Question

I have encountered a problem when simulation transparency in OpenGL.
Here is the scenario:

I have a ship which is represented by a sphere.
Now I though about adding a shield around the ship. I chose a sphere too, but with a larger radius and set the alpha factor 0.5 ( opacity ). However, the shield doesn’t appear and the colors don’t blend( as if it was not there ).

The camera is located in the center of the 1st sphere. I think the problem is that I’m inside the sphere, so opengl will just ignore it(not draw it).

The code looks like this:

//ship colors setup with alpha 1.0f
glutSolidSphere(1, 100, 100); original sphere ( ship )
//shield colors setup with alpha 0.5f
glutSolidSphere(3, 100, 100); //the shield whose colors should blend with the rest of  the scene

I have though to simulate the shield with a parallelepiped in front of the ship.
However this is not what I want…

EDIT: I have found the mistake. I was setting the near param of gluPerspective() too
high, so even though i was setting the alpha value correctly, the camera was in front of the object all the time so there was no way of seeing it.

Answer 1

Seems to work here:

#include <GL/glut.h>

void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0, 0, -5);

    glDisable(GL_BLEND); 
    glColor4ub(255,0,0,255);
    glutSolidCube(1.0);

    glEnable(GL_BLEND); 
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glColor4ub(0,255,0,64);
    glutSolidSphere(1.5, 100, 100);

    glFlush();
    glutSwapBuffers();
}

void reshape(int w, int h)
{
    glViewport(0, 0, w, h);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective( 60, (double)w / (double)h, 0.01, 100 );
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE);

    glutInitWindowSize(800,600);
    glutCreateWindow("Blending");

    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutMainLoop();
    return EXIT_SUCCESS;
}