Controlling physical transparency

- [Instructor] In this chapter on ray tracing techniques, we'll start by looking at how transparency works in a physically based rendering. As we saw previously in the course, in a PDR or physically based rendering, there's no distinction between specular highlights and reflections. They're both controlled by the roughness. Likewise it is with transparency and refractions. In an old school first generation shading model, transparency and refractions are dealt with separately.

But in a physically based rendering, they're the same. I've constructed a test scene that works well for illustrating how transparency works. It's a very simple polyhedral object and it's got an image base lighting set up. I've placed HDR file into the environment. Let's check in on our render set up. Go ahead and open that up, we want to be in target mode active shade. The active render is ART and we're going to render physical camera 001, let's lock that.

And we're just doing a 480 by 480 frame. And our quality is 30 decibels. We'll do a render, okay there's our ideal diffuse material. Now we'll close the render set up dialogue. Go into the material editor. And create a new physical material, found under materials general. Drag that over and then assign it to the object, drag from the output onto the polyhedral object in the view port.

And then double-click on that material in order to load it in the parameter editor. Rename it, glass. And we can choose a pre-set, there are three pre-sets for transparent materials. Thin geometry is really only for single panes of glass. If I choose this, it will load here, but as you can see it's getting us no refractions and it's really unrealistic. Back up in our pre-sets, let's go for, solid geometry.

And now we're seeing a blue tinted glass, with an index of refraction of 1.5. Let's change up that tinting, the transparency color has a slight blue. Click on that transparency color, set the saturation to zero. And the value to one. It still looks blue but that's because the sky and the sea are blue in the environment. Click okay. And we got the index of refraction, it's set up for standard glass. If we want leaded glass, it'll be a little bit more dense, we'll set it to index of refraction of 1.7.

By default, there is a 100% transparency. We can dim that down a little bit and that will allow us to see a little bit of shading on the diffuse surface of the exterior of the glass. Set transparency to 0.9. And that actually dims down the brightness of the object. And that's because the base color is black, and so there is no diffuse component currently. We'll go into that base color and set it to a neutral value of 0.5.

It's not going to subtract nor add very much to the color of the object. Value of .5, click okay. And what we've done here now is give it a little bit of diffuse shading to the object, without changing it's overall brightness. Let's talk about roughness now. Of course that's going to control this shininess of the exterior surface, if we change it here. But there's also a roughness for transparency and that controls the focus or sharpness of the refractions.

To make it clearer to see, I'm going to enable advanced mode up here. And in the advanced reflectance parameters, set a linear curve, give it a custom curve, with a facing value of zero. An edge value of one and slope of .55 And I'm doing that so that we will be able to see the difference between reflections and refractions very clearly in our test image.

Down here, we see a reflection of the column or pedestal. And up here, this is refractions through the glass object. We can control the roughness of each of those independently. Back up in our parameter editor, base color roughness. And that's gonna be just the oranayer blinn shading that we saw earlier. It's just gonna add a little bit of extra side lighting, but we're not really worried about that now.

We're more concerned with reflection roughness and transparency roughness. Right now they're locked, when I change the reflection roughness, I'm also changing transparency. Let's hit reflection roughness to 0.2 and both the reflections and the refractions are now blurry. Now let's try unlocking those two values. Under transparency, click the pad lock. And now we have a separate transparency roughness which is defaulted to zero.

And we can see now that our reflection is very soft but our refractions in the center here are very sharp. And we can of course reverse that relationship very easily. We can set the reflection roughness to zero, so we'll have a very shiny appearance on the surface. But then we can give it a little bit of transparency roughness, let's say .2. And the effect we achieve is a very highly polished piece of frosted glass.

Let's look at this. We've got very sharp reflections, but very soft refractions here in the center. And that's how to work with transparency and roughness in the physical material.