Implementing Physically Based Bloom and Lens Flares

In this lesson, you will learn how to configure the Post Process Volume to generate realistic light scattering and lens artifacts that respond dynamically to your scene's emissive intensities.

The Physics of Bloom

In the real world, bloom occurs when light scatters within the imperfections of a camera lens or the fluid of the human eye. To simulate this in Unreal Engine, we move beyond simple "glow" and look at the Bloom settings within the Post Process Volume.

Instead of a uniform blur, we use Convolution Bloom. This method replaces the standard Gaussian blur with a custom texture kernel. By providing an image that represents the actual shape of an aperture (the "bokeh"), the bloom effect will replicate the physical characteristics of light diffraction.

Crafting Lens Flares

Lens flares are optical artifacts caused by internal reflections within a lens assembly. To keep these physically grounded, we focus on the Lens Flare section of the Post Process Volume.

The engine generates these based on the Intensity and Threshold of pixels brighter than the scene exposure. To make them feel dynamic, we use the Lens Flare Tint and Bokeh Size. When your emissive materials have high values, these artifacts should bloom outward.

You can control this behavior per-material using the Emissive Boost parameter in your Master Materials. If you want a specific object to trigger a more aggressive flare, you might use a Material Parameter Collection to ramp up the Emissive intensity based on proximity to the camera:

// Conceptual logic for dynamic emissive intensity
float3 AdjustedEmissive = BaseColor * (EmissiveStrength * FlareMultiplier);
return AdjustedEmissive;

Implementing Dynamic Response

To ensure your flares react realistically:

1. Set your Threshold: In the Post Process Volume, adjust the Bloom Threshold to match your scene’s high-dynamic-range (HDR) range (usually > 1.0). This ensures that only your intentional light sources produce flares.
2. Apply Aperture Masks: Assign a high-quality, high-contrast black-and-white texture to the Bloom Convolution Texture slot.
3. Control via Exposure: Ensure your Auto-Exposure (taught in lesson 25) is calibrated, as bloom intensity is relative to the calculated scene luminance.

Key Takeaway: Physically based lens artifacts are not global overlays, but light-dependent responses that rely on the relationship between your emissive pixel values, the camera's exposure, and the convolution kernel shape.