Questions about rendering

[Anonymous]

Hello everyone,

I have a few questions to ask in regard to rendering in Maya. I have been using Maya as a student for the past year or so, but I have never played around with the compositing side of things and I can’t seem to get my head around the workflow.

I am trying to render a full 3D scene in Maya with alpha channel, the scene includes a close-up shot of a couple of objects on the table with the ground partial in view. The problem is since each object has its own alpha the channel it gets overlapped by each other resulting in a plain white alpha channel which is useless for compositing?

A lot of the tutorials online for compositing only demonstrate rendering a single object with the passes and alpha channel, my question is……

Q1: If I render each object separately into different passes, they won’t have any influence with nearby objects in the scene I.e reflection/ light bounces from nearby objects and etc because they are hidden when rendering separately. With this method even If I comp each object back together, there will be a slight difference compared to rendering everything together because of the object “influence”. Is that correct?

Q2: Is compositing mostly used for combining real-life footage with CG or with two CG footage or can it be used to edit a single CG footage like grading specific part of an object post-production?

Q3: For full 3D / CG scene do people just use Object ID instead of alpha because of the problem listed above or do people still use alpha for masking?

Q4: How do I specify the colour of object ID in Maya and or with Arnold? For maya, I only see the RGB approach which is limited to 3 channels only. If I use aiUtility in Arnold I can’t seem to change the colour of the object ID?

Thank you in advance.

[sean.heasley]

Hi @Anonymous 

Q1: Sort of. One thing you could do is "hide" the object so it isn't rendered but is still there in the scene and will catch lighting and have influence on objects around it.

Q2: Both

Q3: Also both. It depends on the project and an artist may prefer one over the other but both can be used on the same render. All depends on the look you're going for and how you use each element.

Q4: The aiUtility should work fine. SolidAngle has documentation on this here and this video may help you out as well:

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[Anonymous]

Hi, would you mind elaborate more on question 1 please. I also have another question from playing around with the render passes.

If the object is hidden, why would it still catch lighting from other objects? Or is the object hidden visually but in the renderer all the element is still being rendered as if it is there. Is it some function within maya? I have tried to render a object separately by hiding other nearby objects and it have no other lighting information from the other objects.

Would that reduce render time or it would be the same since it is basically the same as rendering everything together.

In one of my animation, I have a stationary tyre wheel rotating at the same location and I am trying to change the texture of the tyre(i.e logo or stickers) without re-rendering it multiple times for different animation clip. I am not sure if its entirely possible but my theory is to replace the (diffuse) texture of the tyre while keeping/ using the original lighting pass since the object and light source is stationary in terms of translation.

The lighting pass in Maya Arnold seems a lot different compared to the scanline renderer I first used in 3ds max. In scanline renderer I needed to combine diffuse, specular, reflection, lighting and shadow but in Arnold it mostly just need diffuse and specular. The diffuse and specular pass seems to have lighting information on it which is why I am having trouble changing the texture of the tyre while using the same lighting pass(specular pass). The actually Arnold lighting pass I assume is direct and indirect pass but they just look like a beauty pass to me.

Thanks for the object ID! Sorry for the long post. Its just so interesting. 

Thanks

[sean.heasley]

Hi @Anonymous 

Yes you basically summed it up, the object would be hidden visually but in the renderer all the elements will still appear as if it was visible in the scene.

If you're changing something as small as a sticker on a wheel I wouldn't worry too much about the elements of the render changing as they shouldnt be drastic. That said, if it is indeed a noticeable difference that is when you would focus on the compositing aspect to clean it up.