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Texturing

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Instructor: Christopher Rasmussen (cer@cis.udel.edu)

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March 8, 2023 Lecture 14

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Outline

  • Bump maps
  • Texturing pipeline
  • More applications
    • Light maps
    • Environment maps

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Bump Mapping

  • So far we’ve been thinking of textures modulating color and transparency only
    • Billboards, decals, lightmaps, etc.
  • But any other per-pixel properties are fair game...
  • Pixel normals usually smoothly varying
    • Computed at vertices for Gouraud shading; color interpolated
    • Interpolated from vertices for Phong shading
  • Textures allow setting per-pixel normal with a bump map

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Bump map: Example

from wikipedia.org

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Sphere WITH bumps

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Sphere

animated GLSL example

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Bump mapping: Why?

  • Can get a lot more surface detail without expense of more object vertices to light, transform

courtesy of Nvidia

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Bump + color map: Example

Courtesy of http://www.alejandrosegovia.net

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Bump mapping: Example

from MIT CG lecture notes

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Bump mapping: Example

courtesy of A. Awadallah

Height map

Bump texture applied to teapot

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Bump mapping: How?

  • Idea: Perturb pixel normals n(u, v) derived from object geometry to get additional detail for shading
  • Compute lighting per pixel (like Phong)

from Hill

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Bump mapping: Representations

  • 3-D vector m(u, v) added directly to normal n
  • Or: 2-D vector of coefficients (bu, bv) that scale u, v vectors tangent to surface

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from Akenine-Moller & Haines

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Bump representation: Height map f(u, v)

  • Store just scalar “altitude” at each pixel
  • Get bu, bv from partial derivatives:

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    • Approximate with finite differencing

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from Akenine-Moller

& Haines

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Example: Converting height maps to normal displacements (aka normal maps)

Z coordinate set to some constant scale factor; (X, Y) normalized to [0, 1] range.

Right image is mostly blue because “straight up” vector is (0.5, 0.5, 1)

courtesy of Nvidia

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Bump mapping: Issues

  • Bumps don’t cast shadows
  • Geometry doesn’t change, so silhouette of object is unaffected
  • Textures can be used to modify underlying geometry with displacement maps
    • Generally in direction of surface normal

courtesy of Nvidia

from https://en.wikipedia.org/wiki/Normal_mapping

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Displacement Mapping

courtesy of spot3d.com

Bump mapping

Displacement mapping

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Displacement Mapping the Sphere

courtesy of geeks3d.com

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Displacement Mapping – Height Maps

courtesy of artofillusion.org -- Julian MacDonald

Threejs live example

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Texture mapping: Steps

  • Creation: Where does the texture image come from?
  • Geometry: Transformation from 3-D shape locations to 2-D texture image coordinates
  • Rasterization: What to draw at each pixel (since texture coords. are floats)
    • E.g., bilinear interpolation vs. nearest-neighbor

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Texturing: Creation

  • Reproductions
    • Photographs
    • Handpainted
  • Directly-computed functions
    • E.g., lightmaps, visibility maps
  • Procedurally-built
    • Synthesize with randomness, pattern-generating rules, etc.
    • More about this after midterm

courtesy of H. Elias

courtesy of Nvidia

Procedural bump mapping

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Texture mapping applications: Lightmaps

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courtesy of K. Miller

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Idea: Precompute expensive static lighting effects (such as ambient occlusion or diffuse reflectance) and “bake” them into color texture. Then scene looks more realistic as camera moves without expense of recomputing effects

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Texture mapping application: Lightmaps

Tenebrae Quake screenshot

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Texture mapping application: Lightmaps

Textures only

courtesy of https://jbush001.github.io/2015/06/11/quake-lightmaps.html

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Texture mapping application: Lightmaps

Lightmap with bilinear filtering



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Texture mapping application: Lightmaps

Textures and lightmaps combined

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Lightmap example: Diffuse lighting only

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Lightmap example: Light configuration

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Lightmap example: Diffuse + lightmap

Lightmap demo of "Sponza" scene (240K triangles, 5 lights)

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Texturing Pipeline (Geometry + Rasterization)

  1. Compute object space location (x, y, z) from screen space location (i, j)

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list adapted from Akenine-Moller & Haines

courtesy of R. Wolfe

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Texturing Pipeline (Geometry + Rasterization)

  • Compute object space location (x, y, z) from screen space location (i, j)
  • Use projector function to obtain object surface coordinates (u, v) (3-D -> 2-D projection)

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list adapted from Akenine-Moller & Haines

courtesy of R. Wolfe

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Texturing Pipeline (Geometry + Rasterization)

  • Compute object space location (x, y, z) from screen space location (i, j)
  • Use projector function to obtain object surface coordinates (u, v) (3-D -> 2-D projection)
  • Use corresponder function to find texel coordinates (s, t) (2-D -> 2-D transformation)
    • Scale, shift, wrap like viewport transform in geometry pipeline

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list adapted from Akenine-Moller & Haines

courtesy of R. Wolfe

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Texturing Pipeline (Geometry + Rasterization)

  • Compute object space location (x, y, z) from screen space location (i, j)
  • Use projector function to obtain object surface coordinates (u, v) (3-D -> 2-D projection)
  • Use corresponder function to find texel coordinates (s, t) (2-D -> 2-D transformation)
    • Scale, shift, wrap like viewport transform in geometry pipeline
  • Filter texel at (s, t)

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list adapted from Akenine-Moller & Haines

courtesy of R. Wolfe

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Texturing Pipeline (Geometry + Rasterization)

  • Compute object space location (x, y, z) from screen space location (i, j)
  • Use projector function to obtain object surface coordinates (u, v) (3-D -> 2-D projection)
  • Use corresponder function to find texel coordinates (s, t) (2-D -> 2-D transformation)
    • Scale, shift, wrap like viewport transform in geometry pipeline
  • Filter texel at (s, t)
  • Modify pixel (i, j)

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list adapted from Akenine-Moller & Haines

courtesy of R. Wolfe

Rasterization

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Texture coordinates at vertices

  • Polygons can be treated as parametric patches by assigning texture coordinates to vertices
    • The standard OpenGL way

courtesy of

R. Wolfe

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Projector Functions

  • Want way to get from 3-D point to 2-D surface coordinates as an intermediate step
  • Idea: Project complex object onto simple object’s surface with parallel or perspective projection (focal point inside object)
    • Mesh: piecewise planar (default option

with texture coords)

    • Plane
    • Cylinder
    • Sphere
    • Cube

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Planar projector

courtesy of R. Wolfe

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Planar projector

Orthographic projection onto XY plane: u = x, v = y

...onto YZ plane

...onto XZ plane

courtesy of

R. Wolfe

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Cylindrical projector

  • Convert rectangular coordinates (x, y, z) to cylindrical (r, h, θ), use only (h, θ) to index texture image

courtesy of

R. Wolfe

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Spherical projector

  • Convert rectangular coordinates (x, y, z) to spherical (r, θ, Φ), use only (θ, Φ)

courtesy of R. Wolfe

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Making environment textures: Cube

  • Cube map straightforward to make: Render/ photograph six rotated views of environment
    • 4 side views at compass points
    • 1 straight-up view, 1 straight-down view

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Skybox demo

cube panorama