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Render passes are way to separate the different parts of a rendered image so that they can be easily composited in another compositing program, such as Adobe Photoshop or Adobe After Effects. I want to demonstrate quickly how you can add render passes to a scene. In this scene, I have my Frankensteins master toy, and it's just got some very simple lighting: a directional light, a spot light with shadows, and some final gathering just to add some bounce light effects. I currently only have the default masterlayer in the scene.
It's the only render layer that exists. So, I am going to add some render passes to this layer. In Render Layer Editor, under the Render tab, I will right-click and do Add New Render Pass, and I'll start by adding a Specular pass, and I'll do another simple one, and I'll do Shadow pass. So, let's just start with those two, and I'll click in here and create a render. So, here is our rendered image, but where are our rendered passes? You'll have to open them up by going to the File menu of the Render view and choose Load Render Pass, and I'll start by loading the Shadow pass.
This is going to open up the second application called imf_disp. If you're using the Macintosh, you might find imf_disp is in the dock, but it might actually be behind Maya. So, just go down to the dock, and click on the icon to bring up imf_disp. But we can see right here we just have our basic shadow, and this is inverted. So, if you bring this in your compositing program, you just want to invert the image, and you have a nice, dark shadow, and composite that on top of the other images. I am going to open up Specular Pass, so I'll just go Load Render Pass > Specular, and now we can see the specular highlights.
And the way that render passes work is essentially - as mental ray is rendering image it uses what's known as a render buffer. So, while it's rendering the image, it's storing the data in a buffer. This is something that you don't see when you're rendering. It's something that's going on behind the scenes. Then what happens is that after it has stored all this information in the buffer, it extracts that information into your image file. So, the render buffer is going to render in the native Maya if format. Then if you tell the Renderer to use TIF, or Targa, or SGI, at the end of the rendering process, it will convert that 'If' into the necessary format.
As you add render passes to render, it's going to tell mental ray to make sure to additional information in the render buffer, then at the end of the process extract that information from the render buffer, and create the different images for the different render passes. So, that's essentially what was going on. So, this is good to know because as you add more and more render passes, you are going to add time to of the overall render, because mental ray is going to be required to add more and more information to the render buffer.
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