technical render question

[zacktown]

Hi all,

 

One of the questions that has always been in the back of my brain over the years has to do with rendering the inside of an object. I will attempt to explain.

 

To simplify, if one renders a reflective surface, say brushed metal, on a cylinder in a simple HDRI environment, it will render pretty quickly (5 or 6 seconds). Now, remove the top cap of the cylinder, put the camera inside and render (30+ seconds). Rendertimes will increase, in my situation, 5x. Sometimes more depending on various factors.

 

I've always simply accepted that "it is what it is". Does anyone know why this is? It's not just vRay, it happpens with all renders that I have used over the years.

 

Any thoughts?

[Francisco Penaloza]

Could you post an image of your scene?? and what's the point of this question?? I mean what the problem you are trying to resolve.

 

If you render a geometry from the inside and that mesh is not solid (double faces/ single face) you'll have Normals facing the wrong way, so your software will not render or it will shade it wrong.

 

If you are looking a chrome cylinder, and render the scene, then the antialiasing will work very little because everything else is empty, just an image in the background.

if you are inside the cylinder, then the software needs to sample the whole scene or what the camera sees, in this case with blurry reflections and glossiness, so of obviously it will take longer to render.

[zacktown]

Hello Francisco,

 

You pretty much understood what my question was. Thank you for the reply and you confirmed what I already suspected. When looking inside and rendering, the glossy reflections have much more to calculate. Depending on the reflection bounces, it seems like it renders a glossy of a glossy, of a glossy... up until the set levels of reflection. I think it would be cool if a cheat was offered for situations like this where accuracy is not important. For now, I am blurring a crisp reflection pass in post. and comping.

[guenther Malek]

Sorry, this is just bla-bla.

 

Vray has double-sided faces by default and Vray material is double-sided by default (unless turned off)

so in- or outside the cylinder does NOT cause shading problems.

image1 - Outside of the cylinder:

the environment casts light upon your cylinder, the cylinder reflects the evironment 1 time,

all the pixels around the object don´t really need much calculation because they just represent the environment image > done in 1,3 sec.

image2 - Outside of the cylinder-Removed cap:

Like image1 + a few light rays enter the cylinder, get bounced back and forth (by default 5 times), reflections too > done in 4,2 sec.

image3 - Inside of the cylinder:

Almost every pixel of the rendered image needs to be sampled several times because of light being bounced around inside the cylinder, blurred reflections all over the image > BAM! 25,3 sec.

 

 

This is why Interior renderings need so much more time to render clearly than Exterior shots.

Image2 is a very good example to show the difference between exterior and interior...

Edited July 13, 2017 by guenthermalek

[ipixelrenders]

There is a more complex gi sceneraio inside the cilynder. Gi calculations are computationally expensive.

[Francisco Penaloza]

Sorry, this is just bla-bla.

 

Vray has double-sided faces by defaultand Vray material is double-sided by default (unless you turn it off)

so in- or outside the cylinder does NOT cause shading problems.

image1 - Outside of the cylinder:

the environment casts light upon your cylinder, the cylinder reflects the evironment 1 time,

all the pixels around the object don´t really need much calculation because they just represent the environment image > done in 1,3 sec.

[ATTACH=CONFIG]55606[/ATTACH]

image2 - Outside of the cylinder-Removed cap:

Like image1 + a few light rays enter the cylinder, get bounced back and forth (by default 5 times), reflections too > done in 4,2 sec.

[ATTACH=CONFIG]55607[/ATTACH]

image3 - Inside of the cylinder:

Almost every pixel of the rendered image needs to be sampled several times because of light being bounced around inside the cylinder, blurred reflections all over the image > BAM! 25,3 sec.

[ATTACH=CONFIG]55608[/ATTACH]

 

Well not really, render a single side plane with a glass material, then render a double side mesh with a glass material, you won't get the same results. Also, other shaders from VRay are affected by normal. The double side check box will indeed flip the normal but it won't create the same result as a real 2 faces mesh. It is designed to help with paper or fabric-like objects.

 

He also mentioned that "it happpens with all renders that I have used over the years." and that not true because each render engine has it own approach to materials. as we all know it.

 

The rest you explained applies correctly.

[guenther Malek]

There is a more complex gi sceneraio inside the cilynder. ......

Wow! (-;

[guenther Malek]

Well not really, render a single side plane with a glass material, then render a double side mesh with a glass material, you won't get the same results. Also, other shaders from VRay are affected by normal. The double side check box will indeed flip the normal but it won't create the same result as a real 2 faces mesh. It is designed to help with paper or fabric-like objects.

 

 

He also mentioned that "it happpens with all renders that I have used over the years." and that not true because each render engine has it own approach to materials. as we all know it.

 

 

The rest you explained applies correctly.

 

 

i take back the "bla-bla"!

You are right with what you say but it seems to me you are mixing up double-sided and double-faced.

 

In my rendered examples i use a basic object that has no thickness (single-faced),

but since Vray Material is double-sided by default and my material is completely opaque

it renders without problem inside and outside of the cylinder.

 

when it comes to Glass or translucent materials, as you mentioned, it matters if an object has a thickness or not.

Glass needs to be double-faced with a certain thickness to render correct refractions

 

BUT as soon as the object is double-faced, double-sided does not matter anymore because what represents the material are the outer faces

 

The "2-sided" checkbox does not actually "flip" the normals, but it applies the material

on both sides of the the single-faced object (like in my rendered example)

 

i will attach a simple sketch that shows the double-faced/sided thing...

