Article by Steve Goodwin, Head Editor and Archivist, with two cents added by the staff at Producers Library.
“What are these huge files you’ve sent me? My editor says he can’t open them…”
So, let’s get this out of the way first. The technical stuff. Digital Picture Exchange (DPX) is a lossless file format for digital intermediate and visual effects, and is the most commonly chosen format worldwide for still frame storage in most digital intermediate post-production facilities and film labs. The file format is used to represent the density of each color channel of a scanned negative film in an uncompressed "logarithmic" image where the gamma (the black transition to white) of the original camera negative is preserved when run through a film scanner.
“Why are there so many of these DPX files and why do they look so washed out?”
Scanning film to DPX files produces a lossless, numbered digital file corresponding to each frame of the scanned film. For example, if the film was shot at 24 frames per second (fps) then there will be 24 separate DPX files for one second of film scanned. That’s a lot of files. One can think of a folder of DPX still frame files as the digital version of a roll of film.
Each DPX file is often over-scanned past the edge of the frame lines, showing the complete image of the film frame, and looks flat and milky when first viewed. Never fear, all the color information (density) from the film is contained within the file and that’s where the color grading process comes in.
The Delight: The Good News
An advantage of the DPX format is that the files can be color graded to suit the specific needs of the production. The color “look” of the film is not baked in (as in traditional Pleistocene Era telecine) so the post house or effects house has complete freedom to color grade and crop the frame as they wish. This is especially vital when matching archival stock footage to current production values of surrounding shots. Though many stock footage websites show only 2K or 4K QuickTime Pro Res files available for older film-based stock footage, (and they may be just fine for your project!) you may want to consider asking for the choice of scanning to DPX files.
Below is an example of a “flat” scanned 35mm Negative frame of a Paris street scene in the 1950s at 4K resolution, the same 4K frame color graded, and then an HD workflow showing a 16x9 crop of the 4K image. The twelve second clip can be viewed at https://producerslibrary.com/preview/V-0904_387.
Working in the HD world, the 16x9 crop of the original 4K image becomes an editorial and aesthetic decision based upon the framing, content, context and composition of the image. DPX files offer the ability to reframe and/or push in (or focus) on a smaller part of the 4K frame due to the relative frame size difference between 4K (4096 x 2160) and HD (1920 x 1080).
For most editing projects, a common approach is to crop and/or resize and color grade the DPX sequence in DaVinci Resolve or similar software, then export a QuickTime file to the project specifications and import that into the editing system. Visual effects work will have a different workflow as the uncompressed 4K files may need to be manipulated and altered prior to grading and output.
Producers Library has supplied many features, TV dramas, commercials, and even documentaries with 2K and 4K DPX files. A recent example is Showtime’s Black Monday, about the stock market crash of 1987. The first episode utilized Producers Library’s extensive archive of that era for a creative VFX shot. The star Don Cheadle arrives at NYC’s Wall Street in a “Lambo Limo”, CGI created for the scene, which was comped into 4K DPX files scanned from the archival camera original 35mm negative. The ability to seamlessly insert actors, background or other CGI elements into vintage footage is just one of many possibilities that DPX files can provide.
The Dilemma: The Not So Bad News
Beware: these are large files and they eat up storage fast. Twenty-four frames of DPX files at 4K resolution is over one gigabyte of storage, and for 6K or higher... well, go ahead, you do the math! Fast computers and throughput are essential to play a DPX file sequence in real time, as well as a ton of hard drive real estate for archival storage.
The bottom line: if you are working at high resolutions, planning to manipulate a shot, use VFX, insert and match to existing camera footage or archiving for the future then DPX files will be the way to go. Just be prepared for large file sizes, time for color correction, longer render times and playback buffering if you are planning on using them in your traditional edit.