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So, what's the difference between render passes and render layers? Well, the way I like to think of it is render layers are a way to organize the different elements of the scene - in other words, the objects, their visibility, and what shaders are applied to those objects, how the lighting works, and all that kind of stuff. So, I can the split the scene itself into different versions. Render passes then split render layers into different elements for compositing - in other words, specular highlights, shadows, ambient occlusion, things like that.
So, render passes are used on render layers. To demonstrate this and make this point a little bit clearer, what I am going to do is I am going to create a render here using the simple scene. So, I have a Frankensteins monster toy here, and I have a chrome ball, and I have some very basic lighting, just a directionalLight, a spotLight with shadows, and some final gathering has been turned on. So, what I am going to do is I am going to first create my render layers. I am going to create a render layer in which the toy is visible, but the sphere is not, and I am going to create another render layer in which the toy is invisible, and the sphere is visible, but the reflection of the toy appears on the sphere.
So, that's the first thing I am going to do, and then what I'll do is split those rendered layers into render passes, so you can see how they work together. I am going to open up the Render Layer Editor here, and we just have the default masterlayer. So, what I am going to do is I am going to select all the objects here that I want: the geometry and the lights. I am going to select them and then going to go to the Render Layer Editor and choose Create Layer from Selected. So, now we have layer1. It looks just like the masterlayer, but that's fine. This is turned on, so I know it's going to render; this has been automatically turned off, so I know it's not going to render.
I'll call this toy, and now I am going to make a similar layer. One fast way to do that is I can just copy this existing layer. I'll just copy layer, right-click over the layer name in the Layer Editor and do Copy layer. Once I let go, it automatically creates a second layer. I am going to double-click on the second layer, and I am going to name this chromeBall. So, now I am going to use render layer overrides to determine what's visible on each layer. So, what I am going to do is the toy render layer, and I am going to turn off the visibility of the chrome ball.
I'll select the chromeBall open up the Attribute Editor and at the nurbsSphereShape1 tab, I am going to scroll down to Render Stats, and I am going to turn off Primary Visibility. You notice this turns orange. This indicates that a render layer override has been applied to the sphere. Since Primary Visibility is off, this means that the sphere won't appear in the render, but it will cast shadows if there are other reflective objects in here. Since I have Visible In Reflections and Refractions on, it will still be visible in those reflections and refractions.
So, I am going to come go back to the channel box, select the chromeBall layer, and I am going to select the monster toy. I am going to do the same thing. I am going to go to the Attribute Editor and turn off Primary Visibility. So, now the toy will only be visible in the reflection on the chrome sphere. Let's go to the toy layer, and I'll create a quick render. If you notice this round part of the shadow here, that's actually the shadow created by the sphere. So, I am going to store this image and minimize this.
I'll select the chromeBall layer, and now I'll create a render from this layer. Now, we can see the chrome ball, we can see the reflection of the toy in the chrome ball, and we can also see the shadow cast by the toy. Now, I have the shadow in another render layer. So, I don't necessarily need to create this, but I just will, just to make the point more obvious. So, now that I have my render layers set up, at this point, I am going to start creating render passes. So, for the toy layer, I am going to select the toy layer, right-click, and just to keep things simple, I will do Add New Render Layer, and I am going to do Specular, and I'll also add a new render layer with Reflection.
I am going to select the chromeBall layer, and I am going to add the same render layers, same render passes, Add New Render Pass > Specular, right-click > Add New Render Pass > Reflection. So let's go back to this layer. I am selecting the toy layer, and I'll create a render. So, here is the render of the toy layer. So, now I am going to go to the File menu and choose Load Render Pass > Reflection.
So, we can see the environment is reflected on the surface, and it's just the reflective qualities of this object. I'll go back to the Render view, File > Load Render Pass > Specular. Now, we can see the specular highlights on the surface of the toy. Now, we can see how this render layer has its own render passes associated with it. I'll store this image, minimize, select the chromeBall layer. This also has a Reflectivity pass and a Specular pass.
So, I'll create a render for this layer. Once again, we see the chrome ball. We see the reflection. So this is essentially of the beauty pass of the chrome ball. If I go to the File menu in the Render view > Load Render Pass > reflection, now I can see just the reflection, and we can see I just have the toy reflected in the chrome ball, so that's isolated for compositing there. If I go to the File menu of the Render view and Show load Render Pass > specular1, and I can see just the specular highlight has been added.
So, now I can bring any of these images into Photoshop or After Effects and start piecing them together to create a composite. Now, I have - the advantage of doing this, to splitting the renders into passes as I have the absolute control over each of the individual elements. I could add glow quality to just the specular highlight and nothing else. I could change the color of the specular highlight, all without having to re-render in Maya - and same with the other render passes that I've created. So, render passes are there to split up individual render layers for compositing.
That's how they work together. Render passes are part of render layers. I would like to add that render passes are only available for render layers that are using mental ray. I'll create a render layer override by clicking on this icon, and if I right-click new Render Layer Override and choose Maya software, or any of other renders, I'd lose the ability to have render passes; they are only for mental ray.
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