Texture Mapping Fall, 2011

Textures Describe color variation in interior of 3D polygon  When scan converting a polygon, vary pixel colors according to values fetched from a texture

The Quest for Visual Realism

Surface Textures Add visual detail to surfaces of 3D objects

3D Rendering Pipeline Texture Mapping

Texture Mapping Overview Texture mapping methods  Parameterization  Mapping  Filtering Texture mapping applications  Modulation textures  Illumination mapping  Bump mapping  Environment mapping  Image-based rendering  Non-photorealistic rendering

Parameterization Q: How do we describe where on the geometry each color from the image should go?

Option: Varieties of projections Cylinder MappingSphere MappingCube MappingPlane Mapping

Option: unfold the surface

Option: make an atlas

Texture Mapping Overview Texture mapping methods  Parameterization  Mapping  Filtering Texture mapping applications  Modulation textures  Illumination mapping  Bump mapping  Environment mapping  Image-based rendering  Non-photorealistic rendering

Texture Mapping Steps:  Define texture  Specify mapping from texture to surface  Lookup texture values during scan conversion

Texture Mapping When scan convert, map from …  Image coordinate system (x, y) to  Modeling coordinate system (u, v) to  Texture image (t, s)

Texture Mapping Texture mapping is a 2D projective transformation  Texture coordinate system (t, s) to  Image coordinate system (x, y)

Texture Mapping Scan conversion  Interpolate texture coordinates down/across scan lines  Distortion due to bilinear interpolation approximation Cut polygons into smaller ones, or Perspective divide at each pixel

Texture Mapping Linear interpolationPerspective-correct interpolation

Texture Mapping Overview Texture mapping methods  Parameterization  Mapping  Filtering Texture mapping applications  Modulation textures  Illumination mapping  Bump mapping  Environment mapping  Image-based rendering  Non-photorealistic rendering

Texture Filtering Must sample texture to determine color at each pixel in image

Texture Filtering Aliasing is a problem Point SamplingArea Filtering

Texture Filtering Ideally, use elliptically shaped convolution filters

Texture Filtering Size of filter depends on projection warp  Can prefiltering images Mip maps Summed area tables

Mip Maps Keep textures prefiltered at multiple resolutions  For each pixel, linearly interpolate between two closest levels (e. g. trilinear filtering)  Fast, easy for hardware u v d

Summed-area tables At each texel keep sum of all values down & right  To compute sum of all values within a rectangle, simply subtract two entries  Better ability to capture very oblique projections  But, cannot store values in a single byte

Texture Mapping Overview Texture mapping methods  Parameterization  Mapping  Filtering Texture mapping applications  Modulation textures  Illumination mapping  Bump mapping  Environment mapping  Image-based rendering  Non-photorealistic rendering

Modulation textures Map texture values to scale factor

Illumination Mapping Map texture values to surface material parameter  K A  K D  K S  K T  n Key-framemodelgeometryKey-framemodelgeometry Decal skin Bump skin Gloss skin WOW!ResultWOW!Result ++

Bump Mapping Texture values perturb surface normals geometry Bump map Stores heights: can derive normals + Bump mapped geometry =

Bump Mapping

Displacement Mapping Normal Mapping Problem  Doesn’t take into account geometric surface depth Does not exhibit parallax No self-shadowing of the surface Coarse silhouettes expose the actual geometry being drawn Displacement Mapping  Displace actual positions from Heightfield Map

Displacement Mapping (Result) Displacement Offset

Environment Mapping Texture values are reflected off surface patch

Cube Mapping Cube mapping is the norm nowadays x y z n eye

Image-Based Rendering Map photographics textures to provide details for coarsely detailed polygon model

Solid Textures Texture values indexed by 3D location (x, y, z)  Expensive storage, or  Compute on the fly  ex) Perlin Noise

Texture Mapping Summary Texture mapping methods  Parameterization  Mapping  Filtering Texture mapping applications  Modulation textures  Illumination mapping  Bump mapping  Environment mapping  Image-based rendering  Volume textures

Example) Quake 2 Models: Piece by Piece Model Key Frames Quake 2 interpolates between key frames

Example) Quake 2 Models: Piece by Piece Knight model’s decal skin Texture coordinates map triangles to skin. Note clever packing of decal skin. Only half face because triangles “mirror” the face.

Example) Standard Quake 2: Model Rendering Texture key-frame with decal skin ++ ResultResult Key-framemodelgeometryKey-framemodelgeometry Decal skin

Example) GPU Bump-mapped: Quake 2 Model Rendering! Bump-map model with bump, gloss, & decal skin Key-framemodelgeometryKey-framemodelgeometry Decal skin Bump skin Gloss skin WOW!ResultWOW!Result ++

Example) Quake 2 Bump Mapping: Rendering Passes   ) + ( (( ) = DiffuseDiffuseGlossGlossSpecularSpecularDecalDecal Final result!

