The anatomy of a fader cap

Base Volume: Begin with a standard rectangular cuboid. The lateral sides (left and right) remain strictly planar and parallel to each other.
The Primary Saddle (Concave): The top face features a deep, concave cylindrical boolean subtraction running along the transverse axis. This creates the central “scoop” or saddle where the finger rests. The nadir of this curve sits precisely at the center point of the top plane.
The Crests and Downward Trail (Convex): The front and back edges of the concave saddle do not end in sharp corners. Instead, they transition into high-radius convex fillets. These rounded crests roll smoothly downward, transforming the horizontal top surface into the vertical front and back faces. This continuous curve creates a soft, sweeping drop-off on both ends of the longitudinal axis.
OLYMPUS DIGITAL CAMERA

Geometry/Displacement: Embedded entirely within the concave saddle are a series of parallel, shallow horizontal striations.
Orientation: These grooves run along the transverse axis (perpendicular to the fader’s sliding direction).
Shading Execution: In a PBR workflow, these should be handled via a normal map or displacement map using a subtle sine-wave profile. They are designed to catch grazing light, creating alternating horizontal micro-bands of soft specular highlights and self-shadowing (ambient occlusion) within the deeper scoop.

UV Projection: The stark indicator line requires a continuous UV projection mapped along the longitudinal axis.
Path of the Wrap: The solid band originates on the flat, lower-front vertical face. It sweeps seamlessly up and over the front convex crest, plunges directly through the center of the ribbed concave saddle (bisecting the horizontal grooves at a perfect 90-degree perpendicular angle), climbs up the rear convex crest, and drops down to terminate on the rear vertical face.
Material Contrast: The painted line must have an independent material ID or a mask. It requires a high-albedo, purely diffuse surface (Roughness 0.9+) with zero specularity, ensuring the line remains readable and does not reflect light the way the base plastic does.

Base Material: Dielectric polymer (injection-molded plastic).

Index of Refraction (IOR): ~1.45
Roughness: Set to medium-low (approx. 0.35). The plastic should exhibit a slight sheen. Specular reflections should be visible but slightly blurred, simulating a micro-texture that prevents a perfect, mirror-like gloss.
Specular Highlights: The primary lighting interaction will occur on the front and back convex crests. These tight curves should act as sharp specular catchers, grabbing overhead lights and creating bright, curved horizontal glints that define the upper boundaries of the fader cap.
Shadowing: Soft shadowing and heavy ambient occlusion should pool in the central concave depression, contrasting with the highly illuminated convex lips.

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