camera image background vs HDRI composite

[modhob]

I'm using blender 2.8 with luxcore. See the images below. The one on the left is done using an HDRI background, and a shadow catcher. The one on the right uses a camera background image and a shadow catcher.

The one on the right, with the camera image, there seems to be an outline around the robot. It is even more pronounced if the robot is further from the camera. The image on the left, with the HDRI as the background and lighting, there is no outline and it looks very good.

Anyone know why the camera image/robot version creates that thin outline around the robot? I would like to use my own background images rather than mess around with trying to create an HDRI image.

[modhob]

as a follow up, if I check 'transparent film' on the camera, it will render without the background image and the outline around the robot is more noticeable. See image. You can see the light outline around it.

[kintuX]

Using "Alpha Over" node works perfectly fine.

& BTW
Worth knowing the difference: Straight VS Premultiplied alphas.

- Straight Alpha versus Premultiplied Alpha (@YouTube; Smoke compositing)
- Straight vs Premultiplied : Understanding Alpha Channels (@YouTube; AE compositing by Hellolux)

Alpha compositing in detail:
- Alpha Compositing/Straight versus prmultiplied (Wikipedia)
- Alpha Compositing (Bartosz Ciechanowski)

[modhob]

Ok thx, will give that a try

[modhob]

Ok so if I render an alpha, save it out, bring it into photoshop and apply over background image it's good.

The problem seems to do with the camera. See my 2 pics. The 1st is what I said above, the 2nd is using the camera background image rendered in Blender.

It is a very subtle difference but things like this bother me. You can see a very faint outline in the 2nd pic. Mostly noticeable on the left side of it's body, the arm and the leg.

Maybe try adding a background camera image to yours and see if you get the same thing?

[modhob]

ok i think i got it working ok. I took the image off the camera property and just used it in the node, where you have your green color.

It looks good, I just can't see the image while I'm editing which makes placement of my objects trial and error.

Also, say I make a material for my robot and it's fine. How do I get back to the composite output tree that you show. I can't figure it out!

btw, here is a video of it: https://youtu.be/Xmz8ruItej4?si=wqd2IIIuUDU-Oyn3

[kintuX]

Below "Compositor" ('editor') w/ enabled "Nodes" or w/ "Compositing" (Layout), I have "Image editor" open.
You can also simply enable a "backdrop" within "Compositor".

[modhob]

Ah got it thx. I thought going to 'Compositing' on the top tab list is where I set it. But now to get back I have to select 'Compositor'....slightly different.

Also, I can't tell in your pic in your Composite node if you have 'use alpha' on or off. But it doesn't seem to make a difference one way or the other in my render. Also, in that same node, I can have alpha and z both set to 1 and it doesn't seem to matter either.

[kintuX]

(In practice) "Compositor" is just an 'editor' (window) when one is working in a certain (single) 'Layout' while 'Compositing' is one of 'Layouts' for when one has its workflow established (task defined).


Seems, unless one's working with multilayered (multipass) images (Cycles/Eevee), those setting tend to have no effect (as if defined based on file format and alpha presence).
(I prefer Natron and/or Fusion)