how to speed up rendering with transparent walls?

[stormchaser]

Hi all, I have a scene where all the walls are semi-transparent, the rendering time is huge for each frame, over half an hour, how do i speed it up? is there a way to optimize rendering time in scenes like these? i tried reducing max transp. level in vray but its still taking too long to render. I am using object visibility to control the transparency of objects (instead of material).

I am attaching a sample image from the rendering. Kindly let me know if you have some way of cutting the rendering time significantly.

[Craig Ramsay]

I am using object visibility to control the transparency of objects (instead of material)[/img][/url]

 

Well if you're using V-ray that could be part of your problem. Have you tried using V-ray materials with transparency instead of object visability? I'm pretty sure that would speed up your rendering.

[stormchaser]

well not in this scene, but i made a 9x9x9 array of teapots and test different options. such as object visibility, vray material with transparency, standard material with opacity, but result was still the same with huge rendering time per frame, the only thing that seems to be working is reducing max transp level. but it does not have that big an effect, unless i reduce it too much which makes my scene look bad.

craig you said vray is part of the problem, we all know vray and opacity do not go well together, but would you say there are other rendering engine which handle opacity better? apart from scanline of course. i wouldnt mind using mentalray for this scene if it reduces the rendering time significantly.

regards and thanks for your response.

[BrianKitts]

Take all the objects that you are ghosting with transparency and in their Vray properties, turn off the "visible to GI" option (huge time saver)

 

The best way to do transparent objects like that would be to leave the object visibility at 1 but set the object's material IOR for refraction to 1, and manage the transparency with the refraction value.

 

The reason vray doesn't like transparency is the same as any GI engine..... when the object is partially visible it has to calculate the lighting for rays passing through the object in addition to the rays partially blocked by the object which will double your calculation time. Stack multiple partial transparencies on top of each other and you'll exponentially increase your rendertime to a unworkable lengths.

 

Sometimes when rendering images like this with Vray, it's actually easier (and quicker) to render two images, one with the walls on and one with them off, and then adjust transparency level in post. Gives you more control and saves the time of multiple test renders to get the materials set exactly how you wanted.

Edited March 9, 2010 by BrianKitts

[Crazy Homeless Guy]

I would have to double check, but using the opacity in the Vray material maps rollout might be faster and have more control than adjusting the transparency/translucency value.

 

I might even try messing around with a standard material for something like this. I think adjusting the translucency value is going to pose another set of problems on top of the speed.

[erickdt]

I'd have to agree with Brian's suggestion to use a VRay material with and IOR of 1.0 and play with the refraction value. I often do this for semi-transparent fabric and it works very well and renders very quickly.

 

E

[dylannewell]

You could try turning off the "cast shadows" in the properties and doing a separate shadow pass without transparent walls. Shadows cast by transparent materials normally slow down render time. I'm not that familiar with vray, but I think this should help a little bit.

[stormchaser]

hey guys thanks for your valuable suggestions, i will try to experiment with all the methods mentioned here, i did try the vray transparency one with ior set to 1 but it did not affect the rendering time much, ill look into it again and post my results.

regards

[BrianKitts]

Got my curiousity going so I thought I'd run a couple tests myself since I have been doing an increasing amount of diagrams like this with transparency lately.

 

I setup a scene with 36 teapots and varied the transparency method to test the render time.

 

0m 39.9s - solid objects

1m 59.9s - visibility = 0.5

1m 45.0s - opacity = 128 RGB grey

1m 57.8s - refraction = 128 RGB grey

1m 32.4s - refraction = 128 RGB grey / GI Off

1m 29.3s - refraction = 128 RGB grey / GI Off / Shadow Off

 

 

I've attached a pdf of the resulting renders so you can see the material conditions in the renderings. The one I suggested before although not the fastest is still my favorite as a balance between render time and appearance.

 

It also occured to me while writing up the results that an RGB value of 128 grey isn't a 50% grey in LWF, but it's close enough that I'm not going to bother rerunning the results, it probably wouldn't affect it that much.

[stormchaser]

great test there brian, its of great help. but i must say that there is not much difference in rendering time. sure cutting 30sec in 90sec is an improvement but it is not drastic.

[BrianKitts]

.....cutting 30sec in 90sec is an improvement but it is not drastic.

 

actually it's cutting 30 seconds off of a 2 min render from your original method... (25% increase)

 

so if you think of full scale renderings........ a 2 hour render you just cut a half hour off your rendering time, I would call that significant.

Edited March 10, 2010 by BrianKitts

[erickdt]

I agree that a 25% reduction in rendering times is very significant especially when you consider that aggregate affect that it would have on a multiframe animation. For example: if you were able to make a 25% reduction in individual frame rendering time on an animation that was taking 30 minutes per frame (according to you original post) for a 300 frame animation you would have reduced the overall rendering time for the entire sequence from 9000 minutes (150 hours) to 6750 minutes (112.5 hours). That's a time savings of over a solid day and a half of render time...

 

On animation projects any reduction in individual frame render time can have a dramatic effect on the overall render time for the project no matter how small that render time reduction seemingly is.

 

E

[stormchaser]

yes you guys are right when you say the difference is significant, but let me tell you why i said it is not great. the animation i am rendering, its taking 2hr to render 1 frame, even if it takes 1.5 hours its useless for me since i only got two pcs and it will take forever to render 20secs of animation, without any transparency its taking only 5mins a frame, so if i talk abt in that kind of a scale of time, the improvement is not great.

that said the test brian did is of great use and i think it will help many others in deciding which method to use when it comes to transparency.

[Crazy Homeless Guy]

Maybe you should try a different route. I quickly set this up in Mental Ray. Not perfect but I am getting 3 minutes 20 seconds per 720p frame. This time was on a laptop that is more than 2 years old with a 2ghz dual core cpu in it.

[stormchaser]

Maybe you should try a different route. I quickly set this up in Mental Ray. Not perfect but I am getting 3 minutes 20 seconds per 720p frame. This time was on a laptop that is more than 2 years old with a 2ghz dual core cpu in it.

what kind of different route?

[Crazy Homeless Guy]

what kind of different route?

 

Using a different render engine, the test above was done with Mental Ray, instead of Vray.

[stormchaser]

can you do me a favor? can you share that scene? id like to test it in vray and compare the render time.

[Crazy Homeless Guy]

can you do me a favor? can you share that scene? id like to test it in vray and compare the render time.

 

 

I can upload it tonight. It is on a different computer.

[Crazy Homeless Guy]

Here is the Mental Ray scene....

[stormchaser]

thanks travis, I will do some tests on your scene.

regards