I made a simple project with a container and water inside it ,Also I used Spotlight , Environment Light and Caustic Light.

After I started to render the scene , I took wonderful problem.

Each frame from 0 to 30 rendered in 7 minute.

But after frame 30 each frame rendered in 1 hour or maybe 1.5 hour !

I was looking a solution for my problem ,every things like Water Mesh , Lights , Camera and others are same with previous frames

After too many test I found out that this problem is for Caustic Light.
In the previous frames , Caustic points generated only on the ground , the Position.Y for all of them was -5.0.

But in the others (After frame 30) , some of the Caustic points generated on top of the water , the Position.Y for all of them was greater than 0.

So my project rendered heavily !

I imported photon map file in the Houdini and set TY for all of the points to -5.0 and saved them to another pmap file.

After I did that ,I loaded this pmap file in the Caustic Light , Then my problem solved and the scene rendered in 7 min !

Now my question is :

Why render time came very long when I have some caustic points above of the ground !?
:cry:

I made a simple example project about this problem for you , Please check it :

In the first image without Box , the scene rendered in 00:49 sec
But in the second image just with a simple Box , the scene rendered in 3:00 min !

Maybe this is not problem or bug and it's a logical matter in the Photon Mapping process , If yes ,So how can we decrease our render time in the some special situations that we need to have some objects on the water :!: :?: :?

Wow I just downloaded the file and I'm kinda mindblown about the difference in rendertimes!

I'm gonna try and make it fast…

I also noticed when you try to change the photon target to include all objectsss except for the BOX object it will still generate caustics for that object.

Even in light linking when you exclude the box from the caustic light.

The first one must be a bug I suppose?
Because its not listening to my photon targets.

What do you think Joker?

Yes , I tested Object target and light Masks without any chance !

Finally I had to made a digital asset for solving this problem temporary !

This digital asset render photon map and save it on the disk ,
Then open this pmap in the Houdini and move all of the upper points on the ground and save them to another pmap file.
Finally mantra render the scene via this edited pmap.
I'll upload it later , but I like to find best solution :-)

I would like some dev to chime in about the reasons for this and whether this could be considered a bug.