Exercise: a trim sheet

A trim sheet is one texture made of horizontal strips that tile left to right, used to texture the edges, bands and panels of many meshes at once. This brief asks you to plan one sheet from a real kit, build it, prove it survives mipmapping, and texture several pieces with nothing else.

Tim Simpson of Polygon Academy writes that over half of junior environment applicants have "ZERO examples of tiling texture use or trim sheet knowledge in their portfolio", and that he would trust an applicant more with "a strong use of trim textures in at least one portfolio piece" (Polycount: TUTORIAL: Trim Textures for environment art…). Read trim sheets first; this page is the practical project.

#The brief

Goal. One 2048 × 2048 px trim sheet for one material family (painted metal, plaster and stone, or wood) at 512 px/m, used to texture at least three kit pieces and one prop with no unique textures.

Start from a kit. Use the pieces from exercise: a modular room, or block out three simple pieces now: a wall section with a skirting board, a door frame, a pillar. Tim Simpson's advice on Polycount is that "the less winging it the better usually", working backwards from designers' proxy models (Polycount: Creating Trim Sheets 4 Part Tutorial…).

#Constraints

#Stages

#1. Planning from the kit

List every edge, band and panel your pieces need, with its real height in centimetres. Convert each to pixels at 512 px/m and round to a power of two. EMC3D's video on trim planning orders the work as a colour overlay plan, then a trim sheet plan, then "Using The Blockout", with a chapter simply titled "Don't Rush!" (YouTube: What Does Good Trim Sheet Planning &…). Tim Simpson's first video in his series is entirely about planning which pieces a sheet must serve (Polygon Academy: Planning & Creating Trim Sheets For Games…).

#2. Building the strips

Choose how to make the content (the views box below compares them): model or sculpt the strips as geometry on a plane and bake them down, or build them in Substance 3D Designer (procedural materials). Tyler Smith's Primary Shapes course on ArtStation Learning starts a stone temple trim from shape design, the functional anatomy of architecture and reference, blocking in forms for the sheet's design in ZBrush (ArtStation Learning: Primary Shapes).

Make the bevel strip carefully. Morten Olsen's "Ultimate Trim" talk on Sunset Overdrive built a whole open world on a specific trim layout and normal map structure (GDC Vault: The Ultimate Trim: Texturing Techniques of…). Artists reproducing it on Polycount found that "the secret to techinque was a PERFECT 45 degree angle on a normal map" (Polycount: The 'ultimate trim' technique from Sunset…).

#3. Texturing and tiling tests

Texture the sheet to physically based rendering values. Then check that every strip wraps seamlessly at its left and right edges: place the sheet on a long plane tiled four times and look for seams. Polynaught spotted seams where the ends of a circular trim met; Tim Simpson replied that "in 99% of cases you will never notice it" (Polycount: Creating Trim Sheets 4 Part Tutorial…), but check the straight strips anyway.

#4. Mapping the pieces

Unwrap each piece so every band of faces becomes a straight, rectangular UV island. Set density first, then move each island vertically onto the strip that matches its height and let it run off the left and right edges, since the strip tiles. Expect to adjust the mesh as well: Tim Simpson warns you will often "squash and stretch parts of your mesh" or add edge loops so pieces fit the trims (Polycount: Creating Trim Sheets 4 Part Tutorial…). If your engine bakes light, give each piece a second UV set for lightmap UVs, because trim UVs overlap. See UV mapping and texel density.

For the bevels, chamfer real edges at 45 degrees and map the chamfer faces onto the bevel strip. On Polycount, huffer warns that bevels "will introduce shading gradients on planar surfaces, unless you use face-weighted normals" (Polycount: The 'ultimate trim' technique from Sunset…). See weighted (custom) normals and bevels and chamfers.

#5. Engine test and presentation

Assemble the pieces under real lighting. Then add variety on top of the sheet with vertex painting and blending and decals, so the same strip does not read as a copy.

#Shipped trim sheets to study

Before you plan, look at how shipped trim sheets are laid out and what they cover. For each, estimate the pixel height of the widest and narrowest strips and guess the world size.

• Measured trims at production scale. Matthew Trevelyan Johns's The Last of Us Part I trims are measured high-poly panels at 512 px per metre, with duplicate sizes for grooves and rivets; he estimates about 80% of his Lab assets used them instead of unique bakes (ArtStation: The Last of Us: Part I...Trim Pipeline).
• A sheet built for one kit. Tim Wilmsen's Baldur's Gate 3 glasshouse post is captioned "Trim + Assets created for the glasshouse set", alongside a tileable (ArtStation: Baldur's Gate 3 - Glasshouse Modular Set).
• Trims that let a kit grow. On the Indiana Jones and the Great Circle tomb kit, other artists "created custom pieces using finalised trims and tilables" (ArtStation: Indiana Jones and the Great Circle - Tomb Kit).
• An older texture sheet. Adrien Thierry's Dishonored Golden Cat post shows one "Texture sheet" beside all the exterior modules it covers (ArtStation: The Golden Cat).

#Further reading and watching

• trim sheets, texel density and tileable materials: the theory behind this brief.