Avid DNx GX A High-Quality, Flexible RGB(A) Codec at Commodity Bitrates, Combining SMPTE ST 2019–1 (VC-3) and SMPTE RDD 50 (DNxUncompressed)
When the tunable compression feature from the Quick-Time Animation codec was withdrawn, it left behind a gap in its wake, which animators have been struggling to fill with alternatives. The natural first choice, commodity video codecs, leads to color deterioration at sharp edges when applied to, e.g., broadcast graphics, virtual productions/volumes, VFX exchange, augmented graphics, or motion graphics. The root cause can be found using $\text{YC}_{\mathrm{B}}\mathrm{C}_{\mathrm{R}}$ sub-sampling as part of the compression, even if the original imagery was in RGB. These artifacts are usually acceptable for direct graphic transmission. However, not if further image processing (warping, blending, compositing) is required. The artifacts can be avoided by using an RGB-based compression approach. Largely unknown, the 444 mode (12/10 bit) of SMPTE VC-3 (ST 2019–1), which supports RGB-based compression, is adjustable to target bitrates down to about 20:1 compression. A minor modification can also be applied to 8-bit levels for even better quality at the same bitrate. Compared to $\text{YC}_{\mathrm{B}}\mathrm{C}_{\mathrm{R}}$ 4:2:2 at the same target bitrate, the compressed images show vastly superior visual quality. The compressed results can be packed into a DNxPacked bitstream format, which is defined in SMPTE RDD 50 Part 1. This allows combining the RGB compressed filler with an RLE compressed alpha channel of completely independent bit depth, bypassing the VC-3 limitation requiring the alpha channel to use the same bit depth as the filler.
- Print ISSN
- 1545-0279
- Electronic ISSN
- 2160-2492
- Published
- 2024-01
- Content type
- Original Research
- Keywords
- vc-3, rdd 50, rgb compression, alpha channel, tunable bitrate
- DOI
- 10.5594/JMI.2024/YQTR8645