OpenGL学习二十三：多重纹理

在进行纹理贴图是，一次是把一副图像应用在个一个多边形上，多重纹理允许应用几个纹理，在纹理操作管线上把它们逐个应用到同一个多边形上使用glGetIntegerv(GL_MAX_TEXTURE_UNITS)查询当前OPENGL最多支持纹理数量

多重纹理的步骤 1.建立纹理单元 纹理图像glTexImage2D 纹理过滤 glTexParameteri(…, GL_TEXTURE_MAG_FILTER, …); 纹理环境应用glTexParameteri(…, GL_TEXTURE_WRAP_S, …); 纹理坐标自动生成（一般不需要） 顶点数组指定（如果需要的话） 每个纹理单位根据它的纹理状态，把原来的片段颜色与纹理图像进行混合，然后，把上述操作产生的片断颜色传递给以下个纹理单元

2.指定纹理顶点 使用 glMultiTexCoord2f (GL_TEXTURE0, 0.0, 0.0); GL_TEXTURE0为0号纹理单元，后面的是纹理坐标 有几个纹理需要指定纹理坐标几次，对应不同的纹理坐标 glMultiTexCoord2f (GL_TEXTURE0, 0.0, 0.0); glMultiTexCoord2f (GL_TEXTURE1, 0.0, 0.0); 使用glTexCoord 相当于使用glMultiTexCoord2f (GL_TEXTURE0,)

激活纹理 为了向每个纹理单元分配纹理详细，使用glActiveTexture ();选择要修改的纹理单元，再次之glTexImage，glTexParameteri，glTexEnv,glTexGen,glBindTexture只影响当前纹理

 

#include "header.h"

static GLubyte texels0[32][32][4];
static GLubyte texels1[16][16][4];

void makeCheckImages(void)
{
   int i, j;
    
   for (i = 0; i < 32; i++) {
      for (j = 0; j < 32; j++) {
         texels0[i][j][0] = (GLubyte) i;
         texels0[i][j][1] = (GLubyte) j;
         texels0[i][j][2] = (GLubyte) (i*j)/255;
         texels0[i][j][3] = (GLubyte) 255;
      }
   }

   for (i = 0; i < 16; i++) {
      for (j = 0; j < 16; j++) {
         texels1[i][j][0] = (GLubyte) 255;
         texels1[i][j][1] = (GLubyte) i;
         texels1[i][j][2] = (GLubyte) j;
         texels1[i][j][3] = (GLubyte) 255;
      }
   }
}

void init(void)
{    
   GLuint texNames[2];

   glClearColor (1.0, 1.0, 1.0, 0.0);
   glShadeModel(GL_FLAT);
   glEnable(GL_DEPTH_TEST);

   makeCheckImages();
   glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

   glGenTextures(2, texNames);
   glBindTexture(GL_TEXTURE_2D, texNames[0]);
   glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 32, 32, 0, GL_RGBA, 
		GL_UNSIGNED_BYTE, texels0);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, 
                   GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, 
                   GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);

   glBindTexture(GL_TEXTURE_2D, texNames[1]);
   glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 16, 16, 0, GL_RGBA, 
		GL_UNSIGNED_BYTE, texels1);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
   /*  Use the two texture objects to define two texture units
    *  for use in multitexturing  */
   glActiveTexture (GL_TEXTURE0);
   glEnable(GL_TEXTURE_2D);
   glBindTexture(GL_TEXTURE_2D, texNames[0]);
   glTexEnvi (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
   glMatrixMode (GL_TEXTURE);
      glLoadIdentity();
      glTranslatef(0.5f, 0.5f, 0.0f);
      glRotatef(45.0f, 0.0f, 0.0f, 1.0f);
      glTranslatef(-0.5f, -0.5f, 0.0f);
   glMatrixMode (GL_MODELVIEW);
   glActiveTextureARB (GL_TEXTURE1_ARB);
   glEnable(GL_TEXTURE_2D);
   glBindTexture(GL_TEXTURE_2D, texNames[1]);
   glTexEnvi (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
}

void display(void)
{
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
   glBegin(GL_TRIANGLES);
   glMultiTexCoord2fARB (GL_TEXTURE0_ARB, 0.0, 0.0);
   glMultiTexCoord2fARB (GL_TEXTURE1_ARB, 1.0, 0.0);
   glVertex2f(0.0, 0.0);
   glMultiTexCoord2fARB (GL_TEXTURE0_ARB, 0.5, 1.0);
   glMultiTexCoord2fARB (GL_TEXTURE1_ARB, 0.5, 0.0);
   glVertex2f(50.0, 100.0);
   glMultiTexCoord2fARB (GL_TEXTURE0_ARB, 1.0, 0.0);
   glMultiTexCoord2fARB (GL_TEXTURE1_ARB, 1.0, 1.0);
   glVertex2f(100.0, 0.0);
   glEnd();
   glFlush();
}

void reshape(int w, int h)
{
   glViewport(0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   if (w <= h)
      gluOrtho2D(0.0, 100.0, 0.0, 100.0 * (GLdouble)h/(GLdouble)w);
   else
      gluOrtho2D(0.0, 100.0 * (GLdouble)w/(GLdouble)h, 0.0, 100.0);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
}

void keyboard(unsigned char key, int x, int y)
{
   switch (key) {
      case 27:
         exit(0);
         break;
   }
}

int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
   glutInitWindowSize(250, 250);
   glutInitWindowPosition(100, 100);
   glutCreateWindow(“多重纹理”);
   glewInit();
   init();

   glutReshapeFunc(reshape);
   glutDisplayFunc(display);
   glutKeyboardFunc (keyboard);
   glutMainLoop();
   return 0; 
}