Texture maps

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					Introduction to Computer Graphics
            Texture Maps
Texture Mapping
Limited ability to generate complex surfaces with
Images can convey the illusion of geometry
Images painted onto
  polygons is called texture
Texture Maps
Chapter 9 of Open GL Programming Guide (Red
Images applied to polygons to enhance the visual
  effect of a scene
  • Rectangular arrays of data
     – Color, luminance, alpha
     – Components of array called texels
         We’ve also had volumetric voxels
Texture Mapping
Texture map is an image, two-dimensional array of color
  values (texels)
Texels are specified by texture’s (u,v) space
At each screen pixel, texel can be used to substitute a
  polygon’s surface property (color)
We must map (u,v) space to polygon’s (s, t) space
       V                       T

           U                   S
Texture Mapping
(u,v) to (s,t) mapping can be explicitly set at
  vertices by storing texture coordinates with
  each vertex
How do we compute (u,v) to (s,t) mapping for
 points in between
  • Watch for aliasing
  • Watch for many to one mappings
  • Watch for perspective foreshortening effects and linear
Example Texture Map

                      Applied to tilted polygon
Example Texture Map

                                 glVertex3d (s, s, s)

              glVertex3d (-s, -s, -s)
Example Texture Map

 glVertex3d (s, s, s)
 glTexCoord2d(5, 5);

 glVertex3d (s, s, s)
 glTexCoord2d(1, 1);
Texture Coordinates
Every polygon has object coordinates and texture
  • Object coordinates describe where polygon vertices are on
    the screen
  • Texture coordinates describe texel coordinates of each
    vertex (usually 0 -> 1)
  • Texture coordinates are interpolated along vertex-vertex
glTexCoord{1234}{sifd}(TYPE coords)
Texture Object
  • An OpenGL data type that keeps textures resident in memory and
    provides identifiers to easily access them
  • Provides efficiency gains over having to repeatedly load and reload a
  • You can prioritize textures to keep in memory
  • OpenGL uses least recently used (LRU) if no priority is assigned
Example use of Texture
 Read .bmp from file
   • Use Image data type
      – getc() and fseek() to read image x & y size
      – fread() fills the Image->data memory with actual
        red/green/blue values from .bmp

   • Note
      – malloc() Image->data to appropriate size
      – .bmp stores color in bgr order and we convert to rgb order
Step 2 – create Texture Objects

glGenTextures(1, &texture[texture_num]);

  • First argument tells GL how many Texture Objects to
  • Second argument is a pointer to the place where
    OpenGL will store the names (unsigned integers) of the
    Texture Objects it creates
     – texture[] is of type GLuint
  Step 3 – Specify which texture
  object is about to be defined
Tell OpenGL that you are going to define the
 specifics of the Texture Object it created
  • glBindTexture(GL_TEXTURE_2D, texture[texture_num]);

    – Textures can be 1D and 3D as well
  Step 4 – Begin defining texture
  • Sets various parameters that control how a texture is treated as it’s
    applied to a fragment or stored in a texture object
  • // scale linearly when image bigger than texture
  • // scale linearly when image smaller than texture
Step 5 – Assign image data
  • glTexImage2D();
   GL_TEXTURE_2D      (2D Texture)
   0                  (level of detail 0)
   3                  (3 components, RGB)
   image1->sizeX      (size)
   image1->sizeY      (size)
   0                  (no border pixel)
   GL_RGB             (RGB color order)
   GL_UNSIGNED_BYTE   (unsigned byte data)
   image1->data       (pointer to the data))
glTexImage2D – Arg 1
GLenum target
    – Provides queries for texture resources
    – Proceed with hypothetical texture use (GL won’t apply
      the texture)
    – After query, call GLGetTexLevelParamter to verify
      presence of required system components
    – Doesn’t check possibility of multiple texture interference
glTexImage2D – Arg 2
GLint level
  • Used for Level of Detail (LOD)
  • LOD stores multiple versions of texture that can be used at
    runtime (set of sizes)
  • Runtime algorithms select appropriate version of texture
     – Pixel size of polygon used to select best texture
     – Eliminates need for error-prone filtering algorithms
glTexImage2D – Arg 3
GLint internalFormat
  • Describes which of R, G, B, and A are used in internal
    representation of texels
  • Provides control over things texture can do
     – High bit depth alpha blending
     – High bit depth intensity mapping
     – General purpose RGB
  • GL doesn’t guarantee all options are available on given
glTexImage2D – Args 4-6
GLsizei width
GLsizei height
  • Dimensions of texture image
     – Must be 2m + 2b (b=0 or 1 depending on border)
     – min, 64 x 64
GLint border
  • Width of border (1 or 0)
     – Border allows linear blending between overlapping textures
     – Useful when manually tiling textures
glTexImage2D – Args 7 & 8
GLenum format
  • Describe how texture data is stored in input array
GLenum type
  • Data size of array components
glTexImage2D – Arg 9
Const GLvoid *texels
  • Pointer to data describing texture map
Step 6 – Apply texture
Before defining geometry
  • glEnable(GL_TEXTURE_2D);
  • glBindTexture(GL_TEXTURE_2D, texture[0]);
  • glTexEnvf(GL_TEXTURE_ENV,

  First argument to function is always GL_TEXTURE_ENV

    GL_TEXTURE_ENV_MODE                    GL_DECAL




    If GL_BLEND selected, second call to   4-float array for R,G,B,A
    glTexEnv() must specify                blend
Alters the size of an image to meet the 2m size
  requirement of OpenGL
  • Scaling performed by linear and box filtering

Use current frame buffer contents as texture
Copy frame buffer to named texture location
Replace a region of current working texture with a
  smaller texture
SubImage need not adhere to 2m size limitation
This is how you add data from your system’s
  camera to GL environment
  • Frame buffer cut and paste possible too
Bump Mapping
Use textures to modify surface geometry
Use texel values to modify surface normals of
Texel values correspond to height field
  • Height field models a rough surface
Partial derivative of bump map specifies change
 to surface normal
Bump Mapping
Displacement Mapping
Bump mapped normals are inconsistent with actual
 geometry. Problems arise (shadows).
Displacement mapping actually affects the surface
multum in parvo -- many things in a small place
A texture LOD technique
Prespecify a series of prefiltered texture maps of
  decreasing resolutions
Requires more texture storage
Eliminates shimmering and flashing as objects
With versus without MIPMAP
Arrange different versions into one block of
Automatically constructs a family of textures from
 original texture size down to 1x1
Advanced Mipmaps

 You can specify additional mipmap levels on
  the fly
   • MIN_LOD may reduce popping
   • MAX_LOD may reduce over compression
 You can specify min mipmap level
   • Useful for mosaicing (Alphabet on a texture)
OpenGL tries to pick best mipmap level
Question: Which texel corresponds to a particular pixel?
GL_NEAREST (Point Sampling)
  • Pick the texel with center nearest pixel
GL_LINEAR (Bilinear Sampling)
  • Weighted average of 2x2 closest texels
  • Average nearest texels from two mipmap levels
  • Average two averaged texels from two mipmaps

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