No Convergence, but very high iterations
kickflipsonly
Posts: 29
I'm new, but trying to understand a weird issue I'm having.
Whenever I try to render an image, as time progresses my Iray iterations will climb into the thousands. And yet, when I check the render progress, it reports that I'm at 0%.
Is it normal to have 3000+ iterations with no convergence/progress on the render?
Here's some background information - I'm doing a character render (lights, camera, character, nothing else). Settings = very high max sample/max time, rendering quality of 50, rendering converged ratio is 99%. I've done character renders in the past with mostly the same settings and had no issues.
Again, I'm new, so maybe I just don't understand what's going on here. Any help would be appreciated.
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Comments
It's the rendering quality - that determines how exacting Iray is about deciding when a pixel is converged, they say that it increases roughly linearly (so you render is taking ~50 times as long if the quality was still at 1). I would try a much more modest increase over the default if you find you need a higher value.
The quality in Iray comes from when the light paths have been calculated. This means that a render is only pretty much done relative to the complexity of your scene. This also means the settings are pretty much useless, in the sense that none of these numbers represent a "done" render. Iray really doesn't know if a render is done. It only knows if it has calculated all of the light paths.
So it pretty much depends on the number and type of lights, the amount of corners and walls, the amounts of reflection and so on how long it takes to look good. A lone figure in an hdri environment takes around 300 iterations in a couple of minutes. A couple of figures in a complex room, with many different lightsources and many corners and walls and a lot of propers, might take a lot longer to look good. In time you learn to estimate the complexity and the render time involved. Like a photographer knowing what exposure he needs to shoot a scene.
The settings therefore are mostly useful in the sense of setting some hard boundaries to the amount of time, iterations, or convergence level you want.
Rendering quality of 50 is a foolish setting. A rendering quality of 1.00 will give excellent renders if you have decent lighting.
All renders in my gallery are at Rendering quality 1. Here's an example at Quality = 1
Personally, I turn time to zero, and quality off, crank up the samples by quite a few zeros, and just stop it when it looks good enough to me. Outdoor lit with hdri is usually done around 3-5k samples, indoor I have sometimes needed to go as high as 50k+ samples until I was happy with it.
I CPU render on aan AMD desktop and set my
Render Quality Enable to ON
Render Quality to 1.0
Render Quality Convergence to 95.0% (If you CPU render setting this above 95% is like punishing yourself - you won't notice huge changes in render quality but the time to render will shoot up dramatically). 50% or 75% I've never tried but if you have and can't see a difference, at least a big difference, and it saves you render time, go for it.
All of the above are defaults; then I set
Maximum Time I change from 7200 (2 hours) to 0 (unlimited)
Maximum Samples (aka Maximum Iterations) I change from 2000 or 5000 to 15000. However, most FHD (1920x1080) will finish in under 2000 iterations. If you have a scene with lots of lights and lots of glass, metal, liquids, and such it could take over 15000 and you still don't get a good converge according to iRay but your eyes see no discernable change either.
The other render settings are mostly to change the visual qualities of your render to some other than clear, crisp focused iRay render, eg Bloom Filter, Guassian, Mitchell, and other radial blur filters (higher numbers are more blurring), Vignetting and so on.
I never use Quality. I render to a set number of itterations based on lighting and the presence of hair. Then I run it through a denoiser and that's that.
FWIW, I never use a Quality value over 4 (stuck using CPU Iray renders for now, so it takes ages) and the final image usually sharpens up nicely over using the default values.
Never mind what percentage it's showing, what does the image actually look like? If it looks finished to the mark 1 eyeball, it is finished. What else are you going to look at it with?
That's a good point I think many of us long-time users have had to discover for ourselves. I've had scenes that resolved adequately in less than 100 iterations, and others that were still obviously nowhere near it after chugging along for the default two hour time limit. Everything depends on exactly what's in the scene, the materials, and the lighting.
There is a way to cheat on the iterations, set quality to 0.0 and time to next week, set the iterations to say 10. Render and see what it's like, if you change the iterations (don't click the scene) you can 'resume' the render and add extra iterations till it's nice. You can control the batch mode of interactive with that as well.
Is it normal that in my render it went pretty quickly to about 14% of convergence and after that, even though iterations keep going, the convergence didn't move up at all? Since it hit 14% it is rendereing for more time and interations that were needed for hiting the 14% and it still didn't go up at all.
It cerrtainly isn't impossible. A pixel is converged, as I understand it, when it reaches a stable value (the narrowness of the margin to count as stable is determinmed by the Render Quality slider). So yes, it may be that 14% of your image is getting plenty of direct light and so reaches a stable state quickly, while the rest is either receiving more indirect light (which takes more iterations to accumulate) or has complext interatcions with the light (scattering, transmission, reflection) which are much more sensitive to the enagle of the incomiong light ray and so, again, take longer to settle.