OpenGL blend problem

Question

I want to draw aquarium (container) standing on table. Walls of aquarium need to be blended.

But, if I draw table first and then aquarium I get:
- looking from the up of the table: ok
- looking from the bottom of the table: wrong, I still see aquarium

If I draw aquariun first and then table I get:
- looking from the up of the table: wrong, aquarium walls have no effect on the table (they affect the backgrounf, though)
- looking from the bottom of the table: wrong, I still see aquarium

There is picture that explains it:

http://img213.imageshack.us/img213/6609/pictureem.jpg

Code: main.cpp

int DrawGLScene(GLvoid)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);

 Cam.SetPrespective(); 

 //----------------
 GLfloat LightPosition0[]= { 0.0, 0.0, 0.2, 1.0f };
 GLfloat LightAmbient0[]= { 0, 0, 0, 1.0f };
 GLfloat LightDiffuse0[]= { 1.0f, 1.0f, 1.0f, 1.0f };
 GLfloat LightSpecular0[]= { 1.0f, 1.0f, 1.0f, 1.0f };
 glLightfv(GL_LIGHT0, GL_AMBIENT, LightAmbient0);    // Setup The Ambient Light
 glLightfv(GL_LIGHT0, GL_POSITION,LightPosition0);   // Position The Light
 glLightfv(GL_LIGHT0, GL_DIFFUSE, LightDiffuse0);    // Setup The Diffuse Light
 glLightfv(GL_LIGHT0, GL_SPECULAR, LightSpecular0);    // Setup The Diffuse Light
 glEnable(GL_LIGHT0);
 glEnable(GL_COLOR_MATERIAL);
 glColorMaterial(GL_FRONT, GL_AMBIENT_AND_DIFFUSE);

 //----------------

 glLightfv(GL_LIGHT1, GL_AMBIENT, LightAmbient);    // Setup The Ambient Light
 glLightfv(GL_LIGHT2, GL_POSITION,LightPosition);   // Position The Light
 glLightfv(GL_LIGHT2, GL_DIFFUSE, LightDiffuse);    // Setup The Diffuse Light


 drawContainer(); //container drawn first in this example
 //.. draw other things
 drawTable();
drawGround();

/*
    //.. draw other things
    drawTable();
    drawGround();
    drawContainer(); //container drawn last in this example
*/
    CheckKeys();
 if(mouse) 
 {
 CheckMouse();
}

return TRUE;
}

void drawTable()
{
 glColor3f(1.0f, 0.5f, 1.0f);

 GLfloat cx = MIN_X+(MAX_X-MIN_X)/2.0f;
 GLfloat cy = MIN_Y-0.1f;
 GLfloat cz = MIN_Z+(MAX_Z-MIN_Z)/2.0f;
 GLfloat cr = sqrt((MAX_X-MIN_X)*(MAX_X-MIN_X)+(MAX_Z-MIN_Z)*(MAX_Z-MIN_Z))*0.85f;
 int th = 5.0f; //centerPieceThickness

 drawCylinder(cx,cy,cz,th,cr);
 glColor3f(0.7f, 0.5f, 0.5f);
 drawCylinder(MAX_X+cr*0.155f, cy-th, MAX_Z+cr*0.155f, 25*th, cr*0.035f); 
 drawCylinder(MAX_X+cr*0.155f, cy-th, MIN_Z-cr*0.155f, 25*th, cr*0.035f);
 drawCylinder(MIN_X-cr*0.155f, cy-th, MIN_Z-cr*0.155f, 25*th, cr*0.035f);
 drawCylinder(MIN_X-cr*0.155f, cy-th, MAX_Z+cr*0.155f, 25*th, cr*0.035f);
}

void drawGround()
{ 
 int th = 5.0f;
 GLfloat cx = MIN_X+(MAX_X-MIN_X)/2.0f;
 GLfloat cy = MIN_Y-0.1f;
 GLfloat cz = MIN_Z+(MAX_Z-MIN_Z)/2.0f;
 GLfloat cr = sqrt((MAX_X-MIN_X)*(MAX_X-MIN_X)+(MAX_Z-MIN_Z)*(MAX_Z-MIN_Z))*8.5f;
 glColor3f(0.0f, 0.6f, 0.0f);
 drawCylinder(cx,cy-26*th,cz,10.f,cr);
}

void drawContainer()
{
    glColor3f(0.2f, 0.2f, 0.2f);
 drawCuboid(MIN_X-0.5, MAX_X+0.5f, MIN_Y-0.5, MIN_Y+0.5, MIN_Z-0.5f, MIN_Z+0.5f);
 drawCuboid(MIN_X-0.5, MAX_X+0.5f, MIN_Y-0.5, MIN_Y+0.5, MAX_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MIN_X-0.5, MAX_X+0.5f, MAX_Y-0.5, MAX_Y+0.5, MIN_Z-0.5f, MIN_Z+0.5f);
 drawCuboid(MIN_X-0.5, MAX_X+0.5f, MAX_Y-0.5, MAX_Y+0.5, MAX_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MIN_X-0.5, MIN_X+0.5f, MIN_Y-0.5, MAX_Y+0.5, MIN_Z-0.5f, MIN_Z+0.5f);
 drawCuboid(MIN_X-0.5, MIN_X+0.5f, MIN_Y-0.5, MAX_Y+0.5, MAX_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MAX_X-0.5, MAX_X+0.5f, MIN_Y-0.5, MAX_Y+0.5, MIN_Z-0.5f, MIN_Z+0.5f);
 drawCuboid(MAX_X-0.5, MAX_X+0.5f, MIN_Y-0.5, MAX_Y+0.5, MAX_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MIN_X-0.5, MIN_X+0.5f, MIN_Y-0.5, MIN_Y+0.5, MIN_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MIN_X-0.5, MIN_X+0.5f, MAX_Y-0.5, MAX_Y+0.5, MIN_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MAX_X-0.5, MAX_X+0.5f, MIN_Y-0.5, MIN_Y+0.5, MIN_Z-0.5f, MAX_Z+0.5f);
 drawCuboid(MAX_X-0.5, MAX_X+0.5f, MAX_Y-0.5, MAX_Y+0.5, MIN_Z-0.5f, MAX_Z+0.5f);

 glEnable(GL_BLEND);
 glDisable(GL_DEPTH_TEST);
 glColor4f(0.5f,1.0f,1.0f,0.12f);

 drawCuboid(MIN_X,MAX_X,MIN_Y,MAX_Y,MIN_Z,MAX_Z);

 glDisable(GL_BLEND);
 glEnable(GL_DEPTH_TEST);


}

Answer 1

I suspect that the problem is caused by the glDisable(GL_DEPTH_TEST) in drawContainer(). I suggest commenting out the GL_DEPTH_TEST lines -- does the aquarium appear correct with respect to the table and floor in that case?

Answer 2

For opaque (non-transparent polys): Enable GL_DEPTH_TEST, and render all these.

For transparent polys: Keep GL_DEPTH_TEST but disable depth write withglDepthMask(GL_FALSE)and then sort the transparent polygons from back to front before rendering.

See also the FAQ