Chapter 2: Leathers & Composites — Stiffness and Crease Logic

Created by Sarah Choi (prompt writer using ChatGPT)

Leathers & Composites — Stiffness and Crease Logic

Leather and composites sit at the boundary between soft goods and hard surfaces. They bend, hinge, and hold memory in ways plain cloth cannot, and they interface with textiles, hardware, and armor to define believable silhouettes. For character concept artists, understanding stiffness and crease logic turns decorative straps, pauldrons, belts, and shells into engineered parts that move correctly. For production, this knowledge informs seam placement, rig and cloth‑sim constraints, shader parameters, and wear maps that age convincingly across shots and LODs.

Material Physics in Plain Language

Every bend is a negotiation between modulus (material stiffness), thickness, and geometry. Thin, low‑modulus materials fold into small radii with many micro‑wrinkles; thick, high‑modulus materials tolerate only broad arcs and kink sharply if over‑stressed. Creases are memory events—fiber or matrix reorientation that persists after load—so once a crease sets, it guides future motion. Pattern logic exists to prevent uncontrolled creasing by distributing strain along darts, gussets, cutlines, and hardware that reroute load paths.

Leather Anatomy and Behavior

Leather is an anisotropic sheet built from collagen bundles. The grain side is dense and abrasion‑resistant; the flesh side is more open and stretchy. Growth marks and backbone create directional behavior: hides stretch more toward belly and neck, less along spine. Temper describes initial hand feel from board‑stiff to buttery, and finishing oils change how quickly the leather “breaks in.” Vegetable‑tanned leathers start firm, accept molding when wet, and form crisp, permanent creases with high burnish at fold crests. Chrome‑tanned leathers begin supple, resist water set‑molding, and develop softer, rounded creases with less polish. Split suedes and nubucks add nap that blurs highlights; micro‑directional brushing flips tonal read with gesture. When sketching, emphasize the grain/flesh distinction at cut edges, let early folds be broad arcs, and tighten the radius only where repeated motion or intentional scoring would create a hinge.

Stiffness, Thickness, and Bend Radius in Leather

Bend radius is the core design variable. A 2–3 oz leather (≈0.8–1.2 mm) can roll into small radii suitable for glove seams; 8–10 oz (≈3.2–4.0 mm) wants big arcs for belts and armor. Skiving—thinning edges—allows hems and rolled edges to fold without cracking, while leaving center thickness maintains structure. Reinforcing with interfacing or lining raises effective stiffness; perforations or decorative tooling lower local stiffness and can become unintended hinge lines. Crease logic follows repeated load: elbow straps wrinkle in ladder sequences, boot vamps crease over metatarsal lines, and belt holes elongate into teardrops as fibers yield around hardware. Depict these with directional micro‑cracks, shine halos on convexes, and dark sweat/oil staining where skin contact is frequent.

Pattern and Construction Choices for Leather

Leather prefers fewer, larger panels with strategic shaping. Darts and gussets manage curvature where molding alone won’t suffice. In belts and harnesses, keep long grain parallel to load paths and avoid bias‑like cuts that encourage growth. Edge treatments—bevel, burnish, paint—signal quality and alter silhouette crispness. Stitching introduces seam swell and a rhythm of compression marks; double‑needle saddle stitching reads robust and resists peel. Rivets, Chicago screws, and grommets transfer point loads but require washers or leather doublers to avoid tearing. Closures scale up: hefty zippers, buckles, conchos, and toggles that index by feel through gloves. Place holes away from edge stresses and mirror reinforcement symmetry so reads stay orderly at distance.

Composites: From Flexible Films to Armored Shells

Composites combine fibers with a matrix. Fiber direction dictates strength; matrix binds and spreads load. Thermoplastics (Kydex, ABS, polycarbonate) soften with heat and can be vacuum‑formed; they support living hinges when thin. Thermosets (epoxy with carbon or glass) cure permanent, light, and rigid, with minimal elastic recovery and catastrophic crack propagation when over‑bent. Fabric‑reinforced laminates (ballistic nylons coated with TPU/PVC) live between cloth and shell, behaving like waterproof skins that crease into large, shallow kinks and hold fold memory as whitening along the bend. Articulate fiber logic in your designs: long fibers run along expected load paths on pauldrons and greaves, while ±45° plies manage torsion on forearm guards. Visually, carbon twill reads as small, repeating diagonal checks with coherent, mirror‑symmetric highlights across panels; fiberglass appears milkier with less distinct weave at distance.

Crease and Hinge Logic in Composites

Rigid laminates do not “crease” so much as hinge at designed joints or fail along stress risers. Introduce segmentation and living‑hinge bridges where motion is required—over elbows, knees, and obliques—using thinner webs or elastomeric couplers. Avoid sharp internal corners in panel cutouts; filleted radii prevent cracks. Thermoplastic shells will take a permanent kink if folded hot or overstressed cold; depict this as dull, whitened lines with micro‑crazing. Flexible coated fabrics form boxy memory kinks near strap anchors and roll into tubes at hems; show tape overlays and welded seams that control these hinge lines. Document minimum bend radii in callouts to guide riggers and cloth‑sim: large arcs for rigid plates, medium arcs for coated shells, small radii for elastomer bridges.

