Renders Vary Greatly at Different Resolutions

[MattFink]

When working on sandblasted material I noticed that my 1k and 4k square renders had a huge variance in appearance. I found that it is the bump in the material. At 1k it does not seem to consider it, at 4k it is spreading the light how I would expect it. This has been an ongoing problem (I have seen this in at least since Vray 3.4 in Maya 2015), currently working in Maya 2018 with Vray 3.60.03. We've seen this repeatedly across multiple projects over a few year, usually fixing it by relying more on Reflective Glossiness than bump. But for certain materials the bump is paramount.

At our studio all our client renders are required to be 4k square, we light the images while IPRing at 1k so this is an issue, we need our IPRs to match closer to the final image. This also is an issue when we move to animations that are usually 1080, again resulting in less light spread on specific materials than desired.

The material in question uses a Maya Leather procedural set to a very small size to create the surface grain, with the bump multiplier at .002.

I've done some tests to see when these variations occur. If I use Maya's post scale on the camera and reduce the size to 25% the size of the subject in the render matches the 1k render but the lighting/material matches the 4k render. Alternatively, if I zoom out the camera to reduce its pixel space to 25% the lighting/material matches the 1k render. I have tested this with a Normal Map instead of a procedural map as a bump and found that the same issue does not occur, it renders the same at every resolution.

I'd appreciate any insight into this.

Thanks!

[joconnell]

Give the car paint flakes map a go for this, it's hard for renderers to distinguish between noise and things it should filter versus texture detail. It's the same problem with the filtering of pore maps on character skin as it gets further from camera too!

[MattFink]

I see what you're saying for metals with very fine surface texture, but that solution wouldn't work in all scenarios, such as when we have to use MT (mold tech) plastics. This occurs across the board when using Maya procedurals as a bump. I'm not sure noise filtering can be blamed for the massive discrepancy in lighting. The there are areas that get more than 10% brighter on the higher resolution image. On 1k renders you'll have round areas with deep, dark low lights, but on 4k they'll be washed away completely as the bump starts to be considered in the spread. I've attached a quick IPR comparison to illustrate this.

[joconnell]

Well if you're using a bump, it's making the surface look at different directions of light and you're going to get glints from some parts of the surface facing more towards a light and getting more highlight burn and other areas looking more away from the light an getting no highlight. As the camera moves away, the fine detail averages out into more similar colours, so less variation and likely less parts of the object turned towards the light. As a test it'd be interesting to put your bump map into a vray extratex map and have a look at the results both at 4k and 1k - I suspect it all comes down to the fine detail being averaged out. In a 4k render you'll have 16 different pixels / possible directions for the surface to turn towards or away from the light with your bump, all of those get averaged down to just 1 pixels at 1k. Is the size of one of those grains in the paint big enough to take up a whole pixel by itself or are a few grains stuffed into the space of a single pixel?

[MattFink]

That's exactly the problem I was describing, it's not considering sub-pixel surface information correctly when using procedural maps in Maya as a bump (height). If I switch to a texture normal map I get identical results no matter what size I render the image. The short of it, if I render an image at 1000x1000 and again at 4000x4000 and shrink the 4000x4000 to 1k in photoshop they should look the same, at least extremely close. I encourage you to open both of the renders I attached in the original post and compare them. The ONLY different between them was that one was rendered 1024x1024 and the other was rendered 4096x4096 yet they display a massive, unacceptable difference in light even on large flat areas.

What let's me know something is going wrong is that I can get the 1k render to match the 4k by increasing the bump multiplier 4x (.002 to .008 in this case)

[Muhammed_Hamed]

I think using the bump delta scale option, you will be able to match your renders between 4k and 1k
You will need to change it based on your resolution, to make sure your results are matching.
I've heard that Arnold has this behavior as well, it is meant to make bump mapping render faster at some situations.
Working on high res, I tend to use render region, so that I'm 100% sure what I see is what I will get at final rendering. I barely scale down the resolution for IPR
This is also the case with normal maps and VRscans. I really hope that this could be improved somehow, like linking the bump delta scale to the render res ,so I don't need to do this manually

[vlado]

We are doing some research on this, but in the meantime using the bump delta scale as suggested above should work. Or alternatively, you could use a normal map with filtering disabled.

Best regards,
Vlado

[MattFink]

We are doing some research on this, but in the meantime using the bump delta scale as suggested above should work. Or alternatively, you could use a normal map with filtering disabled.

Best regards,
Vlado

Thanks for the response, I'm glad to hear this is considered a known issue. We've been using normal maps for now but trying to avoid unnecessary external files. Both the bump scale delta and the bump multiplier work-arounds get results we want, with that we can use the global bump mult. to address it. I might be able to script something though to automate it until there is a more thorough solution.

[Muhammed_Hamed]

Is there a global bump delta scale in Vray settings?
That would be very helpful, instead of manually adjusting it for each material whenever I change the render resolution.

[^Lele^]

Is there a global bump delta scale in Vray settings?
That would be very helpful, instead of manually adjusting it for each material whenever I change the render resolution.

Not sure this would work, as the bump scale needed is dependent on the screen-space size for each shader, and bump map type and settings.

[1funk]

Just for reference, I posted about this issue a while ago in the modo subforum, and was asking if there was any progress
https://forums.chaosgroup.com/forum/...der-resolution