/*****************************************************************
  smoothAlpha.c
  by Dave Pape
  9 Feb 2003
  
  This program demonstrates how alpha is interpolated (smooth
  shaded) the same as the regular RGB color components.
  By drawing a square with alpha=1 at the lower vertices, and
  alpha=0.1 at the upper vertices, we get a square whose
  transparency changes smoothly across its face.

*****************************************************************/
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <GL/glut.h>
#include <GL/gl.h>

float viewRotY = 0, viewRotX = 0, cameraDistance = 10;

void drawEverything(void);
void initLight(void);
void checkGLError(char *);
void key(unsigned char k, int x, int y);
void specialkey(int k, int x, int y);


int main(int argc, char *argv[])
    {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
    glutInitWindowSize(512, 512);
    glutCreateWindow(argv[0]);
    glutDisplayFunc(drawEverything);
    glutKeyboardFunc(key);
    glutSpecialFunc(specialkey);
    glutMainLoop();
    return 0;
    }


void drawEverything(void)
    {
/*************** CODE OF INTEREST ******************************/
/*    Here we define the two colors for our semi-transparent   */
/*    object - one with alpha=1, one with alpha=0.1            */
    GLfloat red[4] = { 1, 0, 0, 1.0 };                       /**/
    GLfloat green[4] = { 0, 1, 0, 0.1 };                     /**/
    GLfloat white[4] = { 1, 1, 1, 1 };
    glClearColor(0.5, 0.7, 1.0, 0.0);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_DEPTH_TEST);
    glEnable(GL_CULL_FACE);
    
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(30.0, 1.0, 1.0, 100.0);
    glMatrixMode(GL_MODELVIEW);

    glLoadIdentity();

    initLight();

    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    
    glTranslatef(0.0, 0.0, -cameraDistance);
    glRotatef(viewRotX, 1.0, 0.0, 0.0);
    glRotatef(viewRotY, 0.0, 1.0, 0.0);

    glMaterialfv(GL_FRONT, GL_DIFFUSE, white);
    glPushMatrix();
     glTranslatef(0.0, 0.0, -5.0);
     glutSolidSphere(2.5, 16, 8);
    glPopMatrix();    
    
/*************** CODE OF INTEREST ******************************/
/*    Here we draw a square, applying the materials with       */
/*    different alphas at to different vertices.               */
    glBegin(GL_QUADS);
     glNormal3f(0.0, 0.0, 1.0);
     glMaterialfv(GL_FRONT, GL_DIFFUSE, red);                /**/
     glVertex3f(-1.0, -1.0, 0.0);
     glVertex3f( 1.0, -1.0, 0.0);
     glMaterialfv(GL_FRONT, GL_DIFFUSE, green);              /**/
     glVertex3f( 1.0,  1.0, 0.0);
     glVertex3f(-1.0,  1.0, 0.0);
    glEnd();

    glutSwapBuffers();
    
    checkGLError("end-of-frame");
    }


void initLight(void)
    {
    GLfloat white[4] = { 1, 1, 1, 1 };
    GLfloat lightPos[4] = {-1, 1, 1, 0};
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glLightfv(GL_LIGHT0, GL_DIFFUSE, white);
    glLightfv(GL_LIGHT0, GL_POSITION, lightPos);
    }


void checkGLError(char *prefix)
    {
    GLenum err = glGetError();
    if (err != GL_NO_ERROR)
        printf("%s GL error '%s'\n",prefix,gluErrorString(err));
    }


void key(unsigned char k, int x, int y)
    {
    if (k == 27)
        exit(0);
    else if (k == '-')
        cameraDistance += 1;
    else if (k == '=')
        cameraDistance -= 1;
    glutPostRedisplay();
    }


void specialkey(int k, int x, int y)
    {
    if (k == GLUT_KEY_LEFT)
        viewRotY += 3;
    else if (k == GLUT_KEY_RIGHT)
        viewRotY -= 3;
    else if (k == GLUT_KEY_UP)
        viewRotX += 3;
    else if (k == GLUT_KEY_DOWN)
        viewRotX -= 3;
    glutPostRedisplay();
    }