Rendering Image plane with no Alpha

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  10 October 2012
Rendering Image plane with no Alpha

What am I missing here!? Trying to output something simple has got me stumped!

I wish to render my backplate behind my CG into RGB and want to have the image plane contribute 0 to the alpha. I cant get it. I want to render over the backplate as its for my VRay materialwrapper's RGB shadows. If i turn off the image plane I will end up capturing shadow detail over an env map instead.

To confuse matters - In my VFB I get colour information of my shadows and GI in my alpha channel, which would mean I dont need to render over the backplate at all (its the shadow colour that im trying to preserve). Unfortuinately I dont have a compositor handy right now but pretty sure saving this out will strip it down to one channel though. Never seen something called alpha contain colour?!

I can get what im after if I use projection on geo card - buts its just a pain for something I just want my image plane to take care of.

Can anyone help shed some light on this stuff. Im positive im being stupid and missing something.

Ive whipped up a quick little pic showing what i mean - exaggerating the shadow colour using a strong tinted GI.

Thanks in advance
"Yeap, im certain - I spent 50hrs making sure it was a cow"
  10 October 2012
Did you try to put a plane with "useBackground" shader between your objects and the image plane? This at least should you help to set the image planes alpha to zero.
  10 October 2012
Yep that would do it. Similar process as projecting on a plane and using matte opacity. Pretty hacky really.

I have made progress though. If I delete the imageplane all together - I can map my image in the background slot of the environment section in the render globals. This is where i got stuck last time with mapping it to the gate but just realised can switch this to screen in the placement node and it lines up perfectly. I can then check show in viewport to see it in my view.

Problem is though that the image renders differently than the original picture. At first I though it was just needed sRGB correction (which it did), but when I applied this - although better it still looks different. I have to then color gain it up 3.5 to match. This im guess is to do with the Physical camera exposure.

If this is the case It would be great if vray had a bypass for camera exposure of the background plate and also a pixel match - so it does not resample the image. Loved this feature of the mip_cameramip in mental ray.

Any body have any insight why the VFB shows colour alphas?
"Yeap, im certain - I spent 50hrs making sure it was a cow"
  10 October 2012
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