JVET-AM0101 AHG9: On Lens Optical Correction SEI message [L. Kerofsky, Y. He, S. Zhao, M. Karczewicz (Qualcomm)]

This contribution proposes the following changes to the LOC SEI message to increase clarity and resolve some typos

1. Add syntax elements for resolution at which LOC is indicated. This is needed to convert the optical center, specified in LOC as a fraction of picture resolution, to a pixel value.
2. Add data length of loc_focal_parameters_present_flag. Corrects an omission.
3. Add independent focal lengths for horizontal and vertical dimensions to support pixel aspect ratios which are not one or when the focal lengths are estimated and not identical
4. Define the normalization process used in converting pixel coordinates into normalized coordinates used by the distortion models. Camera calibration offsets values by the optical center than scales by the focal length. The defined normalization uses the focal length expressed in units of pixels rather than in mm.
5. Modified semantics of loc_tca_model_id correcting a naming typo.

The semantics refer to pixels, but should probably refer to luma samples.

It was suggested that it could be optional to send one vs two focal length parameters, when the x and y are equal.

JVET-AM0181 was noted as also having some similar information signalled for a different SEI message, and has an option to signal either one or two focal length parameters.

The existing design expressed focal length in millimeters. This contribution proposes using samples and explicitly signals the source picture width and height, and defines a normalization process using those parameters.

There is an error in the normalization process of local_pic_width_in_luma_samples.

The proposed focal signalling is in samples which are integers, so can’t have fractional samples. Is that prevision sufficient?

Proponent said that many applications of interest provide focal length data in pixel units.

It is suggested the link between the sensor and processing domain is not clear.

Further discussion requested.

Further discussed 1115 Tuesday 1 July 2025.

Decision (editorial): Add editorial bug fixes from items 2 and 5 from the -v2 of the contribution.

JVET-AM0101 AHG9: On Lens Optical Correction SEI message

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