Rendering alpha of a single object

Wintersun

Hello There.
I've encountered a problem hoping you guys can help me.
I have to render a 360 view of a car with 8 different colors. and I want to render the color changing part separately so I can render static (in the means of color) objects only once. So the best thing came to my mind was to use the invisible to camera check and use the alpha of the car body, the problem is in this method alpha doesn't consider order of objects and I yet get the other side of the car body in alpha which in a beauty pass should not be seen. I dunno if I make myself clear enough. I need a method to render beauty and alpha pass of a single object yet have the effect of the surrounding objects on it as well.
Sorry can't tell it better.

tadpole3159

What software and rendering plug in are you using? In 3ds max you have an object ID. Just give your car paint one ID and everything else another and use that as a mask
http://youtu.be/oSdYyCO8Gyg
Put a hue and sat slider on your diffuse pass and bingo, one multicoloured car

Wintersun

@tadpole3159 Well That was helpful, thank you. I'm using 3ds max and vray, Ok I have to ask, I'm rendering Alpha, Diffuse, reflection, specular and GI passes. You think I should add any other one? and the other thing is different color will have different GI pass, I mean depending on the color, What do you suggest? Thank you very much I'm gonna try the video at home in about 2 or 3 hours after I get off from here.

Eric Chadwick

Look up Matte properties in V-Ray, this will probably allow you to do what want to do. You access it through the V-Ray Object Properties when you right-click on an object.
http://docs.chaosgroup.com/display/VRAY3/Object+and+Light+Properties

Also check out Render Mask
http://docs.chaosgroup.com/display/VRAY3/Render+Mask

Wintersun

Thank you. Wow I tired The render mask feature and it is amazing, but my problem is it's a 360 view, It has to put every frame on a specific vfb. I mean the angle changes each frame how can I tell it to for example save 24 frames camera rotation now put the rendermask on its own specific angle that it belongs.

Eric Chadwick

Render out each pass as its own sequence of images. Then composite them together.

Best thing would be to use After Effects or some other video compositor. If you don't have those, then you can use 3ds Max's material editor to do this, it's just a bit clunkier.

In the Compact material editor, make a Composite map, and load each sequence in there as layers, then set their blending modes. If you right-click on the sample sphere, you can choose Render Map to output the composite result...
http://help.autodesk.com/view/3DSMAX/2016/ENU/?guid=GUID-2252D417-CBB3-46E5-B305-9F0167A86A48

tadpole3159

Your going to have to composite everything together in after effects. Render out the passes in this video and you should be set. Don't forget to add in your object ID mask as well.
http://youtu.be/6n4EwVrqIg0
As for GI colour I would render the car as a mid grey colour. That way you won't gett any mismatching colours spreading around in your render after you change colour

Wintersun

Ok Thanks to both of you, I did render the car body separately from rest of the stuff with an alpha of -1 for the car and +1 for the rest of stuff! now the problem is I see a blacky edge around my object like alpha is one pixel short the real object, is i because I didn't use exr format? is there any other way to fix it? Thanks in advance.

Eric Chadwick

That's most likely the fault of your compositing software. What are you using? The alpha needs to be premultiplied.