No prepasses in a render farm ?

[Macho3001]

Have a couple of questions for you Damien

While rendering (IRmap -3,0; or any min-max rate ranging from very low to very high settings) over a render farm using 7 systems running DRSpawner, the renders, rather than sequentially going through lower to higher samples, go only through prepass 1 of 1 & go on to the rendering phase. I have tried various (higher) settings including oversampling to the rates of 0,4 to check, but the results' the same, only prepass 1 of 1. The problem goes away when i lose the 'farm' and render only on the host machine. It is noteworthy to mention that renders come out the same in both cases, only difference being that the farm does it in 3 minutes as compared to 39 mins for a high res 3076x2304 image.

Q1: Why doesn't the render require additional prepasses when using a render farm;
Q2: Is it normal ?

To crosscheck on a different platform, I rendered with VRay for Max on Max 2009 too with the same result. (By the way, VfSU rendered FASTER than its Max counterpart, both in the standalone & farm renders... cheers !)

In line with your 'involved' responses in all threads (& many thanks for this), a techinical reply from you is solicited & would be highly welcome.

Thanks.

Macho

[dalomar_cg]

Re: No prepasses in a render farm ?

Well since you asked so nicely

The issue is not the the renderfarm is only doing 1 pass, its that its doing all it needs to within 1 pass. The way that the IR process typically works with just the host machine is that it will successively proceed through each of the passes that you have set up (-4 then -3 then -2...). This however, is not a requirement, and V-Ray can calculate all of these passes at the same time if it so chooses (try disabling Multipass and you'll see what I mean).

Bucket Artifacts, and the time it takes to minimize their effect, is the main disadvantage to the single pass method. Bucket artifacts are "inconsistencies" based on a given bucket having minimal scene information around the edge of the bucket. This means that in order to counter act this issue, V-Ray has to load extra scene information and also calculate a little beyond the bucket to make sure that the samples within the current bucket are calculated accurately. When rendering in multiple passes, other than the first pass (which is always the quickest anyway), there is always some information outside the bucket. So V-Ray won't have to go beyond the bucket for very much information at all.

So now that we understand a little bit more about the difference between calculating with a single pass and multiple passes, lets bring DR back into the picture. One of the main slow downs with DR is actually the time it takes to send the scene and information back and forth to the spawners. Therefore, when rendering with DR, we have to minimize the amount of information that is transfered, as that will be a significant factor in the time taken for the render. Multipass will require sending bucket information to the server for each pass, collecting the calculated IR data per pass, and "compiling" the bucket information together per pass. These are issues that we don't have to deal with when its just our computer. Single pass requires those operations be done only once. This minimizes transfer as well as the work that the host computer will have to do. Because of this, single pass is the ONLY* method that will be used when rendering with DR.

So there's your explanation as to why it doesn't require additional passes, but that statement itself is a little misleading. Its all calculated, just differently depending on the situation. And as to whether this is normal...yes, perfectly so.

(By the way, VfSU rendered FASTER than its Max counterpart, both in the standalone & farm renders... cheers !)

Wow...thats pretty surprising. Question though...did you include scene parse time in regards to the times from SU? Many times thats really where are main speed issues are.


*With DR, you can use bucket mode, which renders the IR map and the image for a given bucket all within the same time. This does cut the amount of data transfer in half, but does significantly increase bucket artifacts (reconstructing the IR map is incredibly susceptible to bucket artifacts). This means the edges of the bucket needing even more coverage. Basically, its not highly recommended to use Bucket mode in any situation, so just pretend I didn't even put this side note in here and go back to your naive days of not even knowing about the troubles of bucket mode...how I long for those days.

[thomthom]

Re: No prepasses in a render farm ?

Very interesting. I'd noted that behaviour myself.

[Macho3001]

Re: No prepasses in a render farm ?

Damien

Thanks for the detailed and very informative reply.

(By the way, VfSU rendered FASTER than its Max counterpart, both in the standalone & farm renders... cheers !)

Wow...thats pretty surprising. Question though...did you include scene parse time in regards to the times from SU? Many times thats really where are main speed issues are.

Yes.. I did take into account the initial scene parse time

Macho