Editing Frank Miller's Sin City - Part 2
FRANK MILLER'S SIN CITY represents a new kind of film in digital filmmaking. This article describes in practical terms what the 4:4:4 HD filmmaking revolution is about and the workflow that filmmakers use to create digital films of striking image quality.
by Robin.Rowe@MovieEditor.com
Contents
- The New Digital Pipeline
- Understanding DI Technology
Sony HDC-F950 Camera
Understanding DI Technology
It seems daunting, all this new technology. Let's break it down.
The human eye, film, and digital sensors each respond to light in different ways. The eye and film see colors on a logarithmic scale, but digital sensors output linear color ranges like computers use. NTSC television cameras use a luminance channel and two half resolution color channels - 4:2:2 Y'CbCr. Computers like three full color channels - 4:4:4 RGB.
DPX is a logarithmic image format used for digital intermediate. As a file format it's something like TIFF, but because DPX is in log space it can efficiently hold data from film scans. HD can be converted into DPX with a device such as RaveHD. The RaveHD DDR has a dual-link SDI input. SDI is a wire, sort of like Firewire. A single channel of SDI carries 4:2:2 and the second channel of SDI is needed to carry the additional 0:2:2 to make 4:4:4. You can plug a Sony HDCAM SR deck into a RaveHD, and convert HDCAM SR tape output into sequentially numbered DPX frames to be served by RaveHD over NFS (Linux) or Samba (Windows). RaveHD is fundamentally a Linux PC with an AJA io board and RAID storage. RaveHD isn't your only option for converting between HDCAM SR and DPX. SIN CITY also used Discreet Smoke and Quantel.
Now let's talk about bits. DPX files are typically 10-bit. Computers don't handle log data easily, so as DPX files are read into memory software converts them to linear. To accommodate this mathematical expansion into linear space another 2 bits of capacity are needed. But, 12-bit linear is unwieldy. Computers like data to be 8-bit, 16-bit, or 32-bit chunks. So, DPX usually gets converted up to 16-bit linear for visual effects. As with audio, more bits per channel is better with HD images. The HDCAM SR format records 10-bit streams from the F950 (although the camera has a 12-bit DA). To hold 10-bit 4:4:4 RGB required a breakthrough in recording tape technology. HDCAM SR has double the data density of HDCAM.
One more thing about bits, DPX may not use the entire range. Mathematically, a 10-bit number can range from 0 to 1023. But, dynamic range encoded into DPX is often from 60 to 900. Because SMPTE reserves some numbers, the dynamic range of HD CAM SR is from 4 to 1019.
Let's not forget resolution. Troublemaker production supervisor Keefe Boerner notes, "Troublemaker only worked at HD resolution 1920x1080, which is almost 2k resolution." The reason it isn't exactly 2k resolution is that the Sony F950 and other HD cameras are ATSC standard at 1080p. (The p stands for progressive, which means no temporal interlace artifacts as with 1080i television images.) Standard 2k film scans are 2048x1556. Does that mean HD sports less resolution? Not really. By the time an audio track is added to the edge, the 35mm image is typically reduced to 1828 pixels wide. By the time 1828 is cropped to 1.85:1 aspect, it's just 988 pixels tall. Bonus points if it didn't confuse you that television standards are named for the vertical resolution, but film standards list horizontal resolution.
The Digital Cinema Society (DigitalCinemaSociety.org) recently met in a session titled, "HD 4:4:4 - Where do we put it?". The new generation of HD cameras produce gorgeous images, but data storage is a challenge. "Sony waited until they had a complete system including the HDCAM SR recorder before bringing the HDC-F950 to market", notes Sony marketing manager Rick Harding. SIN CITY used the SRW-5000 HDCAM SR studio deck, as did STAR WARS EPISODE 3. This deck uses MPEG-4 studio profile compression at 440mbs with a compression ration of about 4:1. (The 880mbs Sony SRW-1/SRP-1 HDCAM SR field recorder was not yet available.) For comparison, DV is 25mbs and Sony's new MPEG-2 IMX format is 50mbs. At 24fps the longest HDCAM SR tape lasts 50 minutes.
Surprisingly, the DI revolution is both advancing and holding back the digital cinema revolution. When you're already digital with DI, it seems logical to go digital through to projection. At IVC High Definition Data Center in Burbank, color grading decisions for feature films such as ICE AGE are made using color accurate HD digital projection. The technology is ready in the screening room, but digital cinema deployment is slow because of high costs equipping theaters and concerns of obsolesce in rapidly evolving projection technology.
What's ironic is the tremendous improvement DI is bringing to film prints, making a proven technology look much better. "Major films are first generation prints from DI on polyester film stock that wears like iron", notes IVC marketing VP Dick Millais. HD and digital intermediate are producing much better film images than ever, whether projected digitally or from 35mm film.