How to use alpha transparency in OpenGL?

Question

Here's my code:

void display(void);  int main(int argc, char** argv) {     glutInit(&argc, argv);     glutInitDisplayMode(GLUT_SINGLE|GLUT_RGBA);     glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);     glEnable( GL_BLEND );     glutInitWindowSize(600,600);     glutInitWindowPosition(200,50);     glutCreateWindow("glut test");     glutDisplayFunc(display);     glutMainLoop();     return 0; }  void display() {     glClear(GL_COLOR_BUFFER_BIT);     glPointSize(8);     glBegin(GL_POINTS);     glColor4f(.23,.78,.32,1.0);     glVertex2f(0,0);     glColor4f(.23,.78,.32,0.1);     glVertex2f(0.1,0);     glEnd();     glFlush(); } 

The problem is that these two points appear identical (even when I set the alpha to 0). Is there something I missed to enable alpha transparency?

Answer 1

have you glEnable'd alpha blending? And have you set up your blend parameters? You can't just set the alpha you need to setup various other parameters in OpenGL.

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); glEnable( GL_BLEND ); 

Answer 2

Just a guess, but could it be that you dont have a background color ? So, when your rendering the second vertex which has alpha 0.1, there is no background to compute the proper color ? Just a guess, been years since i used opengl.