On Wed, Mar 13, 2019 at 10:41 AM Pradeep Ramachandran < [email protected]> wrote:
> On Tue, Mar 12, 2019 at 10:21 PM Vittorio Giovara < > [email protected]> wrote: > >> >> >> On Tue, Mar 12, 2019 at 12:11 PM Aruna Matheswaran < >> [email protected]> wrote: >> >>> Thanks for the pointers, Vittorio. CTA-861.3-A specification states that >>> if both MaxCLL and MaxFALL are signaled as 0, the rendering device shall >>> interpret it as unknown. >>> >> >> Thanks for your response, I am aware of this and it logically makes sense. >> >> With this reference, x265 by default is signaling 0 for both MaxCLL and >>> MaxFALL with the assumption that any logical implementation of the >>> specification would ignore them. >>> >> >> This part I don't understand. The possibility of avoiding sending this >> SEI is just one if clause, what is the purpose of encoding an empty >> message? Is it a requirement for some other specification? Does it serve a >> private x265 use? Nothing wrong in either, but please have it documented >> somewhere. >> >> The problem we see now is that your renderer interprets 0 content light >>> levels as valid values and displays too dark or too bright pixels. Whereas >>> a few other renderers don't accept NULL entries for content light levels >>> and expect 0 content light level as a signal to disable/ignore their usage. >>> >> >> Unfortunately though it is not *my* renderer, but it's the renderer of >> some tvs and devices in the wild, over which I have no control. >> >> Will introducing *an additional param flag to enable signaling of only >>> mastering display metadata *fix your problem? With this, renderers >>> which don't accept NULL content light level entries shall use the default 0 >>> signaling. On the other hand, renderers which treat 0 content light level >>> as valid entries shall disable signaling them via the additional flag. >>> Please share your thoughts on this suggestion. >>> >> >> This would kind of work but I do not believe it's a proper solution. At >> most, the default behavior should be the one of least expected surprise: if >> message is empty just don't encode it. Then if a sensible usecase really >> exists, there should be an option to force encode light level even if >> empty. However it's still unclear why you would need to that in the first >> place, as trusting decoders to do the right thing is not very efficient and >> leads to a catch-a-mole experience. >> > > We have other users who've come back to us with the report that that > unless maxCLL and maxFALL are signalled as (0,0), their decoder/renderer > is decoding this as an invalid HDR10 stream. (My email earlier about > non-HDR10 streams was incorrect; please ignore that.) Your use case is that > your decoder interprets (0,0) as a valid value and renders the pixels > incorrectly! As this SEI message is pass-through for the encoder, we just > went back to the standard and did what we thought was the right > interpretation of the standard, and that was to signal *all* HDR10 params > when *any* HDR10 param was non-zero. And we had another request from a user > asking for having the ability to always signal HDR10 SEIs even when they > were zero and that is why we added the --hdr option. (In hind-sight, we > should've called this --hdr10, but we will live with it for now.) Now, your > use-case is that you want a sub-set of the HDR10 SEIs to be signaled and > not the others. Maybe adding separate flags for force-signalling them > separately is the best option here, but so many flags isn't a good thing! > A couple of points here: - it's not "my decoder", but decoders installed on *some* tvs and *some* devices. I have no control over those devices and I can't even gather data about which devices these are - I am not using the --hdr(10) option from the command line interface, this all comes from the API. While I can expect some kind of automatic when using the CLI, the API itself should not "surprise-encode" messages that weren't explicitly enabled (especially if empty - hdr10 is mostly a commercial term, it's not a real "standard" per se but a collection of specifications stitched together. There is no such thing as "invalid hdr10 stream" because there is no conformance to adhere to: decoders or renderers needs to apply whatever information is present in the stream, to the best of their support. Some perform better some perform worse - I disagree with limiting the number of "so many flags": this is a video encoder which is not a simple thing to begin with, so exposing more knobs to allow more in-tune configuration to "expert" users is actually appreciated (to a limit) - I agree --hdr should have been called --hdr10 but it's never too late to add/deprecate that, especially when major bumps are around ;) -- Vittorio
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