un multiplied alpha from VRIMG file

[tricky]

I'm rendering to a VRIMG file. I then use Shimakaze's vrimg2exr conversion tool to extract the RGB and alpha image. I open these in Photoshop CS2 and comp them onto my background.

I have a nasty bright edge to my geometry where it overlaps the background. It is particularly apparent with fine details such as trees and branches.

How do I get vrimg (or an element within it) to render out 'unmultiplied' so my comps will be cleaner?

[jacksc02]

Have you tried doing a "defringe"?

[tricky]

Yep. Rubbish.

[joconnell]

Photoshop is oddly the worst program I've used for alpha channels. A lot of it depends on what your objects are rendered against - if you're going against a black background you're fine, but otherwise it causes all of those halo effects you're seeing now.

[Morne]

Just to be clear - Chris and John, you're saying that Tricky's problem is not VRay, but his post program (photoshop) in this case. Would he then get better results with say After Effects even if it is just 1 frame, or will all the Adobe packages give the same result? (unless the background is black)

[glorybound]

I have great success using ProEXR. My alpha is transparent and I past my background in a layer below. When you open the EXR you pres alt (I think) and you can specify how it handles your alpha channel.

[tricky]

I have great success using ProEXR. My alpha is transparent and I past my background in a layer below. When you open the EXR you pres alt (I think) and you can specify how it handles your alpha channel.

So is that using ProEXR in Photoshop then? What version of PS are you using? I don't really want to purchase anything to do this:- I would have thought it is quite a straightforward issue. I willrender with a black background in the meantime (its sometimes a 'visual' pain in the neck rendering on a black background as lighting is more difficult to judge correctly).

If VRIMG2EXR were able to save to a PNG file, wouldn't that solve the problem???

[joconnell]

Just to be clear - Chris and John, you're saying that Tricky's problem is not VRay, but his post program (photoshop) in this case. Would he then get better results with say After Effects even if it is just 1 frame, or will all the Adobe packages give the same result? (unless the background is black)

When you render something in 3d that has an anti-aliased edge, you're getting part of the object, and part of the background it's rendered against mixed together in those soft edge pixels. When you import something into photoshop, it doesn't have controls to tell it what colour background your 3d was rendered against so when it makes the edge pixels smi transparent, it can mess up the colours quite a bit. After effects has options on whether the edges were pre multiplied or straight and also has options to pick what colour the object was rendered over too.

[raymarcher]

C'ant you divide (blending mode) your image with your alpha channel?

[glorybound]

I have Photoshop CS5