I am starting a new thread to discuss pre-processing of video. In this context, I am talking about converting from a higher bitdepth down to a lower bitdepth. In the case of BD and HD DVD, it is often 10-bit 4:2:2 to 8-bit 4:2:0.
Below are two examples taken from the Elephants Dream project. You can download the publically available 8-bit uncompressed PNG files from their website. http://media.xiph.org/ED/1080-png/
Sorry, I don’t recall the exact frame number. I am not sure the process they used to create their 8-bit PNGs. Elephants Dream does have some pretty severe banding throughout.
I was able to obtain the original 16-bit OpenEXR files from them. In my two examples, the same path was used.
1. Open EXR file in After Effects CS3
2. Disable all processing
3. Export as 16-bit TIFF
4. Use Microsoft xScaler to convert from 16-bit TIFF (48-bit RGB) into 8-bit RGB (24-bit RGB)
5. The images were also converted from PC to Video levels.
In the left image I used one of the xScaler dither modes. In the other I used the xScaler rounding mode. There are a lot of different techniques to dither an image, this is simply our favorite. Don and I spent a lot of time working on this technology. It was originally written about five years ago.
The area to see the banding is on the light gradient to the right of the image.
It is simple to maintain this level of detail on a static image. The problem comes when there is motion. In this particular part of Elephants Dream, the image pans to the right. The dither does make compression more difficult in this example. I have other examples, like Coral Reef, where it actually becomes easier to compress.