Hacker News new | ask | show | jobs
by omoikane 1049 days ago
Is there a reason for using LAB as opposed to YCbCr? My understanding is that YCbCr is another model that separates luma (Y) from chroma (Cb and Cr), but JPEG uses YCbCr natively, so I wonder if there would be any advantage in using that instead of LAB?
1 comments
TylerE 1049 days ago
The Y in YCbCr is linear, and is just a grayscale image. The L channel in lab is non-linear (as are A and B), and is a complex transfer function designed to mimic the response of the human eye.

A YCbCr colorspace is directly mapped from RGB, and thus is limited to that gamut.

LAB can encode colors brighter than diffuse white (ala #ffffff), like an outdoor scene in direct sunlight.

Sorta HDR (LAB) vs non-HDR (YCbCr).

This image (https://upload.wikimedia.org/wikipedia/commons/thumb/f/f3/Ex...) is a good demo, left side was processed in LAB, right in YCbCr). Even reduced back down to a jpeg, the left side is obviously more lifelike, since the highlights and tones were preserved until much later in processing pipeline.

link
aendruk 1049 days ago
The description included with that image conflicts with your account:

> An example of color enhancement using LAB colorspace in Photoshop (CIELAB D50). Left side is enhanced, right side is not. Enhancement is "overdone" to show the effect better.

And per the original upload the “enhancement” demonstrated is linear compression of the a* and b* channels—

https://upload.wikimedia.org/wikipedia/commons/archive/f/f3/...

—the effect a divergence from the likeness of life at least as I’ve experienced it.

link