Author Topic: Grainy image from Corona Sun  (Read 887 times)

2023-01-12, 07:42:08

cor

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Happy New Year everyone!

To make it easy to understand I recorded this short video (https://youtu.be/gF8eJ5H_HnE) of what is happening. Basically, we have an interior scene with some sunlight casting into the space. When we play with the light mixer in some combination we see grainy areas right where the sunlight is casted. What are we doing wrong or what setting are we missing. The denoising is helping a bit but it seems unusual that this graininess is showing up only at the sun lit areas?

2023-01-12, 10:24:12
Reply #1

romullus

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Hi,

This is expected behaviour. Lightmix is not designed to support extreme changes in light intensity. You can read more about it here (particularly "what are the limitations?" section): https://support.chaos.com/hc/en-us/articles/4528606952849-How-to-use-interactive-LightMix-in-Corona-for-3ds-Max-
I'm not Corona Team member. Everything i say, is my personal opinion only.
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2023-01-12, 13:07:51
Reply #2

TomG

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Corona is adaptive in terms of how it samples lights, giving priority to lights that have the most impact on a particular area. This means the sun takes priority, being the brightest, so other lights in that area where the sunlight land get less processing/attention. This means if you remove the sunlight, you will find the area noisy underneath because the rest of the lights weren't given as high a priority.

If you intend to use LightMix to truly control the end result, it's often best to make all lights a similar or identical intensity (including the sun), and set them to white, to give maximum control in the LightMix result.
Tom Grimes | chaos-corona.com
Product Manager | contact us

2023-01-12, 18:54:02
Reply #3

cor

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Wow, thank you so much for this information! We were pulling out our hair for weeks before we figured to reach out for help. We will run a test and set all lights to the same white intensity and see what happens when we change the intensity and color in the LightMix. Have a fabulous week!

2023-01-12, 19:28:49
Reply #4

TomG

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Hope the test goes well! To avoid further frustration, remember that there are still limits on what you can do even with all lights the same color and intensity; changing things by too much (either brighter or dimmer) can still result in noise and so need more time/passes/stronger denoising. Exact results are scene dependent, but LightMix does have its limits (once you get into things like making something brighter by a factor of 10 or more, as a very very rough guideline!) It should give you the widest range of changes you can make though, if you set the scene up that way.
Tom Grimes | chaos-corona.com
Product Manager | contact us

2023-01-20, 14:43:46
Reply #5

cor

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Thank you Tom. I have a quick follow-up question: We got these weird artifacts along the perimeter of the window where we have the background image, see attached. Is this because we tinkered with the LightMix while it was rendering in the background or was it because we stopped the rendering, allowed it to finish denoising and then continued rendering from file at a later time? What causes this?

2023-01-20, 18:40:55
Reply #6

TomG

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To be honest, not seen anything like that before. Changing LightMix while rendering shouldn't have made any difference. The second case of restarting a render sounds more likely - which denoiser did you use, Corona HQ (where I wouldn't expect this) or one of the AI ones? That background HDRI also looks surprisingly grainy, is there anything odd about it? Was it a straight HDRI used as env and seen directly, or was one HDRI or Sun/Sky used for the Env, and another for Direct Visibility?
Tom Grimes | chaos-corona.com
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2023-01-23, 20:50:10
Reply #7

cor

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We use Corona HQ denoiser. The background is a simple image from our 360 panorama (https://3600.s3.us-east-2.amazonaws.com/Melrose/2022-03-04%2017.34.36.jpg).  The image quality is bad but we used it so that the client can feel his own environment. We are fine with it not being sharp but that outline is weird. We put it on a 'light' plane to give the light. We have a separate sun with environment. This is our approach in this project. And we did not succeed with creating HDRI background from our own background picture.

2023-01-27, 20:00:24
Reply #8

cor

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Tom, just checking in, please see the previous response regarding:
which denoiser did you use, Corona HQ (where I wouldn't expect this) or one of the AI ones? That background HDRI also looks surprisingly grainy, is there anything odd about it? Was it a straight HDRI used as env and seen directly, or was one HDRI or Sun/Sky used for the Env, and another for Direct Visibility?

What could we be missing here?

2023-01-27, 20:05:07
Reply #9

TomG

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Sorry, not sure - sounds like the scene might need to be sent in, unless anyone on the support team has seen something like this before. Only thing I am aware of is that putting an image on a light plane and using it as a main light source (actually emitting light) is generally a bad idea for performance, but not sure it would have anything to do with these artefacts. It would be better to use a plane with a material set NOT to emit light as the visible background (and shadowcasting off) so that it does not contribute any lighting to the scene, with the environment being the sole source of lighting being cast into the room. But I don't see how that would affect the artefacts :)
Tom Grimes | chaos-corona.com
Product Manager | contact us

2023-01-27, 20:07:01
Reply #10

cor

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I see.. We will run some tests. Have a great weekend!