Log and RAW workflows

A new trend in camcorders is their ability to record data from several on-board codecs. For example, take AVC-Intra and AVC-Ultra. The announced Cinema Camera from Blackmagic (BMCC) carries this trend further by supporting two compressed codecs — ProRes 422HQ and DNxHD — plus uncompressed RAW. ProRes, in turn, can be had with either of two gamma curves applied: REC709 (the standard gamma for HD recordings) and logarithmic (“FILM” mode). Figure 1 shows the BMCC’s recording options.

Post-production workflows for these advanced cameras will be more complex than those for ENG/EFP camcorders using traditional compression systems. These workflows require specialized knowledge and can be time consuming. This article will present a brief overview of log and RAW workflows.

DeBayered — REC709 — Compressed

Figure 2 shows a schematic illustrating the data-flow within a BMCC. The right path shows its 15.81mm x 8.88mm (1.78:1) CMOS sensor whose 12-bit serial-RGB signal is deBayered with scaling from 2432 x 1366 photocites to a 1920 x 1080 HD frame. REC709 gamma is applied to this 4:2:2 Y’CrCb signal that is then compressed by a ProRes 422HQ or DNxHD (220Mb/s) codec. The encoded 10-bit data are recorded to an SSD.

Because recordings are standard ProRes or DNxHD, they can be edited as would any other compressed format. They require no special post workflow.

DeBayered — Log — Compressed

Again referencing Figure 2, the right path shows that the serial-RGB signal from the 12-bit sensor is deBayered to 1920 x 1080 pixels. Logarithmic gamma is then applied to this 4:2:2 Y’CrCb signal that is then compressed by a 10-bit ProRes codec. The 10-bit ProRes encoded data, carrying slightly less dynamic range than the 13-stops claimed to be carried by the camera’s RAW data, are recorded by an SSD.

Log ProRes is imported into your NLE the same way  REC709 ProRes is imported. Begin by appending footage into a timeline. The log difference is seen when viewing a clip either from a source Event or in a Timeline. As shown in Figure 3, a frame from log ProRes appears washed-out. (Image courtesy of the Diamond Brothers.)

To restore contrast, software such as FXFactory sold by Noise Industries is employed. Drag the Curves function from the Natress Levels and Curves collection onto a clip.

As shown by Figure 4, you first set Log Mode to “Log to Video then Curves.” With this setting, log ProRes is mapped into video space so contrast is restored. You have a choice of the floating-point mapping function to use. Set Log to Video Curve to one of three curves: Log Cineon To Video (LogC); Log B To Video; and Log A To Video. Each creates a subtly different look. I prefer using Log A, although others may prefer Log C.

A curve with five nodes appears superimposed over the visible frame. (See Figure 5.) These nodes appear in the waveform monitor as five luma peaks. One disregards them when looking at the signal.

Figure 5 displays the image that is the result of pushing the right-most nodes higher thereby compensating for the underexposed shot.

The final step is to click the Show Curves button to disappear the superimposed curve. You can now proceed with editing the footage.