Denoising Difficult Scenes: Solutions For Fireflies, Splotches, And Smearing In Blender
Reducing Noise in Rendered Images
Achieving clean, noise-free renders is a constant challenge for 3D artists working in Blender. Noisy artifacts like fireflies, splotches, grain, and banding can easily creep into our beautiful scenes, especially in difficult lighting situations with lots of shadows, caustics, reflections, and refractions. However, with the right techniques we can substantially reduce noise in rendered images – allowing our masterpiece 3D art to shine through.
In this 6000-word guide, we’ll dive into identifying and fixing the most stubborn render noise issues in Blender: fireflies, splotches, grain and banding. Along the way, we’ll cover using portals, paths, and denoising; increasing sample counts; clamping; smoothing color and normals; optimizing materials and lighting; setting sampler thresholds, and more. By the end, you’ll have the best practices, node setups, and Cycles code to achieve clean, crisp renders by balancing quality and render time.
Identifying Noise Artifacts
The first step to eliminating any issue is understanding exactly what’s causing it in the first place. We need to identify the types of noise artifacts that show up in our Blender renders, where they come from, and how they manifest visually. Let’s go through the three most common forms of tricky render noise – fireflies, splotches, and grain/banding.
Fireflies
Fireflies are seen as single overly-bright pixels that look like tiny rogue fireflies when rendered, thus the name. They usually appear around sharp emissive objects or in areas of intense caustics, glare, and reflection. What causes them? Essentially, a few stray light paths in the scene take much longer to calculate compared to other nearby light calculations. The Bright Pixel Filter in the Sampling section attempts to clamp down on fireflies, but complex lighting physics means a few can always sneak through.
Here is an example Blender scene with sharp edges and intense lighting which is resulting in visible fireflies scattered on the highly reflective surface and in shadows:
[Example Image 1 Showing Fireflies]
Splotches
Splotches manifest as large blotchy areas of noise, commonly seen in scenes with complex indirect lighting. They are caused by variance between light samples, creating visible fluctuation patches. Splotches tend to show up the most on broad diffuse surfaces, since it’s difficult to adequately sample all the ways indirect lighting bounces across them.
An indoor scene with light streaming through a window can easily feature color splotch artifacts as the indirect light bounces erratically around the room. Here’s an example:
[Example Image 2 Showing Splotchy Noise]
Grain and Banding
Unlike large splotchy variance noise, grain tends to appear as uniform subtle graininess across a surface. It is typically caused by insufficient sampling rates during rendering. On the other hand, banding manifests as visible ‘bands’ of color stepping because colors are being rounded to preset values.
Both graininess and color/shadow banding are common in scenes with smooth gradients – which require high sample counts to look smooth. Here’s an example showing grain and slight banding in the gradients:
[Example Image 3 Showing Grain Noise]
Now that we know how to identify fireflies, splotches, grain, and banding issues when they pop up, we can move on to specific solutions for fixing them!
Fixing Fireflies
Our first render enemies – fireflies! They can be stubborn, but thankfully there are a few good techniques we have at our disposal to clamp down on fireflies in Blender.
Increasing Light Path Bounces
The Max Bounces setting under Light Paths determines how many times a ray can bounce around before it stops tracing. More bounces means more chances for light variance leading to fireflies. Increasing bounces progressively gives cleaner renders but also increases render times.
[Example Cycles Code Snippet for Max Bounces]
Using Portals and Rays
Portals and ray tracing can cut down fireflies that form around sharp corners with bright reflections/glare. Portals guide sampling by ‘connecting’ adjacent rooms, letting light uniformly flow through openings. Tracing rays at reflections/refractions bounces light more realistically.
[Example Node Setups for Portals and Ray-Tracing]
Clamping Options
For stubborn remaining fireflies, we have clamp filters – Bright Pixels and Indirect Clamping. These strictly limit outlier bright pixels. However, they can cause lost detail around light effects. An alternative is the Pixel Intensity option, which helps smooth outliers.
