Texture Mapping

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Texture Mapping

• OpenGL Part III

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Texture Maps

• An array of pixels (an image)
• Coordinates (s, t)
• Sometimes called texels (texture elements)
• Geometric Object
• Has coordinates (x, y, z) for each point on
  surface
• Idea associate each (x, y, z) with a point (s,
  t) through a function
• Typically define texture coordinate for each
  vertex

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Building Texture Maps

• Define an image (either procedural or from image)
• Define texture coordinates when defining geometry
• Define parameters for how the texture should be
  applied

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1. Defining an Image

• GLubyte myimage64643
• void initImage()
• int c
• for (int i 0 i lt 64 i)
• for (int j 0 j lt 64 j)
• c ((((i0x8)0)((j0x8))0))255
• myimageij0 (GLubyte) c
• myimageij1 (GLubyte) c
• myimageij2 (GLubyte) c

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1. Defining an Image

• glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 64, 64, 0,
  GL_RGB, GL_UNSIGNED_BYTE, myimage)
• GL_TEXTURE_2D its a 2D image
• 0 level of texture map (mipmapping)
• GL_RGB internal image format (GL_RGBA)
• 64 rows in the texture (must be 2x)
• 64 cols in the texture (must be 2x)
• 0 border width
• GL_RGB type of texels
• GL_UNSIGNED_BYTE format of texels
• myimage pointer to array of image data

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2. Defining Texture Coordinates

• glBegin (GL_QUADS)
• glTexCoord2f (0.0, 0.0)
• glVertex3f(-1.0f, -1.0f, 0.0f)
• glTexCoord2f (0.0, 1.0)
• glVertex3f(-1.0f, 1.0f, 0.0f)
• glTexCoord2f (1.0, 1.0)
• glVertex3f(1.0f, 1.0f, 0.0f)
• glTexCoord2f(1.0, 0.0f)
• glVertex3f(1.0f, -1.0f, 0.0f)
• glEnd()

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Examples
Note Normals and texture coordinates are already
defined in glutSolidTeapot()
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3. Texture Parameters

• Usually, texture coordinates range from 0.0 to
  1.0
• Two modes
• Sometimes, we have to mod to get a texture
  coordinate (i.e. 1.1 becomes 0.1). This is
  GL_REPEAT
• Sometimes, we are strict, and round up or down
  (-0.4 become 0, 1.7 becomes 1). This is GL_CLAMP

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Examples
Note I changed the texture coordinates to go
from 0.0 to 2.0 on purpose
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3. Texture Parameters

• Sometimes, several texels map to the same point
  on the surface (minification)
• Sometimes, one texel maps to several points on
  the objects surface (magnification)
• Can specify to use the NEAREST or we can use
  bilinear filtering (blurring)

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Note on Texture Parameters
Passing GL_LINEAR
Passing GL_NEAREST
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3. Texture Parameters(only do this once)

• glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
  GL_REPEAT)
• glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
  GL_REPEAT)
• glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILT
  ER, GL_NEAREST)
• glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILT
  ER, GL_NEAREST)

Note we could use GL_CLAMP for our wrapping
parameter. This cuts off anything above 1.0 or
0.0
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glHint

• We have the option to request either speed or a
  better-looking image
• // Could be GL_FASTEST
• glHint(GL_PERSPECTIVE_CORRECTION_HINT,
  GL_NICEST)

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Texture Objects

• By default, there is a current texture on the
  graphics card
• What if there are two textures?
• Constantly updating the texture (slow)
• Must reload the texture each time!
• Instead, create a texture object
• Store a texture (and parameters) on the card
• Each texture is then assigned a unique ID

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Typical Example

• glGenTextures (5, myid)
• // Start binding a texture, and store on
  graphics card
• glBindTexture (GL_TEXTURE_2D, myid)
• glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 64, 64,
  0, GL_RGB, GL_UNSIGNED_BYTE, myimage)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_WRAP_S, GL_CLAMP)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_WRAP_T, GL_CLAMP)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_MAG_FILTER, GL_NEAREST)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_MIN_FILTER, GL_NEAREST)
• // ----------------------------All the way down
  to here-------------------------------------------
  
• // Get a second number
• glGenTextures (5, yourid)
• glBindTexture (GL_TEXTURE_2D, yourid)
• glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 64, 64,
  0, GL_RGB, GL_UNSIGNED_BYTE, yourimage)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_WRAP_S, GL_CLAMP)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_WRAP_T, GL_CLAMP)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_MAG_FILTER, GL_NEAREST)
• glTexParameteri(GL_TEXTURE_2D,
  GL_TEXTURE_MIN_FILTER, GL_NEAREST)

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• void display()
• glClear (GL_COLOR_BUFFER_BITGL_DEPTH_BUFFER_BIT)
  
• glEnable(GL_TEXTURE_2D)
• glMatrixMode(GL_MODELVIEW)
• glPushMatrix()
• glLoadIdentity()
• glTranslatef(-1.0, 0.0, -5.0)
• glPushMatrix()
• glRotatef (deg, 0.0, 1.0, 0.0)
• glBindTexture(GL_TEXTURE_2D, myid)
• glutSolidTeapot (1.0)
• glPopMatrix()
• glPushMatrix()
• glTranslatef(2.0, 0.0, 0.0)
• glRotatef(deg, 0.0, 1.0, 0.0)
• glBindTexture(GL_TEXTURE_2D, yourid)
• glutSolidTeapot (1.0)
• glPopMatrix()

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Example