Unit
- UV coordinates definition: UV coordinates are a 2D coordinate system assigned to the vertices of a 3D model, mapping each point on the surface to a specific location on a 2D texture image. The U and V axes correspond to the horizontal and vertical directions of the texture, while the third axis W is reserved for advanced multi-texturing.
- Range and normalization: Standard UV coordinates range from 0.0 to 1.0, where (0,0) is the bottom-left corner of the texture and (1,1) is the top-right. Values outside this range cause the texture to repeat or clamp, depending on the wrapping mode set in the rendering engine.
- Relationship to vertex positions: Unlike world-space or object-space coordinates, UVs are independent of the model's 3D position. A vertex can have the same 3D location but different UVs if the surface is split by a seam, enabling sharp transitions in texture mapping.
- Per-face vs. per-vertex assignment: UV data is typically stored per face vertex, meaning each vertex of a triangle can have its own UV coordinate. This allows for discontinuous mappings across edges, which is essential for creating seams.
- Texel correspondence: Each UV point maps to a specific pixel (texel) in the texture. The density of UV space determines how many texels cover a given area of the 3D surface; higher density yields more detail but requires larger textures.
- Common software representation: In 3D applications like Blender, Maya, and 3ds Max, UVs are displayed as a 2D grid overlaid on the model's surface. Artists edit UVs in a UV editor that shows the unfolded layout of the model's surfaces.
- Importance in rendering: Modern rendering engines sample textures using UV coordinates. Without correct UVs, textures will not align with the model's geometry, causing visual artifacts such as stretching, repetition, or misalignment.
- Historical context: UV mapping originated in the 1970s with early texture mapping techniques developed at the University of Utah. It became standardized in the 1990s with the rise of consumer 3D graphics cards, allowing real-time texture mapping in games.
Design Principle
Principle: UV Continuity and Discontinuity
Definition: UV continuity means that adjacent faces share the same UV coordinates along their shared edge, resulting in a seamless texture. Discontinuity occurs when faces share a 3D edge but have different UVs, creating a visible seam. Choosing continuity or discontinuity depends on the surface type and texture requirements.
Application: For organic models like characters, use continuous UVs across smooth surfaces to avoid visible seams. For hard-surface objects, place seams at natural edges where the eye expects a break, such as the transition between a car door and its frame. Discontinuity allows sharp texture changes, like a sticker on a smooth surface.
Quick Revision
- UV coordinates map 3D surfaces to 2D textures using U (horizontal) and V (vertical) axes.
- Standard UV range is 0.0 to 1.0; values outside cause repeating or clamping.
- UVs are stored per face vertex to allow sharp seams between faces.
- Texel density is the number of texture pixels per unit area on the 3D surface.
- Continuity produces seamless textures; discontinuity creates visible seams at chosen edges.