Physically based bloom

[StarLederer]

Objective

• Make the bloom effect energy conserving
• Make the bloom effect impossible to misconfigure
• Add a physically based alternative to threshold to preserve existing flexibility

Explanation

What is bloom

Bloom occurs in real life when light passes through an imperfect medium such as a lens and slightly scatters around before continuing on its way to the sensor or the retina. Bloom is effectively just a blur effect created by imperfections in glass or human eyes.

Motivation

To emulate it in a natural-looking way it should be energy-conserving and based on its real-life counterpart. The existing implementation is quite close but it does not emulate the scattering perfectly and is certainly not energy-conserving. As a result, the current implementation is easy to configure in an obnoxious way and ruin the look of the game.

Examples

Bloom example with existing implementation:

Bloom example in this PR:

Lighting example with existing implementation of bloom:

Lighting example with bloom in this PR:

Note: The existing implementation could look closer to this PR if the intensity is turned down, however, the existing implementation never results in a natural look at high intensity values, that is what the examples try to highlight.

Existing implementation with intensity of 1 (not related to a physical parameter):

Bloom in this PR with intensity of 1 (light is equally as likely to exit at any point of the lens). This is still WIP since the whole screen should be the same color

Solution

• Calculate mip amount differently so that there the screen can be averaged to a single color at intensity of 1.
• Interpolate between each mip based on bloom intensity rather than adding them together.
• Implement "deblooming" (same as tinting the lens and increasing the exposure; that way light that scatters more loses energy. This restores the artistic control previously provided by threshold but in a physically-based manner)

---

Changelog

Changed:

• Bloom has a more natural look.
• Deblooming parameter replaces threshold and knee.

Migration Guide

• Remove threshold and knee fields from all instances of core_pipeline::bloom::BloomSettings.
• Optinally use the deblooming filed to approximate the look threshold values above 0 created.

[pkupper]

For your awareness: There is already another open pull request adressing some of the issues you brought up in your YouTube comment:
#6677
It also implements ACES tonemapping which also contributes to better looking bloom.

[StarLederer]

Thanks! I'll close this PR and and see if my contribution can be useful in #6677 instead