Iray allows light to bounce around a scene when creating an image. I made a test setup for looking at adjusting the Max Path Length setting under the Optimization tab in the render settings. The light can't get from the left sphere (the only light emitter) to the right sphere, unless it takes an indirect path. The Environment is set to Scene Only and Ground off. The camera headlamp is set off.
The default setting for Max Path Length is "-1" which from what I can tell, allows for the maximum amount of light ray bounces. In my test images, it made for the most light wrapping around the block to get to the other side.
Max Path Length of 1 results in an all black image, except for the all white sphere on the left.
Max Path Length of 3 shows some light wrapping around, but is is clearly limited in being able to get around the block.
Max Path Length of 5 and above shows a result similar to the result for "-1" which is shown in the image. There are a few more light rays that get around with a higher MPL, but it is subtle, and you wouldn't notice it without being able to flip between the images.
So, what sort of performance hit does increasing the ray bounces cause? In 3Delight, there is a huge performance hit for increasing the number of reflections from reflections.
I waited for Iray to reach a number of iterations higher than 300. I recorded the number of iterations, and the time it took (all images were over 5 minutes to render). Then I calculated the number of iterations per second (higher is faster).
MPL .... iterations/second ---------- quality
1 ........... super fast ----------- very very bad 3 ........... 1.02 ------------------ decent 5 ........... 0.97 ------------------ good 7 ........... 0.97 ------------------ good 9 ........... 0.96 ------------------ good 11 .......... 0.98 ------------------ good -1 .......... 0.97 ------------------ good (the best quality)
Now, there is some error in my measurement, due to trying to read the iteration number, and time at the same time, and I did do a bit of web surfing while waiting for the renders, so really anything with MPL of 5 or higher took essentially the same amount of time. And was only about 5% slower than the inferior MPL 3 image.
Now this scene is very simple, and there may be more of a performance hit for a more complex scene, but I would recommend leaving Max Path Length at the default setting of -1 which gives the best image quality, and surprisingly is not noticeably slower than the other settings.
Next time you buy a graphics card, whenever that time may be, get a Nvidia card to take advantage of Iray. Nvidia made Iray, so they make Iray for free, but then you need their cards to fully utilize it.
You can get results with just CPU, but it takes a while, or takes some tricks.
Hey, do you know any of those tricks? I've been struggling lately with optimizing my iray renders. I've never used this engine before and i don't have nvidia card so my renders take literally ages. If you know some good tips to lower the time of render or know some site with such tricks, could you share it with us?
One trick is to render the image much larger, and then use post work to scale it back down. So if you want a 1500x1500 image, render it at 3000x3000. It will be grainy, but in postwork give it a 1 pixel radius Gaussian blur, and then scale it down to 1500x1500.
Another trick is to render the whole scene to OK quality. Then go back and use the spot render tool (set to New Window in tool settings) to render the important parts at a higher quality. Since it is working on a smaller area, it goes a lot faster.
Brightly lit scenes with light coming from multiple directions will render to a higher quality faster.
Finally, just accept that your work is going to have graininess to it, cause you don't have the Nvidia card.
The material is set to not reflect, but it does scatter the light that hits it. The scene is symmetrical, except for one sphere is set to emit light, and the camera is at an angle to show the geometry.
I was going to show a bunch of images showing how the number of light bounces affects the image, but for images with 5+ bounces they were almost identical. And then, because all the renders for 5 bounces or more rendered in essentially the same amount of time, I recommended using the 'unlimited' bounce setting, as that gives the best quality without a performance hit.