Mar 10
25

9. OpenGL Blending

6 Comments



Blending consists of taking your normal RGB values and adding what is called A
for Alpha. Our RGB function becomes RGBA where the Alpha value monitors
the opacity of transparency of the current colour.

Now to get this option into our OpenGL application we need to add the A value
to our window. To do this just change the line:
glutInitDisplayMode (GLUT_DOUBLE);
to include “| GLUT_RGBA)” somewhere on the end.

Now that our window supports Alpha values we want to be able to use the
blending function built into OpenGL. To do this we enable the blending
just like any other option with:
glEnable(GL_BLEND);

So we now have blending enabled. But how do we use it?
We need to set up how the current objects are going to be blended.
I am using the blending function:
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
Which takes the current colour, and blends it with the colours behind it
according to the Alpha value we set.

Now to set the Alpha value we need to change our current colour line which
looks like: glColor3f(1, 0, 0);
to make it look like:
glColor4f(1, 0, 0, 0.2);
Here the fourth value of 0.2 is our opacity level. With 1 being the highest
(fully opaque), and 0 being the lowest (fully transparent, will not be visible).

And that is all that there is.

Just remember that objects that are drawn on the
screen are drawn in the order in which you choose to draw them. So if you
draw a square (square1), before another square (square2), then if square1
is brought infront of square2 while square1 has an alpha value, then
square1 will not show square2 behind it.

If you have any queries feel free to email me at swiftless@gmail.com

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54. 		    #include <GL/gl.h>
#include <GL/glut.h>

GLfloat angle = 0.0;

void cube (void) {
    glRotatef(angle, 1.0, 0.0, 0.0);
    glRotatef(angle, 0.0, 1.0, 0.0);
    glRotatef(angle, 0.0, 0.0, 1.0);
    glColor4f(1.0, 0.0, 0.0, 0.2); //set the color and alpha of 
the cube
    glutSolidCube(2);
    glColor4f(0.0, 1.0, 0.0, 0.5); //set the color and alpha of 
the cube
    glutSolidCube(1);
}

void display (void) {
    glClearColor (0.0,0.0,0.0,1.0);
    glClear (GL_COLOR_BUFFER_BIT);
    glEnable(GL_BLEND); //enable the blending
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); /
/set the blend function
    glLoadIdentity();  
    gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
    cube();
    glutSwapBuffers();
    angle ++;
}

void reshape (int w, int h) {
    glViewport (0, 0, (GLsizei)w, (GLsizei)h);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    gluPerspective (60, (GLfloat)w / (GLfloat)h, 1.0, 100.0)
;
    glMatrixMode (GL_MODELVIEW);
}

int main (int argc, char **argv) {
    glutInit (&argc, argv);
    glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGBA); //set the
 alpha buffer
    glutInitWindowSize (500, 500);
    glutInitWindowPosition (100, 100);
    glutCreateWindow (“A basic OpenGL Window“);
    glutDisplayFunc (display);
    glutIdleFunc (display);
    glutReshapeFunc (reshape);
    glutMainLoop ();
    return 0;
}

Related posts:

  1. 10. OpenGL Scaling
  2. 26. OpenGL Vertex Alphas
  3. 5. OpenGL Color (Version 2.0)
  4. 27. OpenGL Basic Shadows
  5. 28. OpenGL Basic Reflection

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This entry was posted on March 25, 2010 at 4:14 am. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

Posted in OpenGL by Swiftless 6 Comments

6 Responses to "9. OpenGL Blending"

  • Benjamin says:
    June 8, 2011 at 1:02 am
    Reply

    I can’t read the code… the advertisements kind of push it all into the right and then it’s cut off so basically I just see this:

    |Line 21: glClear (GL_COLOR_BUFFER_ …

    this is NOT a good thing… because I’m actually trying to follow a tutorial, not GUESS what is actually there. Why is it like this anyway? :] Would you please fix it?

  • oldnewbie says:
    March 13, 2011 at 8:38 am
    Reply

    Just remember that objects that are drawn on the
    screen are drawn in the order in which you choose to draw them. So if you
    draw a square (square1), before another square (square2), then if square1
    is brought infront of square2 while square1 has an alpha value, then
    square1 will not show square2 behind it.

    I think this is the heart of this tutorial and I couldnt get it.

  • kalin says:
    January 18, 2011 at 8:45 am
    Reply

    lol

    #include”GL/glew.h” not # include “GL/gl.h”

    • Swiftless says:
      January 19, 2011 at 10:28 pm
      Reply

      Hey kalin,

      This was an old tutorial back before I started tidying up code and moved to glew. Although with the #include “GL\glut.h” line, the #include “GL\gl.h” line becomes redundant.

      Cheers,
      Swiftless

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