of an object. It’s often noticed in computer generated graphics or captions that have sharp edges and lines. You can see the Gibbs effect around the letters below – slight blotches or blurring around the edges.

You can minimize this effect by avoiding hard edges and sharp text in your menus and pictures. PhotonTV, for example, has an antialias option for text that smoothes the captions and reduces the Gibbs effect. Ther are also softening or blurring filters that can reduce the effect on hard-edged graphics. Of course, the tradeoff is that you lose the sharpness of the edges. In the graphic below, PhotonTV has anti-aliased or smoothed the edges of the text. The edges are less sharp, and less “mosquitos” can be seen buzzing around the edges.

Blockiness: MPEG compression works by encoding blocks of data, and these blocks can become visible as artifacts in some situations. The two screens below are snapshots taken during a quick transition. The PhotonTV encoder is able to encode moving transitions and retain most of the quality of the still pictures, while the encoder on the left isn’t able to encode the moving picture without introducing blocks.

Single frame snapshot of fast (0.5 second) push transition
Output from standard MPEG encoder	Output from PhotonTV encoder

There are a couple ways to prevent blockiness:

1) Make sure that the encoder you are using is good at encoding both still pictures and moving transitions. A true test of a good encoder is its ability to encode fast moving transitions – create a test disc, play it on your PC with Windows Media Player or similar, and slowly scan through the transition period between slides. The difference in quality between encoders in different photo VCD/DVD products will become very apparent.

2) If you have to use a poor encoder, you can reduce the effect of blocks somewhat by slowing down transitions and using soft edges and blurs instead of hard-edged wipes and push transitions. Of course, the best solution though is to use a good encoder.

Recompression: Last but not least, for the best possible quality try to avoid recompressing images. Photo VCD/DVD software needs to encode or compress them to MPEG format - there’s no way you can avoid that compression. Since you can’t avoid compressing once to MPEG to make a VCD, try to start with uncompressed images or at least images with slight JPG compression. Heavily compressed JPG images that are compressed again to MPEG just don’t have much of a chance of coming out very well on the TV.

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