Trigger recompression for J2K/DCP content

[Carsten]

Usually, when feeding J2C sequences or DCPs into DCP-o-matic, all processing is bypassed and content is copied/wrapped without alteration when creating the new DCP. That is a useful feature.

Recently we received a series of ads with a greyish background that should be black, but isn't. For the fun of it, I tried to correct this using only DCP-o-matic (instead of converting it to MP4/Prores and adjusting it in a video editor).

For this, I loaded the full DCP into DCP-o-matic, and created a custom color correction, changing only input gamma. Interestingly, DCP-o-matic applied the gamma change and recompressed the images as I had hoped for. The effect of the gamma change is clearly visible in player and video analysis, although I wasn't able to screen the modified clips yet back-to-back with the original on our DCI projector. So, not sure yet wether my attempt to correct the greyish background gave acceptable results. But, I learned that you can force DCP-o-matic into recompression by setting a custom color correction for DCP content. Will this work for J2C series as well? I currently don't have any at hand.

Now - these specific ads were in black and white, and aside from the gamma change, I didn't notice any other side effects of this method. However, when I tried it with a colored movie trailer, the resulting new DCP showed a noticeable color shift even when I did not adjust gamma intentionally - that is, I entered a 'simple' input gamma of 2.6, and left the 'inverse 2.6 gamma correction on output' checked. It seems that in this case, the XYZ conversion (probably based on an RGB input color space) is still performed. Which looks acceptable for black and white footage, but will be completely off for colour content.

The idea for this came when occasionally people experienced issues with DCPs created with DCP-o-matic, using MXFs or J2C series that had been created with other software that doesn't conform J2C or MXFs to strict DCI specs. By forcing DCP-o-matic into recompression, without changing other parameters, one could correct that issue by creating a new, DCI conformed J2C. So far, my only suggestion to force recompression was to add 'invisible' burn-in subtitles, or to apply minor cropping. I suggested a simple 'recompress' checkbox somewhere in the GUI earlier on Mantis - that could also be used to recompress the full project intentionally (also full recompress for unchanged content (e.g. with partial burn-in subtitles).

- Carsten