#
# Copyright (c) 1993-1997, Silicon Graphics, Inc.
# ALL RIGHTS RESERVED
# Permission to use, copy, modify, and distribute this software for
# any purpose and without fee is hereby granted, provided that the above
# copyright notice appear in all copies and that both the copyright notice
# and this permission notice appear in supporting documentation, and that
# the name of Silicon Graphics, Inc. not be used in advertising
# or publicity pertaining to distribution of the software without specific,
# written prior permission.
#
# THE MATERIAL EMBODIED ON THIS SOFTWARE IS PROVIDED TO YOU "AS-IS"
# AND WITHOUT WARRANTY OF ANY KIND, EXPRESS, IMPLIED OR OTHERWISE,
# INCLUDING WITHOUT LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR
# FITNESS FOR A PARTICULAR PURPOSE.  IN NO EVENT SHALL SILICON
# GRAPHICS, INC.  BE LIABLE TO YOU OR ANYONE ELSE FOR ANY DIRECT,
# SPECIAL, INCIDENTAL, INDIRECT OR CONSEQUENTIAL DAMAGES OF ANY
# KIND, OR ANY DAMAGES WHATSOEVER, INCLUDING WITHOUT LIMITATION,
# LOSS OF PROFIT, LOSS OF USE, SAVINGS OR REVENUE, OR THE CLAIMS OF
# THIRD PARTIES, WHETHER OR NOT SILICON GRAPHICS, INC.  HAS BEEN
# ADVISED OF THE POSSIBILITY OF SUCH LOSS, HOWEVER CAUSED AND ON
# ANY THEORY OF LIABILITY, ARISING OUT OF OR IN CONNECTION WITH THE
# POSSESSION, USE OR PERFORMANCE OF THIS SOFTWARE.
#
# US Government Users Restricted Rights
# Use, duplication, or disclosure by the Government is subject to
# restrictions set forth in FAR 52.227.19(c)(2) or subparagraph
# (c)(1)(ii) of the Rights in Technical Data and Computer Software
# clause at DFARS 252.227-7013 and/or in similar or successor
# clauses in the FAR or the DOD or NASA FAR Supplement.
# Unpublished-- rights reserved under the copyright laws of the
# United States.  Contractor/manufacturer is Silicon Graphics,
# Inc., 2011 N.  Shoreline Blvd., Mountain View, CA 94039-7311.
#
# OpenGL(R) is a registered trademark of Silicon Graphics, Inc.
#
# alpha.c
# This program draws several overlapping filled polygons
# to demonstrate the effect order has on alpha blending results.
# Use the 't' key to toggle the order of drawing polygons.
require 'opengl'
require 'glu'
require 'glut'
require 'mathn'
include Gl,Glu,Glut

$leftFirst = GL_TRUE

# Initialize alpha blending function.
def init
  glEnable(GL_BLEND)
  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
  glShadeModel(GL_FLAT)
  glClearColor(0.0, 0.0, 0.0, 0.0)
end

def drawLeftTriangle
  # draw yellow triangle on LHS of screen

  glBegin(GL_TRIANGLES)
  glColor(1.0, 1.0, 0.0, 0.75)
  glVertex(0.1, 0.9, 0.0)
  glVertex(0.1, 0.1, 0.0)
  glVertex(0.7, 0.5, 0.0)
  glEnd()
end

def drawRightTriangle
  # draw cyan triangle on RHS of screen

  glBegin(GL_TRIANGLES)
  glColor(0.0, 1.0, 1.0, 0.75)
  glVertex(0.9, 0.9, 0.0)
  glVertex(0.3, 0.5, 0.0)
  glVertex(0.9, 0.1, 0.0)
  glEnd()
end

display = Proc.new do
  glClear(GL_COLOR_BUFFER_BIT)

  if ($leftFirst)
    drawLeftTriangle()
    drawRightTriangle()
  else
    drawRightTriangle()
    drawLeftTriangle()
  end
  glutSwapBuffers()
end

reshape = Proc.new do |w, h|
  glViewport(0, 0,  w, h)
  glMatrixMode(GL_PROJECTION)
  glLoadIdentity()
  if (w <= h)
    gluOrtho2D(0.0, 1.0, 0.0, 1.0*h/w)
  else
    gluOrtho2D(0.0, 1.0*w/h, 0.0, 1.0)
  end
end

keyboard = Proc.new do|key, x, y|
  case (key)
    when ?t,?T
      $leftFirst = !$leftFirst
      glutPostRedisplay()
    when ?\e  # Escape key
      exit(0)
  end
end

# Main Loop
# Open window with initial window size, title bar,
# RGBA display mode, and handle input events.
glutInit
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB)
glutInitWindowSize(500, 500)
glutInitWindowPosition(100, 100)
glutCreateWindow($0)
init()
glutReshapeFunc(reshape)
glutKeyboardFunc(keyboard)
glutDisplayFunc(display)
glutMainLoop()
