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V-Ray supports the rendering of caustics effects by using one of the following techniques: photon mapping (New Map mode) or progressive. Alternatively, you can upload a pre-made photon map file.

The new Progressive caustics Progressive caustics solver uses advanced sampling techniques, and is able to trace as many photons as required without suffering the memory constraints of traditional Photon Mapping techniques.

It is loosely based on two papers: one on on Progressive Photon Mapping from Knaus and Zwicker, and another on on Metropolis-guided caustics tracing from Šik and Krivánek.

The other method, Photon Mapping, is a two-pass technique.

Caustics parameters are available in the GI tab of the V-Ray render settings.

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Parameters

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Enable Caustics – Turns rendering of caustics on and off.

Mode – Specifies whether caustics should be calculated using a traditional Photon map or the new Progressive algorithm.

New map – Causes a new photon map to be generated. The new map will overwrite any previous photon map left over from a previous rendering.
From file – V-Ray will not compute the photon map but will load it from a file.
Progressive – The Progressive method doesn't need most of the controls used for traditional Photon Mapping, so the controls are grayed out. Only the Search Dist control in the render settings and the Multiplier V-Ray property in lights, and in the advanced mode of the caustics panel, have an effect on the result.

Progressive

Uses advanced sampling techniques and is able to trace as many photons as required without suffering the memory constraints of traditional Photon Mapping techniques. See the Progressive Caustics advantages and limitations for more information.

Multiplier – Controls the strength of the caustics. It is global and applies to all light sources that generate caustics. If you want different multipliers for the different light sources, use local light settings. 

Search distance units – Determines in what units the Search distance parameter is specified - World units or Pixels. The Progressive mode works only with Pixels.

Search Distance (World) – Specifies the initial photon lookup radius in world units.

Search Distance (Pixels) – Specifies the initial photon lookup radius in pixels.

Max photons  – Maximum number of photons within the search area circle that will be considered when rendering the caustics effect on a surface. Smaller values cause less photons to be used and the caustics will be sharper, but perhaps noisier. Larger values produce smoother but blurrier caustics. The special value of 0 means that V-Ray will use all the photons that it can find inside the search area.

Max density – This parameter allows you to limit the resolution (and thus the memory) of the caustics photon map. Whenever V-Ray needs to store a new photon in the caustics photon map, it will first look if there are any other photons within a distance specified by Max density. If there is already a suitable photon in the map, V-Ray will just add the energy of the new photon to the one in the map. Otherwise, V-Ray will store the new photon in the photon map. Using this options allows you to shoot many photons (and thus get smoother results) while keeping the size of the caustics photon map manageable. 

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The value set initially is automatically reduced by the solver during successive passes, by up to a fourth of the initial value. For example, with the default value of 4, the search radius after being reduced becomes 1 pixel, allowing for detail to be resolved with that accuracy. If the initial radius is set to 16, the final radius is 4 pixels making the caustics look blurrier.

As of V-Ray 5 update 1, it is recommended to leave the radius at its default value of 4.

 

For the New map mode, when V-Ray needs to render the caustics effect at a given surface point, it searches for a number photons on that surface in the area surrounding the shaded point (search area). The search area is a circle with its center at the original photon and a radius equal to the value. Smaller values produce sharper, but perhaps noisier caustics; larger values produce smoother but blurrier caustics.

 

Caustics Photon Map File – Only active when the Mode is set to From File. Specifies the Photon Map file to be used to generate caustics.

Auto save – When enabled, V-Ray automatically saves the caustics photon map to the file specified in the Auto save file field when rendering is complete.

Auto save file – Specifies the file to which the caustics photon map is saved when Auto save is enabled. Click the Image Added button to specify the file name. 

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Example: Search Distance

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For this example, the light used in the scene is V-Ray Sun. The Caustics method is set to to Progressive and the Search Distance values are pixels. The V-Ray Sun has its Photon emission properties adjusted as so: Caustics Subdivs are at their default value of 1500; the Caustics Mult is set to 2 and the Photon Radius is set to 30 cm.  

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Search Distance = 1

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Search Distance = 16

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Search Distance = 32

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Tips when working with progressive caustics:

Required:

  • The Affect Shadows should  option should be off in refractive materials if they are to generate caustics.. The option can be found in the V-Ray Material's Refraction options, as well as in the V-Ray Node Material options.
  • The Affect Channels option should The Affect Channels option should be set to Affect all Channels in refractive materials to ensure the Caustics render element contains also the ones fallen behind refraction.
    Not doing so leaves photons fallen behind refractive surfaces inside the Refraction RE, but not inside the Caustics one. The Affect Channels option can be found in the V-Ray Material's Reflection options, as well as for in the V-Ray Node Material options.
  • If present, the VRaySun's photon emit radius should Photon Emission Radius should be set to a value large enough to encompass the relevant parts in the scene.

Optional:

Lower the Min. Shading Rate.
Photons are cast for each camera ray, not for the secondary ones. The more camera rays are cast, the more photon passes are achieved.
A value of 3 is fine if rendering with the rest of the scene, but a value of 1 could be used if rendering specific caustic passes (for example, a caustics pass scene prepared with the purpose of maximizing caustics' calculation speeds).

  • Set a higher min AA subdivs Min Subdivs value.
    A higher min AA subdivs Min Subdivs value (16, 24, even higher.) means there will be a longer phase of fixed sampling with no adaptation to the photon casting, so the screen coverage of samples will remain more consistent, allowing for quicker filling of, for example, big flat areas or long thin lines.
    The later phase of rendering, with adaptation, will then just bring noise down to ensure Noise Threshold is respected.

Managing Expectations:

  • The progressive Progressive caustic solver is "fire and forget".
    This means that it can be turned on in any scene, and expected to trace photons on any specular or refractive surface.
    It doesn't, however, mean that doing so will result in an image converged to perfection within a short time.
    The approach, novel as it is, is still photon-map based, and judgement should be used to ensure quick, noise and frustration-free results.
  • It renders with the beauty.
    The solver does indeed render happily alongside a beauty pass, storing caustics in the appropriate Render Element (see requirements tips above!), but that may not be the best solution as far as performance is concerned.
    It's suggested to consider preparing a dedicated caustic rendering pass if the photon tracing is particularly extensive, and/or if sequences are to be rendered.
    That way, it will be possible to optimize the caustics calculation separately from the requirements of the beauty pass.

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Progressive Caustics Advantages

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