For the skin, we decided to apply an SSS material with 2 layers of textures: one sub-dermal, with veins and hints of bones, and a top layer with a lot of dirt and scratches. This was particularly important to get the final aspect of dirty and slimy skin. This couldn't feel too plastic or it wouldn't be icky enough. The textures on the hands, ears and noses were tailor made for each model, so no dirty pattern would be recognizable.
Creating the most appropriate skin textures
SSS skin parameters I
An essential part while considering SSS is scene units: in our scene, the main rats had approximately 6 inches. We did some skin materials with slightly different settings for the following parts: bodies, hands, ears, tongues, tails and noses. A very important aspect of these materials was the scatter radius setting. For the parts where the SSS effect needed to be more apparent (such as ears and hands) a bigger radius was chosen (4cm and 3cm, respectively). The bodies had a lower radius (0,5cm), for a more subtle effect.
Scatter radius setting for the ears
SSS skin parameters II
The previous textures were then applied to the diffuse color and SSS color slots. In order to have a more realistic effect and to enhance the forms, trace reflections was activated. The diffuse amount was tuned and ended in 0.8 in all of the materials, in order to make a good balance between the diffuse and subsurface textures and colors. A procedural bump was created to make the skin more realistic through a Cellular map independent of UVs. We also put it in the specular amount slot this same map with more contrast to further break the specular lighting.
Assigning textures to color slots and tweaking the parameters
The final image size was very large (9000 pixels wide) and so we needed a renderer that would be able to capture the amount of details with enough speed. We opted for V-Ray for all the renders. When the lighting was set, we surprisingly didn't need indirect lighting, so a good amount of speed was achieved through that.
When setting the renders, we divided the render in some parts to make the image as editable as possible: the outside street (buildings, pole), the background, the furthest rats, the scenery rats (which were interacting with it), the middle-ground rats, the main rats (the 4 big ones) and the foreground pipes.
For the fur, we tried mental ray but in the end went for V-Ray, since the fur rendered with more information. Every fur was rendered separately from the rats to allow a better degree of editing. The fur had different passes, one of them allowing for selection of hairs when in Photoshop. We rendered 2 distinct Light Select passes (to separate the background lights from the ones from below), Specular, Wire Color and Reflect Glossiness passes (the latter 2 being the selection passes).
Using V-Ray to render all the components in the scene
An important aspect when choosing render passes is information. Even weird render elements can end in interesting results. Another important part is time. Every render pass adds a bit to the final render time.
We did a lot of tests with low res renders in order to choose which of them were best suited for editing. For the environment we rendered Diffuse, different lighting passes, different shadow passes, light select passes (to separate the front blue light from the other lights), different Reflection passes, Specular and ZDepth passes.
The rats were rendered with two lighting setups: the background and below lights. They had the same passes of the environment plus the SSS and Reflection Filter passes. Both the rats and the environment had two Ambient Occlusion renders, with different radius values.
The various render passes for one of the rats
Small webs between the sewer pipes and mold on the walls were added using textures extracted from the photos taken in Step 2. The smaller cracks were made using 2 Curve Adjustment Layers (one for darkening and the other for lighting the image). Putting two brushstrokes together ? one on each layer mask ? created a naturalistic bevel effect.
The minute cracks in the pipe-work were made in Photoshop
Post-production ? rats
One of the several consequences of living in a sewer is crawling among mud all day, so the rats would have to lose their fresh from render, 3D-clean appearance. A textured brush with low opacity can be used in a Solid Color Layer with a Bevel & Emboss setting. Each brushstroke will then create its own bevel effect and the low opacity will prevent the strokes from blending too fast with each other, creating a richer texture.
"Those are not friendly, cuddly cartoon rats so a few pieces of their ears have been bitten off in post-production as well"
Several areas of fur were erased, showing the skin beneath (rendered separately, actually), simulating a piece of skin or fur that could have been lost to scabies or in a fight. Those are not friendly, cuddly cartoon rats so a few pieces of their ears have been bitten off in post-production as well. Look really close and you'll find a few ticks and lice, too.
Because the ears, hands and tails were modeled separately from the main bodies, it was necessary to create a transition from these areas to the rendered fur, including the naturally sparse hairs that rats have in these areas. These extra hairs were drawn one by one to create an especially organic feel. The whiskers were drawn in Photoshop as well.
Adding the dirt effects gives a more convincing scene
The volumetric light was simulated in Photoshop, to have a better control of the light sources according to the design of the ad layout, respecting areas like customer logo and slogan.
The whole environment was darkened a little to help to better define the rats' silhouette and bring them to the foreground. On the same note, a small glow was added to simulate a rim light around the rats' edges and help separate their figures from the background (create a selection of the rats, and then expand and feather this selection). We also added an atmospheric perspective to the buildings in the street to make them recede further into the background (using Curves Adjustment Layers and lowering the opacity of the sky).
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