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OpenGL学习（七）纹理映射
纹理映射
一m*n的像素数组，我们并不将其看做有离散元素构成的数组，而是将其视作一个连续数组。该数组中的任意一点通过变量s和t来定义。则每个坐标(s,t)都对应一个像素值。现在考虑一个三维空间中的一个几何对象。其表面上的每一点都对应于三维世界坐标系中的一个坐标(x,y,z),如果能通过一对函数映射将对象坐标系的每一点(x,y,z)与纹理坐标中的一点(s,t)建立关联，则可用纹理图像中的颜色或灰度值来确定对象表面上可见点的颜色。
OpenGL对此问题的解决方法是强制应用程序为每个顶点都定义纹理坐标。然后插值。
纹理图的创建
void glTexImage2D(GLenum target,GLint level,GLint iformat,
GLsizei width,GLsizei height,GLint border,
GLenum format,GLenum type,GLvoid* texels)
//该函数依据类型为type,格式为format的数组texels创建一个二维的大小为height*width的纹理。该纹理的边界被指定为b个纹理元宽。图像的数据格式为iformat。level用于多级渐进纹理。
使用纹理前先开启该功能:
glEnable(GL_TEXTURE_2D);
注：纹理图像的维数必须为2的整数次幂。
同理创建一维纹理和三维纹理也是用一样的方式glTexImage1D()和glTexImage3D().
纹理坐标
纹理坐标也是OpenGL状态的一部分。与顶点一样，其内部形式也是四维的：
void glTexCoord<1234>(sifd)(type scoord,.....)
void glTexCoord<1234>(sifd)v(type* v)
二维情况下，我们假定glTexCoord()指定的纹理元素组在一个连续矩形中，该矩形中的任意一点的为奴隶坐标都位于区间(0,1)内。所以左下角坐标为(0,0)，右上角坐标为(1,1)
纹理参数
void glTexParameter<if>(GLenum target,GLenum name,type value)
void glTexParamer<if>v(GLenum target,GLenum name,type value)
//target为GL_TEXTURE_1D、GL_TEXTURE_2D或GL_TEXTURE_3D，将参数name设为true
这些必要的参数决定了当s,t,r或q的值超过(0,1)时做何处理以及如何运用采样和滤波
对于超出(0,1)的有两种处理方式：
GL_REPEATE:若该值为正，则直接使用其小数部分，若为负，则取其与其相加为正的最小整数。
GL_CLAMP:若为负，强制取0，若超过1，强制取1。
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S, GL_REPEATE)
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T, GL_REPEATE)
glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
绘制一个旋转的纹理立方体：
#include <GL/glut.h>
GLfloat vertices[][3] = {{-1.0,-1.0,-1.0},{1.0,-1.0,-1.0},
{1.0,1.0,-1.0}, {-1.0,1.0,-1.0}, {-1.0,-1.0,1.0},
{1.0,-1.0,1.0}, {1.0,1.0,1.0}, {-1.0,1.0,1.0}};
GLfloat colors[][4] = {{0.0,0.0,0.0,0.5},{1.0,0.0,0.0,0.5},
{1.0,1.0,0.0,0.5}, {0.0,1.0,0.0,0.5}, {0.0,0.0,1.0,0.5},
{1.0,0.0,1.0,0.5}, {1.0,1.0,1.0,0.5}, {0.0,1.0,1.0,0.5}};
void polygon(int a, int b, int c , int d)
{
/* draw a polygon via list of vertices */
glBegin(GL_POLYGON);
glColor4fv(colors[a]);
glTexCoord2f(0.0,0.0);
glVertex3fv(vertices[a]);
glColor4fv(colors[b]);
glTexCoord2f(0.0,1.0);
glVertex3fv(vertices[b]);
glColor4fv(colors[c]);
glTexCoord2f(1.0,1.0);
glVertex3fv(vertices[c]);
glColor4fv(colors[d]);
glTexCoord2f(1.0,0.0);
glVertex3fv(vertices[d]);
glEnd();
}
void colorcube(void)
{
/* map vertices to faces */
polygon(0,3,2,1);
polygon(2,3,7,6);
polygon(3,0,4,7);
polygon(1,2,6,5);
polygon(4,5,6,7);
polygon(5,4,0,1);
}
static GLfloat theta[] = {0.0,0.0,0.0};
static GLint axis = 2;
void display(void)
{
/* display callback, clear frame buffer and z buffer,
rotate cube and draw, swap buffers */
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glRotatef(theta[0], 1.0, 0.0, 0.0);
glRotatef(theta[1], 0.0, 1.0, 0.0);
glRotatef(theta[2], 0.0, 0.0, 1.0);
colorcube();
glutSwapBuffers();
}
void spinCube()
{
/* Idle callback, spin cube 2 degrees about selected axis */
theta[axis] += 0.1;
if( theta[axis] > 360.0 ) theta[axis] -= 360.0;
/* display(); */
glutPostRedisplay();
}
void mouse(int btn, int state, int x, int y)
{
/* mouse callback, selects an axis about which to rotate */
if(btn==GLUT_LEFT_BUTTON && state == GLUT_DOWN) axis = 0;
if(btn==GLUT_MIDDLE_BUTTON && state == GLUT_DOWN) axis = 1;
if(btn==GLUT_RIGHT_BUTTON && state == GLUT_DOWN) axis = 2;
}
void myReshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
if (w <= h)
glOrtho(-2.0, 2.0, -2.0 * (GLfloat) h / (GLfloat) w,
2.0 * (GLfloat) h / (GLfloat) w, -10.0, 10.0);
else
glOrtho(-2.0 * (GLfloat) w / (GLfloat) h,
2.0 * (GLfloat) w / (GLfloat) h, -2.0, 2.0, -10.0, 10.0);
glMatrixMode(GL_MODELVIEW);
}
void key(unsigned char k, int x, int y)
{
if(k == ‘1‘) glutIdleFunc(spinCube);
if(k == ‘2‘) glutIdleFunc(NULL);
}
void
main(int argc, char **argv)
{
GLubyte image[64][64][3];
int i, j, c;
for(i=0;i<64;i++)
{
for(j=0;j<64;j++)
{
c = ((((i&0x8)==0)^((j&0x8))==0))*255;
image[i][j][0]= (GLubyte) c;
image[i][j][1]= (GLubyte) c;
image[i][j][2]= (GLubyte) c;
}
}
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(500, 500);
glutCreateWindow("colorcube");
/* need both double buffering and z buffer */
glutReshapeFunc(myReshape);
glutDisplayFunc(display);
glutIdleFunc(spinCube);
glutMouseFunc(mouse);
glutKeyboardFunc(key);
glClearColor(1.0,1.0,1.0,1.0);
glEnable(GL_DEPTH_TEST);
glEnable(GL_TEXTURE_2D);
glTexImage2D(GL_TEXTURE_2D,0,3,64,64,0,GL_RGB,GL_UNSIGNED_BYTE, image);
glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
glutMainLoop();
}运行得到：
OpenGL学习（七）纹理映射