Free caustics, anyone?

If you’ve ever cursed the gods while trying to obtain good and reasonably cheap caustics in Vray, then rejoice. You will breath a sigh of relief as you go through this video tutorial, found on 3D Total. In it, Ricardo Eloy explains what at first seemed to me like a highly implausible trick to obtain the unobtainable: Crisp, complex and clean caustics in Vray, literally in seconds and using the default quality settings. But it works. It essentially hinges on the curious fact that standard Max lights that have been set to exclude all objects in the scene continue to generate caustic photons, even as they do absolutely nothing else. Although the method has some limitations, it gives you what used to be a very expensive output in Vray basically for free, and as such, it is a tip worth a large multiple of its weight in gold.

Below for some context two images, the first calculated in Vray using the said method (the caustic photon map took, without kidding, seconds to calculate) and the second in Maxwell, which, as you will know, gives you free, physically accurate caustics out of the box (whether you want them or not) at the expense of rather long render times. So which do you prefer?

Vray Render

Maxwell Render


  1. [...] decided to do a test of maxwell casutics after reading some time ago an article of Bertrand Benoit about it. I will not explain the differences between maxwell and Vray because [...]

  2. Ricardo Eloy wrote:

    Wow! I have to say I just read all the comments and the positive feedback just made my day (even my week!). Thank you for sharing my tutorial, Bertrand!
    Rick Eloy

  3. Brodie wrote:

    Cool trick. The glass definitely looks better in Maxwell but I guess the caustics just depend on what you want to get (strong or soft). Slight caveat to the “whether you want them or not” comment regarding Maxwell’s caustics. You can, in fact, turn them off. Under Render Options -> Illumination & Caustics you’ve got some options to turn caustics off.

  4. Thanks for the tip. I had some nice results with this technique on a 60 meters high glass and steel office atrium. It gave a nice extra touch on the lighting with the bounced off caustics. I had to use an insane multiplier though.

  5. Rafa Carvalho wrote:

    He is my teacher! :)

  6. Peter Guthrie wrote:

    I LOVE his voice.

    Verrry nice and verrrry fast


  7. Hi Bertrand,
    great test and surely a nice comparison.
    Vray caustics are a bit strong, but that can be easily controlled.
    Thanks for the heads up!!!

    regarding the glass material critique, this can be nicely controlled by adding a little fog, bumping up the refraction and reflection bounces and adding a dispersion. I think maxwell has this all automatically on or am i mistaken? hence the difference…

  8. Michal Timko wrote:

    I thought it was pure hdri! as far as i know, you own indigo license, can you try Indigo ? Im curious about caustics..

  9. BBB3VIZ wrote:

    Thanks, Michal. Yes, this was done with a spotlight.

  10. Michal Timko wrote:

    Nice test bbb, try this with spotlight, not with hdr.
    I prefer maxwell`s output. glass looks more neutral and caustics are soft. Vray gives you really strong caustics which looks like from the sun or strong light source.

  11. manu wrote:

    I was going to say that exactly: VRay caustics, Maxwell reflections. Glad to know it’s just the hdr.

  12. remKa wrote:

    Anyone who wants to learn how to use caustics should take a look at Francesco Legrenzi’s book :)

  13. cpcdron wrote:

    Sure, Maxwell’s caustics don’t seem very convincing… Vray’s are superb!!! Thank you SO for the tip!! Caustics are so time-consuming…

  14. BBB3VIZ wrote:

    Yes, it was a very quick setup, but I think the glass looks better in Maxwell partly because the hdr is oriented differently. There just aren’t enough reflections in the Vray version. But I must say I prefer how Vray handles the caustics at the foot of the glass – there doesn’t seem to be happening much there in the Maxwell version.

  15. artmaknev wrote:

    I must try this now, I’ve been fascinated with caustics for long time, but the amount of time it takes for trial and error during the process is just super frustrating, thanks for sharing such TOP SECRET magic trick!

  16. Jakub Cech wrote:

    Great share! I have also had hard time searching for good time/quality ratio concerning caustics. To me, caustics with Ricardo Eloy´s method look better. I´m not sure but glass material with vray looks quite weird doesnt it? Is it only because of quick setup? Cheers, Jakub.

  17. BBB3VIZ wrote:

    Would be very curious about what you make of it, Peter.

  18. Peter Guthrie wrote:

    Yes, caustics have driven me mad in the past so will be watching it with interest!

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