Textures in OpenGL

Lecture 32Mon, Dec 1, 2003

Creating an Image Internally

To create a 64 x 64 black and white checkerboard, declare the array

unsigned char checkerboard[64][64][3];

Then, for each i and j, assigncheckboard[i][j][0] = 0; checkboard[i][j][1] = 0;checkboard[i][j][2] = 0;

to set the color to black, or

Creating an Image Internally

checkboard[i][j][0] = 255; checkboard[i][j][1] = 255;checkboard[i][j][2] = 255;

to set the color to white.A clever way to set the color is to set each component to white if and only if

((i & 8) ^ (j & 8)) == 0.

Reading an Image from a File

The RgbImage class has a constructor with prototype

RgbImage(char* filename);filename should be the name of a .bmp file.This constructor will read the RGB values from the file.Note that they are RGB, not RGBA.

OpenGL Texture Functions

The OpenGL texture functions typically set the values of various state variables.Therefore, they can be called in any order, just so all the relevant variables have been set by the time the scene is rendered.

Defining the Current Texture

To define the current texture, write

glTexImage2D(GL_TEXTURE_2D,0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, ptr);

Defining the Current Texture

The first 0 means mipmap level 0.The first GL_RGB is the format to be used in the texture.The second 0 means no border.The second GL_RGB is the format used in the image.ptr is a pointer to the texture. Specifically, a pointer to an unsigned

byte.

Mapping a Texture to a Polygon

Consider the simpler case of mapping a texture to a rectangle.Use the function glTexCoord2f(s, t).The specified texture coordinates (s, t) will be mapped to all vertices drawn, until the texture coordinates are changed.

Example

To map the texture coordinate (0, 0) to the vertex with coordinates (0, 1, 1), write

glTexCoord2f(0.0, 0.0);glVertex3f(0.0, 1.0, 1.0);

Mapping a Texture to a Polygon

To map the entire texture to the rectangle with corners at (0, 0, 0), (4, 0, 0), (4, 4, 0), and (0, 4, 0), write

glBegin(GL_QUADS);glTexCoord2f(0, 0); glVertex(0, 0, 0);glTexCoord2f(1, 0); glVertex(4, 0, 0);glTexCoord2f(1, 1); glVertex(4, 4, 0);glTexCoord2f(0, 1); glVertex(0, 4, 0);

glEnd();

Mapping a Texture to a Polygon

Wrapping a Texture

What should OpenGL do if the texture is “smaller” than the polygon?(How can that be?)In the last example, we could have mapped (0, 0) to (0, 0, 0) (2, 0) to (4, 0, 0) (2, 2) to (4, 4, 0) (0, 2) to (0, 4, 0)

Wrapping a Texture

0 2

0

2

Polygon

?

? ?

Wrapping a Texture

To tell OpenGl to repeat (tile) the texture in each block, we should write

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);

Example: Using GL_REPEAT

0 2

0

2

Polygon

Wrapping a Texture

Another choice, instead of GL_REPEAT, is GL_CLAMP.This causes only the edge pixels to be repeated.

Example: Using GL_CLAMP

0 2

0

2

Polygon

Pasting a Texture onto a Polygon

There are several modes we can use when pasting the texture onto the polygon. GL_DECAL. GL_REPLACE. GL_MODULATE. GL_BLEND.

Pasting a Texture onto a Polygon

We will use GL_REPLACE.That will replace the color of the surface with the color of the texture.If we use GL_BLEND, it will mix the surface color with the texture.

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

Magnification and Minification Filters

The term “filter” comes from signal theory, which provides a mathematical model of the problems faced in discrete sampling.We should tell OpenGL which kind of magnification and which kind of minification we want it to do.

Magnification Filter

When a texture is magnified, the two choices are Use the nearest texel. Use a weighted average of the

nearest 2 2 array of texels (interpolation).

Magnification Filter

To set the magnification filter, we should write one of the following.

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

// orglTexParameteri(GL_TEXTURE_2D,

GL_TEXTURE_MAG_FILTER, GL_LINEAR);

Minification Filter

If we are not using mipmaps, then the minification choices are similar.

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);

// orglTexParameteri(GL_TEXTURE_2D,

GL_TEXTURE_MIN_FILTER, GL_LINEAR);