What is the current purpose of macOS_compatibility_workaround_no_nclx_profile flag?

[bigcat88]

I see that it just tell to not save the nclx profile as macOS can not read it.

But what is the point, if macOS can not read nclx profile, we should be probably ok with that, standard macOS Viewer still can open and display that pictures with the nclx profile.

P.S: and there is no mention anywhere no in heic-enc or heif.h, that you need to set this flag to False, to get lossless encoding.

[farindk]

Apparently, iOS-12 completely refused to load an image if there is an nclx profile attached to it:
#397

I do not know whether this is still the case on some active devices. Probably it is the time to set the default to false now.

[farindk]

Let us examine that systematically. I can think about these cases and intended behavior in my view (which is not necessarily the current behavior):

1. input is YCbCr with nclx, no nclx specified in encoding options -> no conversion, output saves the input nclx
2. input is YCbCr with nclx, different nclx specified in encoding options -> image is converted, output saves nclx from options
3. input is YCbCr without nclx, no nclx specified in encoding options -> no conversion, unclear whether to save nclx
4. input is YCbCr without nclx, nclx specified in encoding options -> conversion with assuming input is standard nclx, save nclx from options
5. input is RGB no nclx specified in options -> convert to YCbCr with default nclx parameters and save those parameters
6. input is RGB with nclx specified in options -> convert to YCbCr with these nclx parameter and save those parameters

Does that make sense or is there any case I missed?

I understand that you are referring to case 3. Here, it's the question whether we should fix the unspecified nclx by specifying the standard or whether we should leave it open. Probably leaving it open makes more sense.

[bigcat88]

Probably it is about the 5 option too, as we convert them with default parameters, then why they should be saved?
Sorry if it is a dumb question, no need to explain fully, just say if it wrong assumption or not.

[farindk]

The parameters are only 'default' in the libheif context. If they are not specified in the file, they are undefined. Hence, when we do any kind of RGB->YCbCr conversion, we should also include the parameters used in the file so that it can be correctly decoded without guessing what the default is.

[bradh]

An exception to that might be for the uncompressed case, where RGB would be maintained unless specifically set in encoding configuration.

[farindk]

@bradh Sure. nclx is only used for YCbCr spaces.

[homm]

Was it fixed in iOS 13 or later? What the latest iOS version with this bug?

[farindk]

I just know for sure that it works with iOS 16. But I guess it has been fixed already a long time ago.