SU Podium Forum
Sign up  |   |   |  Calendar  |  Latest Topics
 
 
 


Reply
  Author   Comment  
drawovis

Avatar / Picture

Registered:
Posts: 125
Reply with quote  #1 
Working on a large model of a industrial complex, I ran into some blotches on my buildings.
terrein-totaal-2018-solid 2018-05-30 21031400000.jpg 
 
A QMC2.0 preset render takes care of the blotches, but takes a long time...
terrein-totaal-2018-solid 2018-05-30 21513900000.jpg 

Remembering Nick's tutorial on Interior renders, I checked my model on reverse faces. But all the faces of the building were turned to front.

building.jpg 
So I checked the whole model again with the Monochrome display: then I saw that the terrain face was reversed..

reverse-faces.jpg

So fixed that and tried a new render:

  terrein-totaal-2018-solid 2018-05-30 21100000000.jpg 
And behold... the problem is gone. So a reversed face in an exterior model can also cause blotches.


bigstick

Avatar / Picture

Moderator
Registered:
Posts: 10,665
Reply with quote  #2 
Hmm - interesting!

Blotches however don't manifest themselves in the same place every time because they are caused by insufficient samples and every single render is different.
The first suggestion with blotches is always to rerender. This may cause a better distribution of samples that avoids the error.

However - it's difficult to argue with the principle of always orientating your faces correctly. Using extruded geometry tends to fix this, and avoids the blotches you get at connected edges with single plane geometry.



__________________

That which does not kill us makes us stronger
-Friedrich Nietzsche

drawovis

Avatar / Picture

Registered:
Posts: 125
Reply with quote  #3 
Here's some more renders with the reversed face on the ground.
blotches.jpg 

The blotches are every render different.
Could it be that the size of the model has some influence? This is a 194Mb model of a complete industrial area.
terrein-totaal-2018-solid 2018-06-03 11325000000.jpg 

When I strip down the model, the problem seems to be not so big.

terrein-totaal-2018-solid-less 2018-06-03 12040600000.jpg

And with face front it seems to be gone all together.
blotch-nieuwterreinparkeren 2018-06-03facefront.jpg 
 



bigstick

Avatar / Picture

Moderator
Registered:
Posts: 10,665
Reply with quote  #4 
It makes sense that the problem is worse with larger models.

Because the problem is caused by not enough samples in the model (the scene is divided into a series of individual sampled cells with the same irradiance values, the number is finite and determined in the presets), the more faces there are, the fewer samples any particular part of the model receives. 

Sampling is different in every render, because the algorithm uses what is called a Russian Roulette method of assigning samples, which is effectively a randomised starting point for each set of samples, which are assigned according to a particular set of rules.

As to why reversed faces are more likely to create sampling errors - this baffles me. Your tests seem to indicate that this is in fact the case.

Interesting!

These were really useful tests, thanks a lot for this 😉



__________________

That which does not kill us makes us stronger
-Friedrich Nietzsche

Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.