What is "Convergence"

Sevrin

felix_nukem said:

Old topic, but confusion is left. Let's see:

 

When rendering a 1920x1080 image, the final image begins with completely white noise of 2.073.600 pixels (1920*1080). %0 converged.

Then Iray tries to shoot "rays" from light sources to the objects where your camera is facing. Every hit pixel on the scene is converted from "white noise" to "illuminated" pixel.

Thus, Convergence percent = Illuminated pixel count by rays / white noise pixels (pixels yet to be hit by rays).

Edit: Also a single pixel's proper (subpixel) illumination happens when it is hit by set of rays with different angles. This depends on the "rendering quality" slider.

Due to how surfaces reflects or absorbs light in the scene, the likelyhood of being hit by the rays will be harder/easier.

For example, if the scene has "dim" light(s), it will be harder to illuminate these surfaces where camera is looking. Because most of the light will be absorbed before illuminating pixels on the scene.

Also, the higher the convergence, the longer it will take iray to find and shoot correct angled rays from sources to those white noise pixels.

I hope this makes sense.

Rays originate at the camera.

Dolce Saito

Thanks Richard for additional information. I tried to simplify it and I think I've overdone it a little bit :)

About the start of the render: As soon as a few iterations are done, the image goes from blank to completely noised one. That was my point :)

WendyLuvsCatz

so why do I keep seeing people in facebook groups do renders for up to 2 days past the point of convergence...

Sevrin

WendyLuvsCatz said:

so why do I keep seeing people in facebook groups do renders for up to 2 days past the point of convergence...

It's because they're measuring convergence from the tip of their noses, rather than from the pupil.  Obviously a common mistake.

Gordig

WendyLuvsCatz said:

so why do I keep seeing people in facebook groups do renders for up to 2 days past the point of convergence...

Shouldn't you be asking those people?

NylonGirl

Gordig said:

WendyLuvsCatz said:

so why do I keep seeing people in facebook groups do renders for up to 2 days past the point of convergence...

Shouldn't you be asking those people?

No, because we want a non-stupid answer. 

SpottedKitty

NylonGirl said:

Gordig said:

WendyLuvsCatz said:

so why do I keep seeing people in facebook groups do renders for up to 2 days past the point of convergence...

Shouldn't you be asking those people?

No, because we want a non-stupid answer. 

In short, "convergence limit reached" isn't a measure of "finished" or "looks good". All it means is that the render has stopped improving — whether or not there are still speckles or other imperfections in the image. If the lighting or materials settings aren't quite right, then you'll need a few (or a lot) more render iterations until you get to the point of "looks good". Anything that affects how light bounces around in the scene can affect how quickly this happens.

Richard Haseltine

SpottedKitty said:

NylonGirl said:

Gordig said:

WendyLuvsCatz said:

so why do I keep seeing people in facebook groups do renders for up to 2 days past the point of convergence...

Shouldn't you be asking those people?

No, because we want a non-stupid answer. 

In short, "convergence limit reached" isn't a measure of "finished" or "looks good". All it means is that the render has stopped improving — whether or not there are still speckles or other imperfections in the image. If the lighting or materials settings aren't quite right, then you'll need a few (or a lot) more render iterations until you get to the point of "looks good". Anything that affects how light bounces around in the scene can affect how quickly this happens.

Convergence Ratio Reached just means that target convergence % of the pixels are, to the limit defined by the render quality, converged. It says nothing of how far from convergence the remaining pixels are - it is quite common for small, complex areas (such as eyes) to remain very noisy and require a lot longer to reach an acceptable quality.

Sevrin

If you render different light sources to separate canvases, you'll often notice that some canvases based on weaker light sources are noisier than others, and Iray may report a low level of convergence even if an all-inclusive beauty canvas looks fine.  So my guess is that Iray checks convergence separately for each light source, and, I dunno, maybe averages, best n-1 out of n?  Anyway, generally if the render looks good it is good.

SpottedKitty

Sevrin said:

Anyway, generally if the render looks good it is good.

Agreed. I like to say that "looks good" is the fourth of the three Iray render stop conditions. And possibly the most significant.