beginning opengl game programming 2004 phần 6 pps

By default, you have to specify all levels starting from level 0 to the level at which the tex￾ture becomes 1 × 1 (which is equivalent to log2 of the largest dimension of the base tex￾ture). You can, however, change these limits by using the glTexParameter() function with its

pname parameter set to GL_TEXTURE_BASE_LEVEL or GL_TEXTURE_MAX_LEVEL, respectively. The

value passed to either of these parameters must be a positive integer.

Mipmapping is first enabled by specifying one of the mipmapping values for texture

minification. Once the texture minification filter has been set, you then only need to spec￾ify the texture mipmap levels with one of the glTexImage3D(), glTexImage2D(), or glTexIm￾age1D() functions, depending on your texture dimensionality. The following example code

sets up a seven-level mipmap with a minification filter of GL_NEAREST_MIPMAP_LINEAR and

starting at a 64 × 64 base texture:

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_LINEAR);

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 64,64,0, GL_RGB, GL_UNSIGNED_BYTE, texImage0);

glTexImage2D(GL_TEXTURE_2D, 1, GL_RGB, 32,32,0, GL_RGB, GL_UNSIGNED_BYTE, texImage1);

glTexImage2D(GL_TEXTURE_2D, 2, GL_RGB, 16,16,0, GL_RGB, GL_UNSIGNED_BYTE, texImage2);

glTexImage2D(GL_TEXTURE_2D, 3, GL_RGB, 8, 8, 0, GL_RGB, GL_UNSIGNED_BYTE, texImage3);

glTexImage2D(GL_TEXTURE_2D, 4, GL_RGB, 4, 4, 0, GL_RGB, GL_UNSIGNED_BYTE, texImage4);

glTexImage2D(GL_TEXTURE_2D, 5, GL_RGB, 2, 2, 0, GL_RGB, GL_UNSIGNED_BYTE, texImage5);

glTexImage2D(GL_TEXTURE_2D, 6, GL_RGB, 1, 1, 0, GL_RGB, GL_UNSIGNED_BYTE, texImage6);

Mipmaps and the OpenGL Utility Library

The GLU library provides the gluBuild2DMipmaps() and gluBuild1DMipmaps() functions to

build mipmaps automatically for two- and one-dimensional textures, respectively. These

Mipmaps 167

Figure 7.6 Mipmaps help control the level of detail for textured objects.

07 BOGL_GP CH07 3/1/04 10:03 AM Page 167

TLFeBOOK

functions replace the set of function calls you would normally make to the glTexImage2D()

and glTexImage1D() functions to specify mipmaps.

int gluBuild2DMipmaps(GLenum target, GLint components, GLint width, GLint height,

GLenum format, GLenum type, const void *data);

int gluBuild1DMipmaps(GLenum target, GLint components GLint width, GLenum format,

GLenum type, const void *data);

One of the nice features about these functions is that you do not have to pass a power-of￾2 image because gluBuild2DMipmaps() and gluBuild1DMipmaps() automatically rescale your

images’ width and height to the closest power of 2 for you.

The following code uses the gluBuild2DMipmaps() function to specify mipmaps in the same

way as the previous mipmap example using glTexImage2D():

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_LINEAR);

gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGB, 64, 64, GL_RGB, GL_UNSIGNED_BYTE, texImage0);

Automatic Mipmap Generation

Extension

Extension name: SGIS_generate_mipmap

Name string: GL_SGIS_generate_mipmap

Promoted to core: OpenGL 1.4

Function names: None

Tokens: GL_GENERATE_MIPMAP_SGIS

As of Version 1.4, OpenGL has introduced a new method for automatically generating

mipmaps with the texture parameter GL_GENERATE_MIPMAP. Setting this parameter to GL_TRUE

will induce a mechanism that automatically generates all mipmap levels higher than the

base level. The internal formats and border widths of the derived mipmap images all

match those of the base level image, and each increasing mipmap level reduces the size of

the image by half. The actual contents of the mipmap images are computed by a repeated,

filtered reduction of the base mipmap level.

