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14-textures.md

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Textures

  • Textures — patterns that could be used to add realism to a scene
  • Texture Elements (Texels) — arrays of textures
  • Texture Coordinates — s and t, used to scale textures according to the interval [0,1]

Texture Mapping in OpenGL

  1. Create a texture object and specify a texture for that object
  2. Indicate how texture is to be applied to each pixel
    • GL_MODULATE: multiply color components
    • GL_BLEND: linearly blend color components
    • GL_DECAL: use texture color in RGB (if with alpha, interpolates texel and fragment color)
    • GL_REPLACE: use the texture color
  3. Enable texture mapping
  4. Draw the scene, supplying both texture and geometric coordinates

Binding and Loading

glEnable(GL_TEXTURE_2D);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
GLUint texName;
glGenTextures(1, &texName);
glBindTexture(GL_TEXTURE_2D, texName);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, ImageWidth, ImageHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, ImageData);

Setting Texture Params

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
// texture magnification
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
// texture minification
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

MIP Maps

  • mipmaps (or pyramids) are pre-calculated sequences of images, each of which is a progressively lower resolution representation of the same image
  • Textured objects can be viewed, like any other objects in a scene, at different distances from the viewpoint — in a dynamic scene, as a textured object moves further away from the viewpoint, the texture map must decrease in size along with the size of the projected image
  • To avoid such artefacts, you can specify a series of pre-filtered texture maps of decreasing functions, called mipmaps
  • To specify these textures, call glTexImage2D() once for each resolution of the texture map, with different values for the level, width, height and image paramters. Starting with zero, level identifies which texture in the series is specified
  • void glGenerateMipmap(GLenum target)
  • void glGenerateTextureMipmap(GLuint texture)