Project 2: Reflection, Refraction, Translucency
For project 2 of technical compositing, the project requires the use of reflections, refractions and translucency as the title suggests. Using these elements within 3D, we will then be compositing the elements over a live plate. We will also be required to utilize camera tracking as the plate will be moving in some way. Using a crystal or some type of detailed, translucent object, the goal of the project is to explore a different approach to shading. I chose to use an Amethyst crystal.​​
Shooting Live Plate
Next, another aspect of the assignment is to capture our own live plates, using a chrome ball and sphere in order to prepare for integration with CG. I knew I wanted to attempt something more interesting in terms of lighting, so I chose a setup which allowed for both warm and cool tones of light to hit the object.
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Because the object will be somewhat transparent, it will be an interesting challenge to have two different colors of backlighting, as well as two different temperatures of shadow.
Live Plate 3D Tracking
Using Nuke's camera tracker, our next goal is to set up a camera to use in 3D software that matches the live camera used. After placing a CameraTracker node, I set the number of tracks or features to 300, set the separation to 6, and checked the "Refine Feature Locations" and "Preview Features" boxes.
With these settings, we can now hit "Track" under the analysis tab, then hit "Solve" once the track is completed. Nuke will then update each tracker with color codes of orange green and red, displaying the accuracy of each tracking point.
Lastly, the goal of the track is to get the error to be close to 1. In the AutoTracks tab, there are options for "Max Track Error" and "Max Error". By decreasing them slowly then deleting rejected points, the error can be brought down to 1.
To now use the tracking data to create a 3D camera, we first need to give Nuke a couple more parameters. By selecting trackers on the ground, which in this case is a desk, we can right-click the points and set them to ground plane. I then go to the CameraTracker tab, select Scene under the Export tab, and hit create. This will set up a scene with Nuke's estimation of the camera.
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In 3D space, I then added a flat plane geometry to act as the top of the desk in the live plate. By manually transforming the plane, I could get a better idea of if the perspective is correct.
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To finish this step, I added a WriteGeo to export the scene as an FBX to bring into Maya.
Shader Development
For this project, we were supplied with a few different selections of crystals with multi-layered cracking, translucent elements and subsurface-like qualities. The usual approach to creating a shader typically lies all within the software itself, whether that be a shader built in Arnold, Karma, etc. However, this project takes an alternative approach, separating all of the intricate details within render layers to be compiled together in Nuke. These layers include: Specular, Fresnel, Subsurface, Transparency, Transparency with fractures, Volume, and the typical shadow and occlusion layers for the environment. Below is the first iteration of these layers:
In Nuke, I can then begin to composite these layers together to build up the amethyst.
From the crystal, it can be observed how the transmissive glass-like quality exists on the outer edges, whereas deeper into the crystal, the subsurface layer may look most accurate. Using the Fresnel layer, which is the blue and red dragon in the middle, I can use a Shuffle node to pull the red and blue as a mask for the subsurface and translucent layers.
SSS
Transmission
Merged
I then began to experiment with using the other layers to help bring out the amethyst-like quality. Since I am mostly just setting up the layers to see how they work together, much polishing still needs to be done.
SSS x Volume
Transmission Fractures
Merged
As a test for now, I also merged the dragon into the scene using the 2 shadow layers and ambient occlusion.
Slap Comp
Graded with shadow and occlusion masks
Caustics
For integrating the dragon into the live plate, not only do we need the shadow layers and ambient occlusion, but for this transparent object, we will need to use caustics. Caustics are essentially the light rays that pass through the object, bouncing through and transferring some of the object color within the shadow. Following Arnold's documentation on the subject, I created another two render layers (one per source light) to composite the elements with more control.
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First, the documentation describes how to set up the two materials to create caustics-- for the ground plane and the object. The documentation first discusses the object material, which is a basic aiStandardSurface with transmission. Then, a facing ratio is plugged into the shader's transmission weight. I also added a 3d texture, plugged into transmission color, to help break up the output caustic a bit more. Along with the shader, we have to check the caustics box in the shader, then in the object, uncheck opaque and primary visibility.
Next, for the ground plane, the documentation uses the aiShadowMatte plugged into the emission color. However, because I want the caustic element to be a mask for compositing, I instead plugged the shadow matte into the base color. Plus, instead of adding color in the layer itself, I can use color correction in Nuke to have better control of the color based on the object. Below, I also included what the render layer output looks like. Since the scene has two main key lights, there are two caustic masks.
Lastly for the caustics, I brought the elements into Nuke to test what the masks would look like in the finished composite. Using a color grade node, I multiplied the live plate by a shade of purple. While the object's shader still needs work, here is what the composite looks like so far. The caustics are also pretty strong, in the future this will need to be toned down with a ramp.
Shader Refinement
Continuing through the project, I also wanted to go backwards and begin refining the shaders used in the render layers to better achieve the amethyst appearance. The main layers to address were the transmission, subsurface and transmission fractures; right now, the layers appear unrealistic and simple.
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Some of the main changes appeared in altering the subsurface radius and color, as well as the transmission depth and color. In addition, I attempted to increase the detail within the transmission fractures. Right now, they appear as flat straight lines cutting into the mesh, whereas in the actual rock, the slices are rough and choppier.
Final Composite