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Oftentimes a render artist may be called upon to provide specific rendered elements that may be needed by compositing artists further down a production pipeline, or indeed it may well be that we need these elements to perform our own compositing operations. In this video, we will walk you through the steps required to create an alpha mask render that we can use a little later in this chapter to perform some basic color correction operations on our final rendered image. Before we create our alpha mask, there is a little bit of scene setup that we need to run through, so let's just come up to our Window menu and come down to the Layers option.
The first thing I want to do in here is just Layer0 as the Default layer. This is so that we can go and turn off our Sky Portals, Patio Glass, and Skylight Glass layers. We've hidden these objects because we don't want to them included in our alpha mask render. Now of course, we could just perform a number of selection operations, but if we know we have objects that we definitely don't want included in our alpha mask, it's a very simple method to just go and turn off their layers. In fact, what we can do now is to just go up to the Edit menu and just use the Select All command.
We could of course use the keyboard shortcut of Ctrl+A as well. By selecting our geometry, what we are essentially doing now is telling Twilight which objects had to be included in our alpha mask render, in other words which objects are to be assigned a pure white color. As you will see in just a moment or two, any objects not included in this selection will ultimately have a pure black color assigned to them in the final render. Another little tweak we may want to perform is to come into our Twilight Render dialog, come to the Camera tab, and just set our Tone Mapping Filter to None.
This just ensures that no tone mapping will interfere with the creation of our mask. We'll of course, need to come to the Render tab, come into the Advance Settings, and choose our Alpha Mask Render preset. And now we ready to start over render, so let's go and click on the Start Render icon. In this instance when we get our Selection dialog, we actually want to say Yes. This means that only the selected geometry will be rendered and anything not included in the selection will again just be a simply assigned a pure black value which worked perfectly for our alpha mask render.
So let's click Yes and let our render start. And as you can see, we get a black-and- white image with all of our geometry assigned to pure white value and our background, or our environment, assigned a pure black color. This of course makes it a perfect mask for over sky and as we say, we will use this a little later on in Photoshop to perform some basic color corrections on the sky. Naturally, we do need to save our image to disk, so I'll click on the Save option, and I'm just going to give these a nice descriptive name. I'll call this Alpha. In this instance we can just Save As a PNG file, click Save, and now we are done.
We have of course only created a single mask image in this instance, but we can create as many images, as many configurations of mask renders as we feel our compositing operations will require. We could even create an alpha mask render for every single object in our scene. That means we could isolate every single object inside of Photoshop and then perform image editing operations own them. Certainly there is no doubt that compositing has increasingly become an essential part of the modern rendering pipeline.
It brings a lot of flexibility to the creative process, as well as potentially saving us having to perform costly rerenders for entire scenes, maybe even entire sequences when we render with Twilight. With that in mind then, it certainly can be said that when we rendering for compositing, the Alpha Mask preset is an essential piece of Twilight functionality.
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