[FFmpeg-devel] clarification on General Description for FFV1 Draft Specification

[Dave Rice]

Hi all,

I’m reviewing the FFV1 Draft Specification [1] and commenting here specifically upon the General Description section and have some questions to clarify the meaning of this section. I’m cross-posting to ffmpeg-devel though responses are welcomed on either list though encouraged on the IETF Cellar working group listserv [2].

The General Description section [3] contains this introductory paragraph:

"Each frame is split in 1 to 4 planes (Y, Cb, Cr, Alpha). In the case of the normal YCbCr colorspace the Y plane is coded first followed by the Cb and Cr planes, if an Alpha/transparency plane exists, it is coded last. In the case of the JPEG2000-RCT colorspace the lines are interleaved to improve caching efficiency since it is most likely that the RCT will immediately be converted to RGB during decoding; the interleaved coding order is also Y, Cb, Cr, Alpha."

Two colorspaces are referenced, YCbCr(with optional Alpha) and JPEG2000-RCT (Reversible Color Transform), but the RCT sentence doesn’t reference planar storage. Does an RCT encoding store with planes and if so how many places are used and what are their names (3 planes or one packed plane)? Also is storage of an alpha plane with RCT planes allowed?

What is the meaning of “line" in the paragraph above? What does "In the case of the JPEG2000-RCT colorspace the lines are interleaved” mean?

Re: "Each frame is split in 1 to 4 planes". I'd like to be more specific. From this reading it seems like 2 planes is possible; however, are 2 plane encodings possible (grayscale with alpha or Y with only Cb)?

Re: "since it is most likely that the RCT will immediately be converted to RGB during decoding”. Is there anyone other conversion possible?

Best Regards,
Dave Rice

[1] https://github.com/FFmpeg/FFV1/blob/master/ffv1.md
[2] Cellar listserv subscription info: https://www.ietf.org/mailman/listinfo/cellar
[3] https://github.com/FFmpeg/FFV1/blob/10fa55b6f70e9bd5c8f2347b455059524b1163a3/ffv1.md#general-description