When environment maps were introduced for illumination purposes, there were a couple of subtle effects:
Turning the background's visibility off now provides an opaque white background (instead of the transparent white background as would be apparent when doing image captures). An invisible background always produced a white background; the color was preserved and only its opacity was modified.
The "invisible" background still provides illumination to the scene (assuming there are objects with PBR materials. Regular phong materials are not illuminated by the environment map. This is new. Generally, this was implemented because it seemed really strange for the background to go from non-white to white and then to see PBR materials all get darker, even when the environment seems to have gone all white.
Should an invisible background truly have those kind of effects? What are the alternatives? There are two independent phenomena in play: what gets rendered in the background, and what is observable by geometry in the scene.
Options for making the background invisible
It renders as opaque white in the background, but omits no light
Highly reflective metallic surfaces would appear black.
All PBR material surfaces would be darker because there would be no accumulated ambient light.
Image captures will have a fully opaque white background.
The background would get "lighter" but the surfaces would get darker.
The background goes transparent black and gives off no light.
The background is "gone" in every sense: not in captured images, not in lighting, everything gets darker one way or another.
The background goes opaque black and gives off no light.
Same as (2), but the background is now present in the captured image.
The background goes "transparent" white and gives off no light.
This represents the pre-environment map behavior.
The background would get lighter, but PBR surfaces would get darker.
Captured images would have no background.
FTR: Making an "invisible" background opaque is probably a waste of time. If the user wants an opaque, solid background, they have the option of setting the color gradient to that solid color. No extra contortions are necessary, so that probably throws out (1) and (3).
(2) seems the most consistent, but constitutes a change in the historical behavior. Turning off the background would no longer be the way to create a simple, white background. Those users who rely on that would have to respell their incantation to setting the two gradient colors to white, instead.
When environment maps were introduced for illumination purposes, there were a couple of subtle effects:
Should an invisible background truly have those kind of effects? What are the alternatives? There are two independent phenomena in play: what gets rendered in the background, and what is observable by geometry in the scene.
Options for making the background invisible
FTR: Making an "invisible" background opaque is probably a waste of time. If the user wants an opaque, solid background, they have the option of setting the color gradient to that solid color. No extra contortions are necessary, so that probably throws out (1) and (3).
(2) seems the most consistent, but constitutes a change in the historical behavior. Turning off the background would no longer be the way to create a simple, white background. Those users who rely on that would have to respell their incantation to setting the two gradient colors to white, instead.