Thursday, August 10, 2006

HVX200 resolution options

In recent releases of all CineForm products, we now offer scaling HD source material to standard sizes of 1280x720, 1440x1080 and 1920x1080 (for those using Prospect HD.) These scaling options where added primarily to support the HXV200, which doesn't record standard HD sizes. This surprises a few people, but the 720p mode on the HXV200 only stores a 960x720 image, and the 60Hz version of the camera in 1080 modes only stores 1280x1080 (rather than 1440x1080 like HDV or the full 1920x1080 of a Viper.) It is not that a lower encoding resolution is a bad thing, as very few the of the sub-$10k HD cameras can resolve close to their recording size, it just makes post more difficult. So the scaling was added to simplify the mixing of HVX footage with HDV/HD footage. Remember CineForm products don't have the baggage of being a native editor, so you can freely mix footage from any source.

For 960x720 source material I suggest up-scaling to 1280x720, that gives you a standard HD picture that matches your distribution format (no formats distribute at 960x720.) So although the up-scale doesn't gain in detail it doesn't lose any either. For post all you additional elements, graphics, effects, titles, etc., can benefit from the additional 33% image resolution, making you final broadcast, HD-DVD, BluRay or film-out master the best it can be. Whereas you lose this advantage would using native editing (it is limited to the 960x720 source format.)

Now for 1280x1080 interlaced projects, I recommend up-scaling to 1440x1080i for Aspect HD and 1920x1080i for Prospect HD. Both 1440x1080 and 1920x1080 are standard distribution formats for MPEG2, VC-1, WMV-9, etc. Many of the first 1080 WM9 clips were 1440x1080 as most PCs couldn't play 1920 clips (machines are faster now, so it is less of an issue.) Again posting in your output resolution and rather than source resolution give you the best results.

So why am I recommending HVX's 1080p modes be scaled down to 1280x720p rather than up. Of course you can scale up to get the most of your titles and effects elements, but there are diminishing returns. An SD project edited at 1920x1080 still looks like SD - you have to have a lot of effects and titling elements to obscure you SD image. Despite the advances in scaling algorithms, image detail can't be increased (only perceived sharpness is enhanced with good scalers.) You are probably thinking, this is not SD, if you scale a 1280x1080 to 1280x720 aren't you losing data? Not if the source contains less detail than the target 720p image, as it does with the HXV200. The camera only uses 960x540 sensors. Due to pixel shifting the camera will resolve a little more than the sensor size would suggest, about 30% more in each direction (testing shows slightly less than this.) But using the 30% estimate the three 960x540 sensors will generate a 1248 x 702 image (pretty closes to standard 1280x720.) Up-scaling this to 1080 doesn't gain any detail, it just uses more disk space.

To prove the 1280x720 is sufficient to contain all the detail from the 1080p modes of the HVX200, I have provided this test example (thanks Mike for this source.) Now this is not a glamorous scene, shoot though a window into a bunch of trees, but this 1080p24 clip is prefect for this demo as it is filled with high frequency details. The source image was converted to an AVI twice, once at it source resolution of 1280x1080 and one was scaled to 1280x720 (during conversion, then up-res'd back to 1080 (using VirtualDub's "resize" with the "Lanczos3" Filter mode.) The fact that is hard to tell the difference is proof that less than 720p detail was in the original scene. Can you tell which image was scaled? Please try this with your own footage.

Image A or Image B
(Note: make sure the browser isn't scaling the image. These are 1280x1080 PNG files)

I do recommend shooting in the HVX's 1080p mode as it does resolve a little more horizontally than then 960x720 can store. In addition the 1080p mode is significantly less compressed. DCT compression like DVCPRO-HD has trouble with excessive detail (like DV did with mosquito noise, and blocking is shadow details), so storing 1248x702 worth of detail in 1080p24's 80Mb/s is much cleaner than 720p24 40Mb/s. So shooting 1080p is a good way to reduce artifacts.

In the end I recommend the 1080p to 720p down-res, as the workflow enhancements of more efficient disk usage, higher editing performance with more real-time operations and square pixel processing, out-weigh the gains of a 1280x1080 (or 1440 or 1920) timeline.

1 comment:

Anonymous said...

Great summary. I am still leaning towards HVX shoot and post 1080p24 with PHD, and color correct in 10bit.
-Christopher Barry