Green/Blue screen effect in Iray?

Silent Winter

Tried searching through Yahoo (google's blocked here) on the daz site but no dice.

In 3DL it was a simple case of cranking up the ambient channel to get a green-screen effect.  E.g. I'd put a green plane (or any other shape with a transmap) against a wall - so I could easily cut it out in postwork and look through the wall to a scene behind.  Even a cartoon effect of a man-shaped hole.

Is there a way to acheive the same in Iray?  A regular green plane is affected by the lighting (so might be too dark to show up or too varied to cut out).  Using an emissive plane will affect the things around it.

Come to think of it - how do they do it in real life for a darker scene? (since Iray lighting is more realistic).

MelanieL

Do you just want to render a character (or prop) in the foreground without any background to add in an image editor?

If so, then you can put nothing in the background (make sure you have "Draw Dome" off in render settings) and save the render as .tif (maybe .png too but I've never tried that) then it saves with a transparent background.

ETA: Ah I just re-read your question and I see that you want to have a wall with a "hole" in it behind your character. Short of taking the wall into a modelling program and making a hole in it, I can't think of how you can do that. Sorry.

Silent Winter

It's using the 'green-screen' effect like they do in movies - it can be done easily in 3DL, just wondering if it can be done as easily in Iray (i.e. what settings would I use on the surface?)

Thanks anyway :)

Richard Haseltine

As MelanieL says, it's far simpler to have no backdrop geoemtry at all and save in a format that suports alpha channels - PNG or Tiff.

Tobor

1. If using the natural transparent (checkerboard) background: if you're making a movie you'll definitely need to save as individual PNG or TIFs as the alpha channel isn't carried over in any of the supported video formats (that I've vfound). You don't really need a green screen because you already have a transparency.

2. If using the Daz Studio background feature: you can select a suitable green color to drop out in your compositing software. I don't believe the background will not influence the colors in the render.

With either of these, the opening in your wall shows either the checkboard background that will render as a transparency with PNG or TIF, or the background color.

Silent Winter

Sorry for the late reply - lost track of what I'd posted.

I can easily get a transparent background.  What I'm wanting to do is cut a hole in a solid object.  Let's say there's a road with a manhole cover as part of the texture (not geometry).  I'd like to put a green circle there, then cut it out in post and add a manhole beneath.  Or there's a wall with no doorway - I could add a green rectangle and cut it out in post to show the interior of the building.

It's doable without the green-screen effect.  Just easier with.

Edit:  I could just cut out a shape - but for complex shapes or getting the angle just right, it's easier with the above.

prixat

Iray's 'Matte shader' should give you the equivalent of cut out shapes:

https://www.daz3d.com/forums/discussion/60697/shadow-catchers-for-iray-hdri

 

Tobor

Prixat is correct that the matte object feature will likely give what you want. It is intended for such matte-in-place effects, but it has some unusual properties in that it's not just a simple green screen for later cutout. 

However, if what you're wanting is to simply create a color key from a surface or as part of the texture, then that's readily done, but there are some concerns. Here's two:

1. The appearance of the color depends on the light falling on it, unless you use an emissive shader, in which case you can cause it to self-light to a controlled level. This won't work as part of a texture as you have to apply the emissive shader to the whole surface (though you can get tricky and make yourself a cutout transparency and/or isolated texture to apply to the Emission channel to control how it lights up). You can also use the geometry editor to reassign portions of the geometry to another surface, then make that surface emissiove. This does require sufficient polygons, though. 

2. The color may "bleed" into surrounding objects, just like real green screen does. Large areas that cast a big green tint will affect the color balance of the whole image, and or create fringing. These are well-known problems in real green screen, and the ways to work around them in Iray are similar.

Anyway, what you're wanting to do I've done with mirrors on cars, and such. I render the scene with a transparent background, but color the mirror to something not in the scene. Green is typical, but there are a lot of blue paint cars, so as necessary I might go with red or something else. 

Finally, though I don't tend to use it a lot, you can apply Material IDs to surfaces, and pick a color to represent them in a canvas render (canvases are under the Advanced Render tab). This render creates a clownish-style color mashup where the different materials have unique colors you can then use to create mattes in Photoshop.

Silent Winter

Great, thanks all :)