Region Render issue

[rachelre]

Hi
I am working with V-ray 5 + Sketchup Pro 2020.
I render with Irridiance Map, Light Cache.
When I render a scene everything OK.
Then I try to render a little area in the VFB image with Region Render,
and unfortunately I get a slightly different tone of the color.
It is minor, but it is different.
What is the reason for this ?
How can I fix it?
I attach a before and after examples, and the settings file.
Thanks
Rachel

[konstantin_chaos]

Hi Rachel,

The reason for this difference is most probably the Irradiance Map primary GI engine.
It evaluates the scene lighting only for the visible parts of the scene and is not 100% accurate.
It produces and interpolated light approximation which differs when you change the camera position and even when you choose a different image area to render.

There are multiple possible solutions for this.
One approach would be to increase the quality of the Irradiance map sampling by bumping up the Subdivisions. You can also try increasing the Min Rate value to let's say -2 or even -1.
This will not get rid of the difference completely but will make it less visible.
Another option is to render and save an irradiance map for the whole image and then load it from a file. This way V-Ray will not recalculate the GI for the new region.
However if you change a material or and object position in the set, the pre-saved GI will become inaccurate and in some situations useless.
And last but not least, you can start using Brute Force as a primary GI engine.
This is the recommended (and default) choice. It's slower to calculate but much more accurate, predictable and stable.

Hope that makes sense,
Konstantin

[rachelre]

Hi Konstantin
Thank you very much for your detailed and well explained answer.
I will try your recommendations.
Thanks Rachel