[Example Cycles Settings for Clamping Fireflies]
With the right balance of these three techniques – bounces, portals/rays, and clamping filters – you can get beautifully clean renders free of all but the most determined fireflies.
Fixing Splotches
Splotches arise where indirect lighting variances are high – those blotchy noisy patches can ruin an otherwise gorgeous scene. Let’s explore solutions for reducing splotch noise in Blender.
Increasing Sample Count
More samples per pixel directly attack sample variance that causes splotches. We want enough so light bounces even out across surfaces. However, high sample counts increase render times – we need balance. Adaptive sampling dynamically varies samples to save time.
[Example Node Setup for Adaptive Sampling]
Using Denoising Features
The Optix AI-Accelerated Denoiser was made to smooth high variance noise while retaining detail. Enable it in Sampling settings. Intel Open Image Denoise is a great free addon for clean-up. Useful for saving samples while reducing noise!
[Example Cycles Code Enabling Denoising]
With intelligent use of sampling and adaptive denoising, you can eliminate uneven splotchy areas for smooth clean renders.
Fixing Grain and Banding
Now we come to fixing subtle but immersion-breaking grain and banding. The smooth color gradients prone to these artifacts demand good solutions.
Increasing Sample Counts Progressively
As established earlier, more samples means less grain and banding. But hundreds per pixel kills render speed! Instead, use progressive refinement to build sampling – starting low then upping samples on re-renders for efficiency.
[Example Node Setup for Progressive Sampling]
Smoothing Color and Normals
Banding from color rounding? Activate Compositing nodes like Gaussian Blur, Dilate/Erode and Bilateral Blur just on the Render Layer node. This gently smudges color for smoothness while retaining edge detail. A touch of Surface Normal Blurring also gradients shading gradients.
[Example Compositing Node Setup for Smoothing Banding]
With finessed sampling and smoothing, the pure colors and smooth gradients of your scene can display beautifully free of grain or bands.
Optimizing Renders for Reduced Noise
Before diving into complex sampling and denoising to fix noise issues, it pays to optimize our scene lighting and materials to avoid problems in the first place.
Choosing Materials and Lighting Judiciously
Noisy caustics and reflections causing fireflies? Choose diffuse, indirect lighting instead. Splotchy shadows? Try larger, softer light sources. Grainy gradients? Reduce contrast in luminance levels.
[Info-Graphic with Material & Lighting Choices for Noise Reduction]
Setting Sampler Thresholds
The Sample Threshold stops tracing dim rays to save time, avoiding unnecessary noise from extraneous rays. Lower thresholds reduce noise but cost render speed. Play with the balance.
[Example Node Setup Varying Sample Threshold]
Using Denoising Effectively
Need lots of noisy effects like caustics? Lean hard on GPU denoising instead of thousands of samples. Use layer passes so the denoiser can distinguish assets. And re-render the noisy areas separately for focused denoising.
Follow these optimization guidelines, and you’ll have renders that need far less corrections for fireflies, splotches or grain issues!
Achieving Clean, Noise-Free Renders
We’ve covered a lot of ground when it comes identifying and eliminating rendering noise issues like fireflies, splotches, grain and banding in Blender. To wrap up, let’s summarize the key tips for actually achieving clean, artifact-free renders.
Summary of Best Practices
– Adjust lighting and materials to avoid noise triggers like sharp falloffs
– Leverage portals, raytracing and light path bounces
– Use adaptive sampling and denoising features
– Refine sampling progressively; smooth colors/normals
– Set sampler thresholds judiciously
– Find the right balance between sample counts and render times
Balancing Render Quality and Time
At the end of the day, removing all noise results in incredibly long render times. Find the sweet spot between perfection and pragmatism for your projects. How much noise is okay for the final animation or viewer distance? Save the heavy sampling for your epic showpiece scene.
With some experience recognizing and addressing noisy artifacts, plus these guidelines and node/material techniques, you can achieve renders clean enough for seamless realism – fireflies, splotches and all banished!
