Tutorials
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Only a Dream
© onlyadreammovie.com
June 07, 2021
In this training, we’ll use a scene from Pixar co-op short film Only a Dream, where we’ll learn to use some of the most effective RenderMan Shot Lighting tools with RenderMan for Maya. During this process, we’ll understand how to make meaningful artistic choices in order to tell the best story possible, while digging into some of the more technical aspects of master and shot lighting.
Lighting Breakdown
Maya doesn't support repathing the supplied Alembic files and might crash Maya, but there is a workaround:
Phew! With that technicality out of the way, let's get on to lighting....
Our main goal as lighting artists is to tell a story, in this case, a coming-of-age thriller. The aesthetic we’re going for is film-noir with an analogous blue/green color palette, in fact, one of the color inspirations was the Chernabog Demon sequence from Disney’s Fantasia.
Amazing and pretty creepy!
In order to achieve our look, we’ll mix a couple of different lighting techniques, including Low-Key and Motivated lighting, which will give the scene a bit of drama while heavily art directing the shot to create the look we need.
Outline:
A useful way to start master lighting is through an environment light. We’ll use a PxrDomeLight with an industrial HDRI map which is full of small and bright lighting features, yet offers dim diffuse lighting. This will give us interesting glints to bring out shiny surfaces.
If light is overpowering, RenderMan offers the decoupling of diffuse and specular contributions under the “Refine” tab in all lights. This is also a useful attribute to keep in mind when creating manual eye highlights for example.
Using light temperature is also a great way to stay within a plausible aesthetic, as it’s based on physical Kelvin color temperatures, which are commonly used to measure the color of incandescent light bulbs.
...but don’t be afraid to add or remove color for a more stylized look...after all, in live action sets, it's very common to color lights with thin plastic sheets called "gels."
Using Maya's viewport render is a great way to quickly iterate
Due to the sampling principles in path tracing, interior lighting is generally less efficient than exterior lighting, because camera rays have a harder time finding light sources when they are hidden from the camera. This is exemplified in our scene with our dome light, which is being covered by the house.
To make this more efficient, RenderMan can use Portal Lights, which communicate to the renderer where the lighting is coming from. We’re using one portal light for each window or door opening, which can have a dramatic effect on sampling quality, effectively making our renders converge much faster.
Using Portal Lights is critical to rendering interiors efficiently
RenderMan’s Preset Browser is not only able to save materials, but lights too! Save entire light rigs and share them across RenderMan bridge products with the click of a button.
Lighting direction is crucial for the mood of the shot, and by using Low-Key lighting we can start to produce a scarier mood. To achieve this, we are pointing lights from the floor towards the ceiling to bring a gradient of light intensity in the walls.
Rect and Disk lights work well for this but their direction might be too obvious, so adding Sphere lights is useful when we want to cover a larger area or create softer shadows with a less obvious lighting direction.
If you’re looking for further realism, try using IES profiles which will create photometric light patterns common in light bulbs.
Use different types of lights for different effects
The Chernabog Demon sequence uses complementary colors to highlight key moments in the storytelling, such as blue and orange tones. We’re doing the same with our Key Light in order to bring out the warm tones in our character’s face. Although we want to create tension with a cooler color palette, we want to make sure our hero doesn't look dead.
The key light is coming from the bottom left, in order to create that Low-Key aesthetic we’re aiming for. We’ve also added a bounce light in the opposing direction to shape our character’s face while avoiding harsh faloffs.
It’s sometimes useful to normalize our lights to avoid intensity changes while scaling the lights during our blocking stage.
Use complementary colors to add aesthetic appeal to your shot
Lighting as a storytelling tool is especially useful to emphasize a character's emotions. One of the most common lighting techniques is using very bright lights in the back of a character in order to separate them from the background with a silhouette. This is a traditional filmmaking technique called rim lighting and it avoids our character getting lost in the frame...especially in dark and creepy rooms like this shot.
To create rim lighting, we’re adding a large rect light behind Lucy to make her pop from the background. We're also using two kicker lights to emphasize the hair specularity, which the director wanted to look electric and synthetic, to highlight the dream scenario. A kicker light differs from a rim in that it also shapes part of the front of the character.
Our kicker light for the hair specularity is giving us unintended highlights in both the eyes and face, to control this, we are removing unwanted contributions of this light via light linking. This can be done with Maya’s Relationship Editor > Light Linking.
Use Light Linking to isolate your lighting
The current setup is going in the right direction, but it’s looking a bit flat, so we need to shape our character to make sure we're achieving the more dramatic film noir aesthetic needed for our shot.
The most common way to manipulate lights in RenderMan is through light filters, which provide a non destructive way to block, isolate, or color lights. Even portals can use light filters!
Light Filters are easy to add to any light
We’re using PxrRod and PxrBlocker light filters in this scene in order to shape a barn door through our character’s eyes, emphasizing her expression of confusion and fright. We can simply right click on our light filter attribute and pick from a list of available options. Using the same tools, we’ll also refine and shape the light contribution of our environment light.
Director's notes:
Light Filters allow us to make non-destructive lighting changes.
It’s very important not to rely on a single frame for lighting and always check the animation sequence for context. In our shot, the character starts out in penumbra, guiding the viewer to the item in her hand. As the shot progresses, we shift focus to the character’s face and her reaction to that element in the story.
To do this, we are animating the light intensity of our low-key bounce light, as well as decreasing the specularity of that light by 80% so it doesn’t overwhelm the eyes with reflections.
To see added detail, make sure you open these 4K images in a new tab.
If you’re having a hard time troubleshooting your render, use the PxrVisualizer Integrator, where you can check geometry normals, wireframes, and other surface issues with ease. The PxrOcclusion integrator is also a great way to check for contact shadows during the layout process. Oh, and switch between all of the Integrators interactively for a great way to refine composition and shot framing!
Besides AOVs and LPEs, RenderMan can output arbitrary light groups by simply assigning a name to the Advanced > Light Group attribute. We can then go to our Render Settings > AOVs tab and create a new channel to display our light group, such as a new beauty or specular pass.
Don’t forget to check “Adapt All” under Render Settings > Advanced, so that your new passes are not noisy. This comes in handy for compositing, or troubleshooting, so that you can isolate lights with any arbitrary lobe, and you can even do this during an IPR session!
If you're going to be using the Cross Frame Denoiser, make sure you prefix any passes with their assigned LPE or AOV name, such as _directSpecular_myLightGroup instead of an arbitrary name. For efficiency, the passes will use the data created in the main _variance beauty pass when denoising.
Light Groups are a powerful tool for compositing
We’re taking advantage of PxrUnified, Pixar’s own studio integrator, which has sophisticated ray termination and path guiding tech. These efficiencies allow us to have higher trace depths for our final frames, improving the look of our hair and skin...and of course more light bounces is something especially useful for making indoor renders looks great, while rendering as efficient as possible
We also have the denoise button checked on in our AOVs > Display tab, which will allow us to use the Nvidia Optix denoiser in the Image Tool (IT) by pressing “N.” This will also take care of the denoise process automatically for batch rendering.
The Optix denoiser needs an Nvidia video card and is thus not available in macOS.
If your computer is struggling with resources, try switching resolutions or adjusting sampling settings interactively!
The PxrUnified Integrator provides cutting-edge light transport
Leif is a RenderMan specialist at Pixar’s RenderMan team, where he loves to showcase some of the most artistic and technical aspects of the renderer.
This asset is available with a Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) License. This allows you to share and redistribute for non-commercial purposes, as long as you credit the original author.
