Environment Mapping

Examples

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MotivationSilver candlestick No appropriate texture for it “environment” map

Simulates the results of ray-tracing without going through expensive ray-tracing computation

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Reflection Mapping (wikipedia)

Standard Environment Mapping in which a single texture contains the image of the surrounding as reflected on a mirror ballCubic Environment Mapping in which the environment is unfolded onto the six faces of a cube and stored therefore as six square textures.

A typical drawback of these techniques is the absence of self reflections: you cannot see any part of the reflected object inside the reflection itself.

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What’s available in OpenGL

SphereMap Cubemap

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Cube Map

Encode the environment with a cube map

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Overview1/2

The texture is a set of six 2D images representing the faces of a cube. These targets must be consistent, complete, and have equal width and height.The texture coordinate (s,t,r) can be generated with (1) reflection map, or (2) normal mapThe (s,t,r) texture coordinates are treated as a direction vector emanating from the center of a cube.

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Overview2/2

The interpolated per-fragment (s,t,r) selects one cube face 2D image based on the largest magnitude coordinate (the major axis). A new 2D (s,t) is calculated by dividing the two other coordinates (the minor axes values) by the major axis value. Then the new (s,t) is used to lookup into the selected 2D texture image face of the cube map.

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Example: (s,t,r)(s,t)

(2,0,1)

Major

sc tc ma

+ry -1 0 2

s = (-1/2 + 1)/2 = ¼t = (0/2 + 1)/2 = ½

st

x

y

z[1/4,1/2]

(s,t,r) need not be normalized!

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Two TexGen Modes

REFLECTION_MAP(s,t,r): vertex’s eye-space reflection vector

NORMAL_MAP(s,t,r): vertex’s transformed eye-space normal

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Reflection Map [Eye Space]

ne

r

Every vertex (fragment) needs to compute the corresponding reflection vector as tex coord. (more accurate but time consuming)

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Normal Map

ne

Use the normal vector (world or eye) to index into the cube map. Quickly get an environment map feel, but not a real reflection.

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Cubemap Textures (ref)

Debug Snow

Red sky

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Other Applications of Cube Maps

Stable specular highlights A better alternative over massive

over tessellation Limited to distant specular lights

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Other Applications (cont)

Sky illuminationDynamic cube map reflectionsPer-pixel shading (w/o shader)

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Front face color

Debug Cubemap Settings

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Other Environment MapsSphere map (OpenGL) View dependent (different sphere map

required for different eye position) Authoring texture images is non-obvious

(require special image warping)

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Spheremap

involves the use of a textured sphere infinitely far away from the object that reflects it. By creating a spherical texture using a fisheye lens or via prerendering or with a light probe, this texture is mapped to a hollow sphere, and the texel colors are determined by calculating the light vectors from the points on the object to the texels in the environment map. This technique may produce results which are superficially similar to those produced by raytracing, but incurs less of a performance hit because all of the colors of the points to be referenced are known beforehand, so all it has to do is to calculate the angles of incidence and reflection.There are a few glaring limitations to spherical mapping. For one thing, due to the nature of the texture used for the map, there is an abrupt point of singularity on the backside of objects using spherical mapping.

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Sphere Map (from Spec)

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Sphere Map

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Sphere Map (cont)

x 0 1 -1 1 1 -1 0 0 1 0y 0 0 0 1 -1 1 1 -1 0 1z 1 1 1 1 1 1 1 1 0 0xh 0 0.707107 -0.70711 0.57735 0.57735 -0.57735 0 0 1 0yh 0 0 0 0.57735 -0.57735 0.57735 0.707107 -0.70711 0 1zh 1 0.707107 0.707107 0.57735 0.57735 0.57735 0.707107 0.707107 0 0m 2 1.847759 1.847759 1.776148 1.776148 1.776148 1.847759 1.847759 1.414214 1.414214s 0.5 0.691342 0.308658 0.662529 0.662529 0.337471 0.5 0.5 0.853553 0.5t 0.5 0.5 0.5 0.662529 0.337471 0.662529 0.691342 0.308658 0.5 0.853553

(x,y,z): the reflection vector r (in eye coord)

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GLSL Implementation

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Generating Sphere Map

Fisheye lens

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FishEye

PS: Filter/distort/spherize

Fisheye lens/camera

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Add Specular with Sphere Map

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Adding Silhouettes with Sphere Map

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[bug?!]

Somehow, the results from ShaderMaker is wrong!?

The result from OpenGL & glslis correct

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