One of the nice features of this parameter is that if you change the texture image data, the

mipmap data is calculated automatically, which makes it extremely useful for textures

you’re changing on the fly.

We are actually discussing texture parameters in the next section. This means you should

read on to find out how to use the automatic mipmap generation functionality of

OpenGL!

168 Chapter 7 ■ Texture Mapping

07 BOGL_GP CH07 3/1/04 10:03 AM Page 168

TLFeBOOK

Texture Parameters

OpenGL provides several parameters to control how textures are treated when specified,

changed, or applied as texture maps. Each parameter is set by calling the glTexParameter()

function (as mentioned in the section “Texture Filtering”):

void glTexParameter{if}(GLenum target, GLenum pname, TYPE param);

void glTexParameter{if}v(GLenum target, GLenum pname, TYPE params);

As mentioned before, the value of the target parameter refers to the texture target and can

be equal to GL_TEXTURE_1D, GL_TEXTURE_2D, GL_TEXTURE_3D, or GL_TEXTURE_CUBE_MAP. The pname

parameter is a constant indicating the parameter to be set, a list of which is shown in Table

7.5. In the first form of the glTexParameter() function, param is a single-valued parameter;

in the second form, params is an array of parameters whose type depends on the parame￾ter being set.

Texture Parameters 169

Table 7.5 Texture Parameters

Name Type Values

GL_TEXTURE_WRAP_S integer GL_CLAMP, GL_CLAMP_TO_EDGE*, GL_REPEAT,

GL_CLAMP_TO_BORDER*, GL_MIRRORED_REPEAT*

GL_TEXTURE_WRAP_T integer GL_CLAMP, GL_CLAMP_TO_EDGE*, GL_REPEAT,

GL_CLAMP_TO_BORDER*, GL_MIRRORED_REPEAT*

GL_TEXTURE_WRAP_R integer GL_CLAMP, GL_CLAMP_TO_EDGE*, GL_REPEAT,

GL_CLAMP_TO_BORDER*, GL_MIRRORED_REPEAT*

GL_TEXTURE_MIN_FILTER integer GL_NEAREST, GL_LINEAR, GL_NEAREST_MIPMAP_NEAREST,

GL_NEAREST_MIPMAP_LINEAR, GL_LINEAR_MIPMAP_NEAREST,

GL_LINEAR_MIPMAP_LINEAR

GL_TEXTURE_MAG_FILTER integer GL_NEAREST, GL_LINEAR

GL_TEXTURE_BORDER_COLOR 4 floats any 4 values from 0 to 1

GL_TEXTURE_PRIORITY float any value from 0 to 1

GL_TEXTURE_MIN_LOD* float any value

GL_TEXTURE_MAX_LOD* float any value

GL_TEXTURE_BASE_LEVEL* integer any non-negative integer

GL_TEXTURE_MAX_LEVEL* integer any non-negative integer

GL_TEXTURE_LOD_BIAS* float any value

GL_DEPTH_TEXTURE_MODE** enum GL_LUMINANCE, GL_INTENSITY, GL_ALPHA

GL_TEXTURE_COMPARE_MODE** enum GL_NONE, GL_COMPARE_R_TO_TEXTURE

GL_TEXTURE_COMPARE_FUNC** enum GL_LEQUAL, GL_GEQUAL, GL_LESS, GL_GREATER, GL_EQUAL,

GL_NOTEQUAL, GL_ALWAYS, GL_NEVER

GL_GENERATE_MIPMAP* boolean GL_TRUE or GL_FALSE

* Available only via extensions under Windows. See the explanation of the parameter in this chapter for details.

** Available only via the ARB_depth_texture and ARB_shadow extensions under Windows.

07 BOGL_GP CH07 3/1/04 10:03 AM Page 169

TLFeBOOK