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Using negative lights to darken parts of scenes selectively

From: Production Rendering Techniques in CINEMA 4D

Video: Using negative lights to darken parts of scenes selectively

Sometimes when you're lighting a scene, everything looks great except for certain portion that is blown out. On real-world film sets, the DP might have a grip place a black cloth on the ground or a wall near the brightened area in order to absorb some of the light. Light works a little bit differently in CINEMA 4D by default. But we do have the tools to accomplish the same end. Let's take a look at this apartment model right here. It's looking pretty good, but the contrast is a little bit low. I've lit the scene using standard CINEMA 4D lights. Now if I compare this scene to a quick GI preview of the scene, which albeit a bit grainy, we see some pretty distinct differences especially in terms of the darker areas.

Using negative lights to darken parts of scenes selectively

Sometimes when you're lighting a scene, everything looks great except for certain portion that is blown out. On real-world film sets, the DP might have a grip place a black cloth on the ground or a wall near the brightened area in order to absorb some of the light. Light works a little bit differently in CINEMA 4D by default. But we do have the tools to accomplish the same end. Let's take a look at this apartment model right here. It's looking pretty good, but the contrast is a little bit low. I've lit the scene using standard CINEMA 4D lights. Now if I compare this scene to a quick GI preview of the scene, which albeit a bit grainy, we see some pretty distinct differences especially in terms of the darker areas.

Our brights are looking fairly well matched, but if you look at the GI version the area under the couch is much darker as well as the area under that little end table. So what we're going to do is use something called a negative light in order to bring down some of that brightness. Now that seems like a contradiction in terms. A light is a bright thing, how are we going to use it to make something dark? Well, let's just start the process, and we'll see how. I'm going to start by creating a light object in my scene, and I'm going to name this Negative Table, and I'm going to use this to add a bit of a shadow underneath that table.

Now as we can see, this light is absolutely blowing out our scene. It's just making everything too bright. So one, let's try and focus it in, so I'm going to change the Type here from an Omni light which emits light in all directions to a Spot light. Now because I want to focus in on a table and the table is rectangular, I'm going to choose a Square Spot. Now we are doing a bit better but I just don't know where my light source is. So I'm going to go into my four-way view, and I see that I've created a light source at the world origin. Well, the center of my room is not there, so I want to take this object, and I want to center it over my table.

So I'm just going to take my Negative Table light, and I'm going to go in the Coordinates Managers so it's easier for you to match my values. I'm going to switch over the X, I'm going to increase the Z, so I'm moving it right over the center of that table and then in my front view I'm going to bring that light up so that it's above that table, and I want to move it to about the center of the room. Now as I look at this, we're doing pretty well. The problem is that it's rotated towards the window. So I'm going to take my R.P, and I'm going to drag that down to a -90 value here.

We're doing pretty well, but the light is not quite the same shape as our table. Let's go ahead and go into the Details tab of our Light, and inside of Details there is something called Aspect Ratio. That allows you to change the relative width and height of your light object. And I'm going to increase the Aspect Ratio here. It looks like a value 2.1 should about do the job, and now I'm just going to render my view. So we've definitely got a light in that area, but it's over exposing our scene as opposed to darkening it. And there's just one more property that we need to change to get that effect to start happening.

So I'm going to take this Negative Table light, go into the General tab and then set the Intensity to a negative value. So I'm going to type in -80%, and now I'm just going to a turn on my Interactive Render Region to focus in on this lower area. The area under the table is looking all right, but it is definitely pretty bright. So I'm going to set the Intensity here to maybe just -20% to just knock down the brightness a bit. That's not quite enough so let's try, -30 might be the final correct value.

Let's go ahead and tweak some of the settings here. I see that it's actually darkening the table top as well, and I don't want that. So I'm going to go into my Project tab, and I'm going to exclude everything but my floor. Well, that seems like a way of doing it, but probably the more direct way is to change the mode from Exclude to Include, and I'm only going to drag in my floor object. Now the floor here is all part of the SketchUp object, so I'm just going to twirl open my apartment model and drag in SketchUp, and that's going to prevent the couch and the table from being included.

Now I'm going to go ahead and duplicate my Negative Table light and name it Negative Couch, and I'm going to my four-way view and just choose my top view here and reposition it so it's resting over my couch. That's looking pretty good and then I'm going to look at it in my side view here, and I'm just going to increase the Y value until the triangle here sort of the cone of my light is intersecting with the floor at about the same width as the couch itself.

So now when I come back to my Perspective view and take a look at this render, the area under the couch is also darker. Now if I recall the area under the couch in my QMC render was quite a bit dark than this, so I'm going to go into my Negative Couch, go into the General tab and set the Intensity to -35, and hopefully we get a result that we'll like. Now the difference between this render and our original render is subtle, but it's real, and I feel like this second version with the negative lights better grounds these objects and adds a bit more contrast to our scene.

If I was to keep doing work like this, I might add a Negative Light near that far left wall and maybe some in the brighter regions of the ceiling and the floor there. Now Negative Lights are a bit of a cheat, but if they get you to the look that you want it's good to know that they are there. So whenever you find that part of your render is blown out, don't limit yourself to the Burn tool in Photoshop, know that you can selectively darken your scene using Negative Lights.

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Production Rendering Techniques in CINEMA 4D

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Donovan Keith
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