Example) More Bump-mapped Knight Examples Different light positions and key-frames All lighting including specular is computed per-fragment!

Example) Bump Mapping Models Viva la difference

OpenGL Texture Mapping Texture Mapping in OpenGL  Allows you to modify the color of a polygon surface  Textures are simply rectangular arrays of data (color, luminance, color+alpha). Individual values in a texture are called texels

OpenGL Texture Mapping : Step Steps necessary to use texture mapping:  Create a texture object and specify the texture  Indicate how the texture is to be applied to each pixel  Enable texture mapping  Draw the scene, supplying both texture and geometric coordinates

OpenGL Texture Mapping : 2D Texture Specification 2D Texture Specification  glTexImage2D(GLenum target, GLint level, GLint internalFormat, GLsizei width, GLsizei height, Glint border, GLenum format, Glenum type, const Glvoid *pixels); target: GL_TEXTURE_2D level: specifies the level of detail when using multi resolution textures. “0” is the base image, “n” is the n-th mipmap reduction image internalFormat: an integer 1 to 4, or one of 38 symbolic constants width, height: the dimensions of the texture (MUST BE power of 2) format: the kind of pixel-data elements type: the data-type of each element pixels: array containing the texture image data Ex) glTexImage2D(GL_TEXTURE_2D, 0, 3, iwidth, iheight, 0, GL_RGB, GL_UNSIGNED_BYTE, image);

OpenGL Texture Mapping : Values for Format and Type Format Constants:  GL_COLOR_INDEX: A single color index  GL_RGB: A red component, followed by green & blue components  GL_RGBA: Like GL_RGB, followed by an alpha component.  GL_RED: A single red-color component  GL_GREEN: A single green-color component  GL_BLUE: A single blue-color component  GL_ALPHA: A single alpha-color component Type Constants:  GL_UNSIGNED_BYTE: unsigned 8-bit integer  GL_BYTE: signed 8-bit integer  GL_UNSIGNED_SHORT: unsigned 16-bit integer  GL_SHORT: signed 16-bit integer  GL_INT: signed 32-bit integer  GL_FLOAT: single-precision floating point

OpenGL Texture Mapping : Texture Coordinates Texture Coordinates  You need to specify BOTH texture & geometric coordinates as you specify the object in your scene glColor3f(1.0f, 0.0f, 0.0f); glBegin(GL_TRIANGLES); glNormal3d(0,0,-1); glTexCoord2d(0,0); glVertex3d(-100,-100,0); glTexCoord2d(1,0); glVertex3d(100,-100,0); glTexCoord2d(0,1); glVertex3d(-100,100,0); glEnd();

OpenGL Texture Mapping : Texture Filtering glTexParameteri( target, type, mode)  Target : GL_TEXTURE_1D, GL_TEXTURE_2D  Type : GL_TEXTURE_MIN_FILTER  Mode : GL_NEAREST, GL_LINEAR, GL_NEAREST_MIPMAP_NEAREST, GL_LINEAR_MIPMAP_NEAREST, GL_NEAREST_MIPMAP_LINEAR GL_LINEAR_MIPMAP_LINEAR  Type : GL_TEXTURE_MAG_FILTER GL_NEAREST or GL_LINEAR Ex) glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MAG_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXURE_MIN_FILTER, GL_LINEAR);

Min / Max Filter : GL_NEAREST Min / Max Filter : GL_LINEAR No mipmappingMipmappping

OpenGL Texture Mapping : Texture Wrapping glTexParameteri( target, type, mode)  Target : GL_TEXTURE_1D, GL_TEXTURE_2D  Type GL_TEXTURE_WRAP_S GL_TEXTURE_WRAP_T  mode GL_CLAMP or GL_REPEAT Wrap S : GL_CLAMP Wrap T : GL_CLAMP Wrap S : GL_CLAMP Wrap T : GL_REPEAT Wrap S : GL_REPEAT Wrap T : GL_CLAMP Wrap S : GL_REPEAT Wrap T : GL_REPEAT

OpenGL Texture Mapping : Texturing Functions Texturing Functions  Indicate how the texture is applied to each pixel:  REPLACE or DECAL: Texture is painted on top of the fragment  MODULATE: Combine texture with fragment color. This technique is useful to combine the effects of lighting with texturing  BLEND: A constant color is blended with that of the fragment, based on the texture value Ex) glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);

OpenGL Texture Mapping : Texture Functions