 

 

!!! Of course, there are differences between render engines but the overall the problem stays the same: interior renderings need much more time to render than exteriors !!!

[notamondayfan]

Sorry, this is just bla-bla.

 

Vray has double-sided faces by default and Vray material is double-sided by default (unless turned off)

so in- or outside the cylinder does NOT cause shading problems.

image1 - Outside of the cylinder:

the environment casts light upon your cylinder, the cylinder reflects the evironment 1 time,

all the pixels around the object don´t really need much calculation because they just represent the environment image > done in 1,3 sec.

[ATTACH=CONFIG]55606[/ATTACH]

image2 - Outside of the cylinder-Removed cap:

Like image1 + a few light rays enter the cylinder, get bounced back and forth (by default 5 times), reflections too > done in 4,2 sec.

[ATTACH=CONFIG]55607[/ATTACH]

image3 - Inside of the cylinder:

Almost every pixel of the rendered image needs to be sampled several times because of light being bounced around inside the cylinder, blurred reflections all over the image > BAM! 25,3 sec.

[ATTACH=CONFIG]55608[/ATTACH]

 

 

This is why Interior renderings need so much more time to render clearly than Exterior shots.

Image2 is a very good example to show the difference between exterior and interior...

 

forget GI, 2 sided materials and all that crap, the difference between image 2 and 3 is the CAMERA ANGLE. In shot 2, approx 25% of the image is the cylinder, where as image 3 it's more like 95%.

 

You're right about interiors taking more time than exteriors, exteriors can have the majority of the shot empty (sky)!

 

As for the difference between images 1 and 2, it's never a good idea to have backfacing normals. What happens if you add a shell modifer to the open object? What is the difference in time?

[Francisco Penaloza]

i take back the "bla-bla"!

You are right with what you say but it seems to me you are mixing up double-sided and double-faced.

 

In my rendered examples i use a basic object that has no thickness (single-faced),

but since Vray Material is double-sided by default and my material is completely opaque

it renders without problem inside and outside of the cylinder.

 

when it comes to Glass or translucent materials, as you mentioned, it matters if an object has a thickness or not.

Glass needs to be double-faced with a certain thickness to render correct refractions

 

BUT as soon as the object is double-faced, double-sided does not matter anymore because what represents the material are the outer faces

 

The "2-sided" checkbox does not actually "flip" the normals, but it applies the material

on both sides of the the single-faced object (like in my rendered example)

 

i will attach a simple sketch that shows the double-faced/sided thing...[ATTACH=CONFIG]55622[/ATTACH]

 

 

!!! Of course, there are differences between render engines but the overall the problem stays the same: interior renderings need much more time to render than exteriors !!!

 

Maybe I should explain better in my first post, @Guether what you write about longer time to calculating because the enclosed sample is correct, no problem with that, when you are in a enclose scene you need to sample the whole thing, as mentioned by dean, not only reflections but glossiness, GI, and everything else, and in this mere case scenario not only the first reflection/GI but also the already blurry reflected lights, reflection that bounces a few times inside this object. So it takes longer to render.

 

Regarding the double side and double faces thing, well here is a little bla bla from Chaos Group

 

And my whole point here is you can't generalize with some functions or properties in this CG environment because it will always vary depending on the software and your scene.

 

In this mere case since it is V-Ray and only if the material is opaque, the Double -sided check box will do the trick. But this does not apply to other render engines, as mentioned suggested Mike at the beginning.

 

The Double-side check box is not a magic click that will fix all your problems because when the material changes properties the results will vary.

and that's my point here, Trying to help @Mike T to not think that only clicking that check box will fix all his problems in the future.

[Tom Sproule]

I agree with Francisco. Just have to add that also we've found that light calculations can increase dramatically when there are objects with holes in them. Have you ever noticed that the light cache is calculating, but you get an incredibly long time estimate to calculate or you get no estimate for a while at all, and then it suddenly changes the amount of time and calculates it quickly. We've done tests and found that the light calculations can be massively decreased in time by ensuring all objects are capped or shelled and there are no open faces. The problem seems to be light bouncing around inside objects. Vray just has a problem calculating the light bouncing on the other side of the normals. So, V-Ray is trying to calculate that, but it doesn't always work. Also, open faces can crash max and vray at render time, not normally, but occasionally.

 

An example of increased render times would be if you are trying to render a room and all of the walls have the tops and bottoms removed. Then at render time the light will go into the walls and other objects and bounce around on the inside. That's just additional polys it's trying to calculate light for. So, it adds time.

 

I'm also not convinced on that all V-Ray materials being double sided by default actually works all the time. I've had to plug the vray material into a Vray 2 sided material on numerous occasions and turn the opacity to black in order to get the render on the back side of the faces to look the same as the front side (normally this is when working with meshes imported from other 3D software) but at time also applies to meshes modelled in max.

[guenther Malek]

i never claimed, "double-sided" is the magic click. it only simulates a "thickness"

I said double-sided only works for opaque materials (unless you want to make paper.etc.)

 

But this was not the main issue.

It was only about inside and outside.

You are right, Tom, what you say about open polygons is true for the reasons i explained in my first post...

It was not about single-or double-sided...

 

(Indeed! Chaosgroup says "double-sided" "...flips the normals..." which is not correct!)