OpenGL学习笔记(9)颜色混合

 

两个图形重叠时

红色三角形在下,绿色三角形在上

颜色混合

比如玻璃,假设玻璃是绿色的,那么我们还可以看到下面的一层.即将两种颜色混合了

glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

如下效果

绿色三角形可以看到红色部分.

这里称底下的红色三角形的颜色为目标颜色(下层),绿色为源颜色(上层)

在OpenGL中使用颜色混合

调用glEnable(GL_BLEND)开始混合功能,glDisable(GL_BLEND)关闭混合功能 
之后使用glBlendFunc 函数,选择混合选项

混合选项

好比画家将两种颜色混合了,但并非是透明的

这个选项用glBlendFunc函数来完成 
glBlendFunc(GL_ONE, GL_ONE);

完整的示例,来自红宝石书 

#include <GL/glut.h>
#include <stdlib.h>

static int leftFirst = GL_TRUE;

/*  Initialize alpha blending function.
 */
static void init(void)
{
   glEnable (GL_BLEND);
   glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
   //glBlendFunc(GL_ONE, GL_ONE);
   glShadeModel (GL_FLAT);
   glClearColor (0.0, 0.0, 0.0, 0.0);
}

static void drawLeftTriangle(void)
{
   /* draw yellow triangle on LHS of screen */

   glBegin (GL_TRIANGLES);
      glColor4f(1.0, 0.0, 0.0, 0.55);
      glVertex3f(0.1, 0.9, 0.0); 
      glVertex3f(0.1, 0.1, 0.0); 
      glVertex3f(0.7, 0.5, 0.0); 
   glEnd();
}

static void drawRightTriangle(void)
{
   /* draw cyan triangle on RHS of screen */

   glBegin (GL_TRIANGLES);
      glColor4f(0.0, 1.0, 0.0, 0.55);
      glVertex3f(0.9, 0.9, 0.0); 
      glVertex3f(0.3, 0.5, 0.0); 
      glVertex3f(0.9, 0.1, 0.0); 
   glEnd();
}

void display(void)
{
   glClear(GL_COLOR_BUFFER_BIT);

   if (leftFirst) {
      drawLeftTriangle();
      drawRightTriangle();
   }
   else {
      drawRightTriangle();
      drawLeftTriangle();
   }

   glFlush();
}

void reshape(int w, int h)
{
   glViewport(0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   if (w <= h) 
      gluOrtho2D (0.0, 1.0, 0.0, 1.0*(GLfloat)h/(GLfloat)w);
   else 
      gluOrtho2D (0.0, 1.0*(GLfloat)w/(GLfloat)h, 0.0, 1.0);
}

void keyboard(unsigned char key, int x, int y)
{
   switch (key) {
      case 't':
      case 'T':
         leftFirst = !leftFirst;
         glutPostRedisplay();    
         break;
      case 27:  /*  Escape key  */
         exit(0);
         break;
      default:
         break;
   }
}

/*  Main Loop
 *  Open window with initial window size, title bar, 
 *  RGBA display mode, and handle input events.
 */
int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize (200, 200);
   glutCreateWindow (argv[0]);
   init();
   glutReshapeFunc (reshape);
   glutKeyboardFunc (keyboard);
   glutDisplayFunc (display);
   glutMainLoop();
   return 0;
}