Wednesday, July 25, 2012

Thoughts on BMCC part 2

Thoughts on Blackmagic Cinema Camera

Dynamic range and encoding options

Previous post discussed the overall story of BMCC and 2.5K bayer pattern CMOS sensor, lets continue in the same line. According to Blackmagic, this camera will have 13 stops of latitude which is great! Why is it great? Because the more latitude you have, the less blown out highlights and black shadow splots you will have. You can always add your clippings and smooth rollouts to your liking but important thing is that you will have the option to get every drop this camera is able to capture.

For recording raw sensor data, CinemaDNG file format is used. It is an Adobe initiated open format for raw sensor data that supports different bayer patterns and bunch of other features. One 2.5K raw file will be about 5MB with 12 bits per pixel. BMCC will use log encoding for raw files and 12 bits are plenty for that. There is a nice discussion about log versus linear in Cineform insider blog. Log encoding preserves more information in shadow areas and is thus more suitable for digital cameras that tend to need some under-exposure to avoid rather ugly looking clipped highlights. On the downside, as log image does not linarly match captured light intensity values but instead contains the derived log-encoded values, it must be linearized with correct operation that suits the log encoding curve. Such curves are usually provided by camera manufacturers in form of simple reversible 1D LUT. When this camera will be released, Davinci Resolve will probably get an update with ACES IDT transform for BMCC camera.

Other encoding options besides CinemaDNG are Prores 422HQ and DNxHD, both in either log encoding for grading or rec709 for fast stuff. Prores is mostly nice feature for FCP users because it suits so damn well with it and Resolve. DNxHD should make both Avid and general Windows/Linux user happy. It is possible to decode Prores on Windows too (Quicktime does that) and with ffmpeg, it is possible to encode Prores on Windows although FCP is said to complain about the files (some missing metadata) although it plays them.

Recording media

Recording media for BMCC is SSD drives which is really nice because they are much cheaper per GB and more readily available than some exotic memory card format. And they are fast too. One 256GB SSD is said to hold about 30 min of RAW CinemaDNG sequences or 2,5h of Prores or DNxHD. SSDs must be formatted in Apple file system but it can be done on Windows too with certain programs. It is said that camera itself cannot format the drive, so no quick-buying these during shoots...

Probably the camera also accepts standard 2.5" HDD drives which are dirt cheap and should be fast enough for Prores/DNxHD 220Mbit/s speed (less than 30MB/s). One 500GB HDD should be able to record almost 5 hours of footage! Someone should try this out!

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