Post-Processing: Bloom, DoF, Motion Blur, Tone Mapping

Cyberpunk 2077 sold 13 million copies in 10 days in 2020 - a PC record. Critics praised its cinematic visuals. The same polygons with and without post-processing look like different games. HDR Bloom, Bokeh DoF, and ACES tone mapping are what separate a tech demo from an experience.

• Cyberpunk 2077: HDR Bloom + ACES tone mapping make neon lights glow without blowing out on night streets
• God of War: cinematic hexagonal bokeh DoF on portrait scenes running in real time
• Unreal Engine 5: Temporal Super Resolution uses the motion blur velocity buffer to upscale 4K from 1080p
• Horizon Forbidden West: per-object motion blur for characters via a separate skinned velocity pass

Bloom: Why Bright Lights Glow

A camera sensor cannot capture the full dynamic range of a sunny day - 100 000:1 versus 255:1 for an SDR screen. Bloom does not 'add glow' - it simulates optical scattering inside the lens that occurs precisely when the scene is in HDR.

**Bloom pipeline:** 1) render scene to HDR buffer (16f RGB); 2) threshold pass - extract pixels with luminance > 1.0; 3) downscale to 1/2, 1/4, 1/8, 1/16; 4) separable Gaussian blur (horizontal then vertical) at each mip; 5) additive upsample back to full resolution; 6) combine with original before tone mapping. Separable Gaussian (two 1D passes) costs O(n) instead of O(n²) for a 2D kernel.

**Dual Kawase vs Gaussian.** Unreal Engine 4 uses Dual Kawase blur instead of Gaussian - it requires half the texture samples for a similar result. Each downscale pass reads 4 neighboring pixels at +0.5 offset, producing a Gaussian-like blur without an explicit kernel.

Depth of Field: Blur From the Circle of Confusion

In a real camera, objects outside the focal plane project not to a point but to a disk - the Circle of Confusion (CoC). The larger the disk, the blurrier the object. GPU DoF reproduces this geometry using depth from the G-buffer.

**Gather vs Scatter DoF.** Gather: each pixel collects contributions from its CoC radius (shown above). Scatter: each bright pixel projects bokeh disks onto neighbors - physically more accurate but requires vertex shader instancing with alpha blend and no depth test. Unreal Engine 5 uses Scatter for bright light sources and Gather for the background.

Motion Blur: Smear From the Velocity Buffer

Without motion blur, 30 fps animation looks choppy even on a fast GPU - every frame is too sharp. A real camera integrates light over the shutter interval (1/60 s), smearing moving objects. The GPU reproduces this in a single post-processing pass.

**Screen-Space Velocity Buffer.** During the G-buffer pass the vertex shader computes the difference between current and previous NDC position for each pixel: `velocity = (currentNDC - prevNDC) * 0.5`. The post-processing shader reads the velocity vector and takes N samples along it, averaging the color. Sample counts: 8-16 for games, 32+ for cinematic rendering.

**Per-object vs camera motion blur.** A velocity buffer derived from NDC delta automatically captures both camera and object motion. But skinned characters need velocity per bone. Unreal Engine maintains a separate skinned velocity buffer for Skeletal Mesh - an extra geometry pass - which is critical for correct blur on animated characters.

Tone Mapping: From HDR to What the Screen Shows

After Bloom, DoF, and Motion Blur the HDR buffer holds luminance values from 0 to 10 000 nits. A display shows 0-255. Tone mapping is a non-linear compression of that range preserving detail in both shadows and highlights. Without it the bright region blows out to white and the dark region crushes to black.

**ACES and Display-P3.** ACES (Academy Color Encoding System) was created by the film industry in 2014. Epic Games switched Unreal Engine to ACES in UE4.15. Modern iPhones use Display-P3 (DCI-P3 with D65) - a color space 25% wider than sRGB. To render correctly, the target color space must be known before tone mapping.

Key ideas

• Bloom: HDR threshold -> downscale mip chain -> separable 1D Gaussian (O(n)) -> additive upsample
• DoF: Circle of Confusion from G-buffer depth -> gather N samples over CoC radius (hexagonal bokeh)
• Motion Blur: velocity = current NDC - previous NDC -> N samples along vector -> average
• Tone Mapping: last step - compress HDR to LDR with S-curve (ACES) -> gamma correction -> display
• Pipeline order: G-buffer -> HDR lighting -> Bloom -> DoF -> Motion Blur -> Tone Mapping -> gamma

Related topics

Post-processing completes the HDR pipeline started in Deferred Rendering.

• Deferred Rendering — G-buffer with depth and velocity is the data source for all post-effects
• GPU Graphics Pipeline — Fullscreen quad render target is the mechanism behind every post-processing pass
• Terrain and LOD — Terrain renders into the same HDR buffer and passes through bloom and tone mapping

Вопросы для размышления

• Why must Bloom be applied before tone mapping and not after? What changes if the order is swapped?
• How does Temporal Anti-Aliasing use the same velocity buffer as motion blur for a completely different purpose?
• ACES is a film-industry standard. What trade-offs does it have compared to Reinhard for games with bright HUD elements?

Связанные уроки

• cg-14 — Deferred Rendering produces the G-buffer that all post-processing effects sample from
• cg-08 — Compute shaders and render targets are the mechanism behind every fullscreen pass
• cg-16 — Terrain LOD renders into the same HDR buffer and passes through the same post pipeline
• dsp-04 — Bloom and DoF are convolution filters: Gaussian and bokeh kernels from signal processing theory
• arch-09-cache