Guide

×1 Install

macOS. Open the DMG and drag Reblend to Applications. It is signed and notarized, so it opens like any other app. GPU rendering needs Apple silicon and macOS 15 or later; there is no Intel build.

Windows. Unzip the archive anywhere and run blender.exe. The build is not code-signed yet, so SmartScreen may show a warning the first time: choose More info, then Run anyway.

Linux. Extract the archive and run ./blender from the folder. It needs glibc 2.28 or later, which every current distribution has.

Reblend uses the same settings folder as Blender 5.2, so preferences, add-ons and recent files are shared with your normal Blender. Files saved by Reblend open in Blender 5.2 without changes; the extra settings are simply ignored there.

×2 Sampling method

Render Properties → Sampling → Method has two entries.

• Path Tracing is Cycles as you know it, and the default. Photon Caustics and Path Guiding build on it.
• Bidirectional also traces paths from the lights. Vertex Merging is an option under it.

Neither is better for everything. Stay on Path Tracing; switch to Bidirectional for caustics seen directly and for small or hidden lamps, and compare at equal render time rather than equal samples.

×3 Bidirectional

Cycles traces paths from the camera and from the lights and connects them. Light that is hard to find from the camera side, such as a lamp behind a shade, a spotlight tucked into a fixture or a caustic on a table, arrives with far less noise, at a higher cost per sample. With it selected a Bidirectional panel appears under Sampling.

• Light Paths: light paths traced per sample, relative to the pixel count. Lower renders faster; higher gives cleaner caustics and cleaner light from small or hidden lamps. Default 0.25.
• Light paths follow the same bounce limit as camera paths, Light Paths → Max Bounces → Total.
• Bidirectional traces its own light from the lamps, so the Photon Caustics and Path Guiding panels are hidden while it is selected.

It works on CPU, Metal, CUDA and OptiX, on one render device at a time. On OptiX the first Bidirectional render after launch can pause for a few minutes while the light-path pipeline compiles; see known issues.

×4 Vertex Merging

A checkbox in the Bidirectional panel. Besides connecting camera and light paths, Cycles also joins them where they land close together, which reaches the caustics seen through glass or water that connections alone cannot: the floor of a pool through its surface, light gathering inside a glass.

• Slightly soft at first, sharpening with samples, at a higher cost per sample. Off by default.
• With it on, Light Paths defaults to 1.0: merges get cleaner with every light path.
• Merge Radius, in pixels, default 2. Lower is sharper at the cost of noise; higher reduces noise in faint caustics spread over a large area.

Files saved with the old VCM method open as Bidirectional with Vertex Merging on.

×5 Photon Caustics

Works with Path Tracing. Render Properties → Light Paths → Caustics → Photon Caustics. Before each batch of samples, photons are traced from the lights through the objects that focus them and stored where they land; the path tracer reads the caustic from that map instead of trying to find it by chance. It is for the sharp caustics of glass, water and polished metal, on surfaces and inside volumes such as light shafts under water. It does nothing for diffuse interiors; that is what Path Guiding is for.

• Photons: more give smoother caustics at the cost of render time and memory. Bounces: two covers a caustic through one glass, four also light that bounces around inside glass or water. Strength: 1.0 is the physically correct result.
• Casters subpanel. Automatic lets the materials decide: anything refractive, and reflections sharper than a quarter roughness, cast photon caustics. Manual lets you pick objects and collections yourself, which puts all the photons where you want them. Either way nothing is lost; what does not cast photons is rendered by the path tracer.
• Advanced subpanel: gather radius, progressive shrinking, samples per map and the like. Normally leave it alone.

Photons come from every light by default. To switch one off, untick Emit Photons in Light Properties → Settings for a lamp, World Properties → Settings → Surface for the world, or Object Properties → Shading → Caustics for a glowing object.

×6 Path Guiding

Works with Path Tracing, on the CPU or on the GPU with Metal, CUDA or OptiX. Render Properties → Sampling → Path Guiding; tick the checkbox in the panel header. Guiding learns during the first samples where the light in a scene comes from and aims paths there. In Blender it exists for the CPU only; Reblend runs both the learning and the sampling on the GPU.

• Turn it on for interiors lit by bounced light, like a room lit through a window. Leave it off in brightly lit scenes, where it costs more than it saves.
• Training Samples: how many samples are spent learning; 0 keeps learning until the last sample.
• Photon Training (GPU only): a head start by tracing photons before the first sample, about a second per render.
• Surface and Volume switch guiding on or off for each.
• It is unavailable when the CPU and the GPU are both enabled in Preferences. Pick one.

×7 Nested Dielectrics

A drink is three objects that occupy the same space: the glass, the liquid and the ice. Cycles alone bends light at every surface it meets, including the surfaces buried inside other objects, so overlapping objects render wrong and modellers cut the meshes apart by hand. Reblend fixes this with a priority per object. Works with every method.

1. Model each object as a closed mesh and let them overlap a little: the liquid slightly into the glass wall, the ice fully inside the liquid. Never let two surfaces sit exactly on top of each other.
2. Give each object its own Glass, Refraction or Principled material with the right IOR.
3. Set Object Properties → Shading → Nested Dielectrics → Priority on each: glass 3, ice 2, liquid 1.

Where objects overlap the higher priority is the medium: the surfaces of the lower one inside it are ignored, its volume is off there, and at every real boundary the light refracts with the ratio of the two IORs, glass to liquid rather than glass to air. Priority 0, the default, means the object takes no part, so scenes that do not use this render exactly as before.

Limits in this version: Shadow Caustics does not know about priorities (Photon Caustics does); shadow rays through refractive surfaces without transparent shadows are blocked as they are in Blender; subsurface, coat and thin-wall materials ignore priorities.

×8 Caustics pass

View Layer Properties → Passes → Light → Caustics. Available with Photon Caustics on, or with Bidirectional. It holds only the caustic light, so you can grade it on its own in the compositor. It is also part of Combined, so subtracting it gives the render without caustics.

×9 Fallbacks and known issues

• When Bidirectional cannot run the render switches back to Path Tracing and tells you: with a shadow catcher, a rolling shutter, a custom camera, depth of field or spherical stereo on a panoramic camera, Open Shading Language on a GPU, or more than one render device. With motion blur, light paths still reach the camera directly, so caustics seen directly remain, but nothing is connected or merged.
• OptiX. The first Bidirectional render after launch can pause for up to six minutes while OptiX compiles the light-path pipeline. It looks stuck; it is not. Later renders start at once, and CUDA has no such wait.
• Windows on NVIDIA. The Windows build shares its GPU code with the Linux build, which has been tested on an RTX 4090, but the Windows package itself has not yet run on an NVIDIA card. CPU rendering on Windows is tested.
• Windows signing. The Windows build is not code-signed, so SmartScreen warns on first launch.
• Experimental. Settings and defaults can change between versions. Check deadline work in stock Blender too.

Found something else? Write to contact@leonidaltman.com with the version line from the About box and, if you can, the .blend file.