Reconsidering handing of black level

[Alex-Vasile]

Currently handling of black levels for the image requires the black level be carrying around and passed to multiple functions, far downstream of the dng to texture conversion.

Option 1: have image_url_to_texture return a float32 texture with the black level already subtracted.

• Pro: simplified pipeline, less textures, no need for extra conversion, much simpler handling of position-dependent black levels (e.g. row/column dependent)
• Con: Certain functions will need to be re-worked since they work on the pixel intensities with the black level included. The increased memory usage of returning float32 textures can be offset by creating them much closer to when they're needed, and doing so one at a time.

The adobe DNG sdk (and likely the open source one) has functionality for querying black level for individual sub-pixels, making that portion of the implementation easier.

Option 2: carry around more information about black level

• Pro: easier to modify into existing code
• Con: Will increase downstream code complexity, will likely increase memory usage (especially for spatially varying black level), and will increase computation time.

_Original public discussion started in https://github.com/martin-marek/hdr-plus-swift/pull/38#discussion_r1359950165_

[Alex-Vasile]

Further consideration, I am not sure the black level is currently being handled correctly. prepare_texture, and the code it was based on, ensures that values are above zero after the black level has been re-added: https://github.com/martin-marek/hdr-plus-swift/blob/8be652ff83854a4d8c25d27b1f1ba78e326a6b3d/burstphoto/texture/texture.metal#L547-L549

While this does prevent negative values in the texture returned by this function, it does not set all pixels below the black level to 0.

I was under the impression that pixels with intensities below the black level should be zeroed out. I.e. the above two lines should be replaced by:

pixel_value = max(0.0f, pixel_value - black_level)*corr_factor + black_level;