In Cortex v1.5.3 b5238 we introduced support for encoding Apple ProRes 4444 XQ QuickTime files in Cortex. (Support for decoding the codec was available in the first v1.5.3 release).
Details about the new codec can be found in the updated Apple ProRes White Paper where they describe the new bitrate:
Apple ProRes 4444 XQ: The highest-quality version of Apple ProRes for 4:4:4:4 image
sources (including alpha channels), with a very high data rate to preserve the detail
in high-dynamic-range imagery generated by today’s highest-quality digital image
sensors. Apple ProRes 4444 XQ preserves dynamic ranges several times greater than
the dynamic range of Rec. 709 imagery—even against the rigors of extreme visual
effects processing, in which tone-scale blacks or highlights are stretched significantly.
Like standard Apple ProRes 4444, this codec supports up to 12 bits per image channel
and up to 16 bits for the alpha channel. Apple ProRes 4444 XQ features a target data
rate of approximately 500 Mbps for 4:4:4 sources at 1920 x 1080 and 29.97 fps
When Apple says “high-dynamic-range imagery” that includes Log-encoded imagery from cameras like the ARRI Alexa camera which also recently announced support for the codec.
As Marc Shipman-Mueller, ARRI’s Product Manager for Camera Systems states in that release:
ProRes 4444 XQ has a higher target data rate of 500 Mb/s that makes it the ideal choice for postproduction that requires visual effects processing or extreme color grading.
So its real purpose is to encode images that are in a Log colorspace which will later be used as a source for Color Correction or Visual FX.
It’s not intended to be a substitute for delivery of viewing copies, offline editorial or REC709 master deliverables. It could be used for that purpose, but the costs of the extra data rate may not be justified by any gain in quality, which you would likely only see if the image is later manipulated.
The normal Apple ProRes 4444 and 422HQ variants still fulfill that role, according to the white paper:
Apple ProRes 4444: An extremely high-quality version of Apple ProRes for 4:4:4:4
image sources (including alpha channels). This codec features full-resolution, mastering quality
4:4:4:4 RGBA color and visual fidelity that is perceptually indistinguishable
from the original material. Apple ProRes 4444 is a high-quality solution for storing
and exchanging motion graphics and composites, with excellent multigeneration
performance and a mathematically lossless alpha channel up to 16 bits. This codec
features a remarkably low data rate compared to uncompressed 4:4:4 HD, with a target
data rate of approximately 330 Mbps for 4:4:4 sources at 1920 x 1080 and 29.97 fps. It
also offers direct encoding of, and decoding to, both RGB and Y’CBCR pixel formats
Apple ProRes 422 HQ: A higher-data-rate version of Apple ProRes 422 that
preserves visual quality at the same high level as Apple ProRes 4444, but for 4:2:2
image sources. With widespread adoption across the video post-production industry,
Apple ProRes 422 HQ offers visually lossless preservation of the highest-quality
professional HD video that a single-link HD-SDI signal can carry. This codec supports
full-width, 4:2:2 video sources at 10-bit pixel depths, while remaining visually lossless
through many generations of decoding and reencoding. The target data rate of
Apple ProRes 422 HQ is approximately 220 Mbps at 1920 x 1080 and 29.97 fps.