Are render passes always rendered?

Question

It would seem that the answer at least partially no, as rendering a simple cube scene with no passes enabled takes 3 seconds, while with all passes enabled takes 9 seconds.

I always imagined that blender rendered the image and while doing so gathered all the information needed for the various passes, then only saved the passes enabled in Renderlayers > Passes.

Is this the case? If not, what does blender actually do?

I think it increases render time. But that time is probably negligible compared to the actual render time. — Vader, Feb 27 '14 at 22:59

score 4 · Accepted Answer · answered Feb 28 '14 at 16:30

The contents of some render passes is always computed as part of rendering the combined pass. It takes little extra time to allocate the pass and store that value, however for a simple render and many passes this overhead will add up too.

For others (e.g. the Motion Vector pass) it would not normally be computed, so it will take up more time.

score 2 · Answer 2 · answered Feb 28 '14 at 00:56

2

Render in passes or layer will always be slower than render everything in one go. The reasons why one would want to use render by passes include:

Easier to change certain things post-render, without having to re-render the entire image. (This is a huge time saver)
Easier to composite other effects in
Lower memory demand

answered Feb 28 '14 at 00:56

Mike Pan

11,990
1
37
58

Thanks for the useful info :) Though I was mostly hoping for information about what blender actually does and why it's slower. – gandalf3 Feb 28 '14 at 00:59

Are render passes always rendered?

2 Answers2