Hybrid Soft‑Hard Interfaces

Most believable kits mix leather, coated fabrics, and rigid plates. The interface needs a grammar. Let plates float on webbing ladders or elastic bridges so motion reads through gaps. Seat plates onto leather yokes with stitch lines and rivet clusters arranged along load paths rather than random grids. Place binding tapes along composite edges to protect cloth and hide ply stacks. Under tension, soft goods should pouch slightly beneath attachments; show shallow “tents” between fasteners. In simulation, convert these choices to constraints: pin groups along stitch lines, limited‑slide constraints under webbing, and collision proxies that match plate curvature so capes and skirts don’t clip.

Synthetics that Mimic Leather

PU and microfiber “leathers” bond a polyurethane skin to a fabric backer. They crease by breaking the coating and whitening at bends, especially at cold temperatures. They resist burnish but scuff into fuzzy mats at high‑wear points. Pattern for knit‑backed faux leather like a stable knit: fewer darts, wider hems, and covered zips that won’t abrade the coating. Depict laminate edges with a telltale layer cake and tighter specular than natural leather; add delamination bubbles at cheap seams to sell age.

Aging, Patina, and Failure Modes

Leather ages by compression polish, oil darkening, dryness haze, and crazing at tight radii. Stitch holes elongate; buckle tongues bruise holes into ovals with a dark rim. Wet‑dry cycles stiffen vegetable‑tanned pieces unless conditioned; salt and sweat etch tide lines. Composites age differently: thermoplastics scratch into bright streaks and may stress‑whiten; thermoset carbon chips at edges, exposing pale matrix beneath; coated fabrics crack along fold memory and lose waterproofing at tape edges. Use environment‑appropriate language: desert dust chalks matte onto everything and fills stitch channels; maritime salt blooms at edges; polar cold makes PU skins glassy and prone to cracking. Repairs follow material logic: leather accepts stitched scabs, riveted plates, and edge repaint; composites receive bonded patches with scarfed edges or tape overlays; coated fabrics take heat‑welded patches and perimeter stitching with reinforcement disks.

Shader and Paint Guidance

At concept‑paint distances, communicate stiffness with highlight size and edge behavior. Stiff leather produces tight, specular ridges at creases and polarized sheen that shifts with view; soft leather shows broader, duller highlights. Expose thickness at cut edges with color transitions—from darker core to brighter burnish at the grain—then glaze in micro‑cracks on over‑bent zones. Carbon fiber demands coherent specular that follows panel curvature without breaking at the weave scale; avoid over‑resolving the twill at medium distances. Thermoplastic shells read with high, crisp specular, subtle orange‑peel at close range, and stress‑whitening at kinks. Coated fabrics exhibit lower‑frequency highlight “rivers” with dull transitions; seam tapes and welds interrupt specular as clean stripes. Reserve visible pores and weave for close‑ups and hero shots so LODs stay clean.

Rigging and Cloth‑Sim Notes

Declare stiffness and plasticity per part. Assign leather moderate bend stiffness with limited plastic deformation to allow crease memory at recurring hinges; raise collision thickness to match real bulk at straps and hems. Drive composite plates as rigid bodies on constraints or bones, not cloth; allow soft‑hard coupling through short elastic joints. Sim coated fabrics with low stretch and higher bend resistance than plain wovens; set high damping so kinks don’t oscillate unrealistically. Provide minimum bend radii, hinge locations, and plate overlap rules in your package so technical artists can build constraints without guessing.

Production‑Side Callouts

On your orthos, annotate leather thickness in ounces or millimeters, temper, tanning type, and any skive zones. Mark stitch type, SPI (stitches per inch), thread gauge, and reinforcement doublers at hardware. For composites, specify layup schedule (fiber orientation per ply), matrix type, nominal thickness, and edge treatments. Call out seam types on coated fabrics—welded vs stitched‑and‑taped—and tape widths. Include exploded views of soft‑hard interfaces with pin locations and allowable slide ranges. Provide variant states that matter to gameplay: belt hole progression, bracer straps cinched/loose, plate segments expanded/contracted.

Case Notes

A desert ranger’s bracer reads plausible because a 6–7 oz vegetable‑tanned cuff is wet‑formed over a dowel to pre‑set radius, edges are skived and burnished, and a floating thermoplastic plate rides on stitched loops to preserve wrist flex. A riot‑control pauldron sells because carbon fiber shells are segmented with elastomer bridges, edges are bound, and under‑plates sit on a padded leather yoke with bar‑tacked webbing that tents fabric realistically. A courier’s synthetic boot convinces because knit‑backed PU shows cold‑crack whitening at vamp creases, the midsole guard is vacuum‑formed ABS with filleted cutouts, and coated‑fabric gaiters retain broad memory kinks with welded seam tapes.

Workflow: From Brief to Package

Translate role and climate into stiffness maps before you draw. Decide which zones must hinge, which must hold shape, and where you can trade weight for mobility. Block silhouettes with plate segmentation and strap vectors that respect load paths. Move to linework that fixes bend radii, skive zones, and attachment logic. Paint material studies at three states—new, broken‑in, and over‑stressed—to lock crease language. Package orthos, exploded views, stiffness/hinge maps, shader notes, and wear timelines so downstream teams can build, rig, and texture without guesswork.

Closing Thought

Stiffness and crease logic are the grammar that makes leather and composites readable at a glance. When your seams, radii, and attachments reflect real material behavior, characters feel engineered rather than decorated—and every motion they make reinforces the story you set in their costume.