Texturing

Textures vs. Images

Drawing an image (glDrawPixels)

Applies transformations to the glRasterPos to find the starting point

After that, skips the 3D rendering pipeline and just copies pixels directly from the image to the framebuffer

Texturing

Is a stage in the 3D rendering pipeline, for polygon data

Determines the color of each pixel when filling in a polygon

Texturing

Applying textures in OpenGL involves the following steps:

glEnable(GL_TEXTURE_2D)

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_FILTER, GL_LINEAR)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR)
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height,
             0, GL_RGB, GL_UNSIGNED_BYTE, imageData)

glTexCoord2f(0,0)
glVertex3f(-1.0, -1.0, 0.0)
glTexCoord2f(1,0)
glVertex3f(1.0, -1.0, 0.0)

Named Textures

Defining a texture transfers the image data to the graphics card

Named textures let you do this once, and then use the texture again later by referring to its ID (its "name").

textureID1 = glGenTextures(1)

glBindTexture(GL_TEXTURE_2D, textureID1)
glTexImage2D(...)
glBindTexture(GL_TEXTURE_2D, 0)

...
glBindTexture(GL_TEXTURE_2D, textureID1)
# Draw textured object

glTexImage2D

glTexImage2D(target, level, internalFormat, width,
             height, border, format, type, pixels)

Underlined arguments are equivalent to those for glDrawPixels

target - GL_TEXTURE_2D
level - used in mipmapping; set to 0
internalFormat - usually the same as format; can be used for optimization
border - normally 0; set to 1 when image data includes a border

Important: The width & height of a texture image must be powers of 2 - i.e., 2, 4, 8, 16, 32, 64, 128, 256, 512, etc.
(except when a border is used - then it's 2 plus a power of 2)

Texture Coordinates

Objects need texture coordinates to define how a texture is applied

Texture coordinates form a coordinate system on the texture image, ranging from 0 to 1 in each direction

They are referred to as S and T
(to distinguish them from X & Y geometric coordinates)

Texture Coordinates

One texture coordinate should be assigned for each vertex

This defines what part of the texture should be used at that point

When a polygon is filled in, the vertices' texture coordinates are interpolated across its surface, to find the texture data for each pixel

Texture Coordinates

A texture does not have to be evenly applied to a polygon.

The image can appear distorted, either as a result of the geometry or the texture coordinates used.

Texture Tiling

Texture coordinates outside the range 0 - 1 can be used to tile a texture

Texture Tiling

Tiling is controlled by the WRAP texture parameter:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
                   GL_REPEAT)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
                   GL_REPEAT)

for no tiling:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
                   GL_CLAMP)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
                   GL_CLAMP)

This document is by Dave Pape, and is released under a Creative Commons License.