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+/*
+ * 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+ * Unpublished-- rights reserved under the copyright laws of the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.
+ */
+#include <GL/glut.h>
+#include <stdlib.h>
+
+static int leftFirst = GL_TRUE;
+
+/* Initialize alpha blending function.
+ */
+static void init(void)
+{
+ glBlendEquation (GL_FUNC_REVERSE_SUBTRACT);
+ glBlendFunc (GL_ONE_MINUS_SRC_ALPHA, GL_SRC_ALPHA);
+ glShadeModel (GL_FLAT);
+ glClearColor (1.0, 0.0, 0.0, 0.0);
+}
+
+static void drawLeftTriangle(void)
+{
+ /* draw yellow triangle on LHS of screen */
+
+ glBegin (GL_TRIANGLES);
+ glColor4f(1.0, 1.0, 0.0, 0.75);
+ glVertex3f(0.1, 0.9, 0.0);
+ glVertex3f(0.1, 0.1, 0.0);
+ glVertex3f(0.7, 0.5, 0.0);
+ glEnd();
+}
+
+static void drawRightTriangle(void)
+{
+ /* draw cyan triangle on RHS of screen */
+
+ glEnable (GL_BLEND);
+ glBegin (GL_TRIANGLES);
+ glColor4f(0.0, 1.0, 1.0, 0.75);
+ glVertex3f(0.9, 0.9, 0.0);
+ glVertex3f(0.3, 0.5, 0.0);
+ glVertex3f(0.9, 0.1, 0.0);
+ glEnd();
+ glDisable (GL_BLEND);
+}
+
+void display(void)
+{
+ glClear(GL_COLOR_BUFFER_BIT);
+
+ if (leftFirst) {
+ drawLeftTriangle();
+ drawRightTriangle();
+ }
+ else {
+ drawRightTriangle();
+ drawLeftTriangle();
+ }
+
+ glFlush();
+}
+
+void reshape(int w, int h)
+{
+ glViewport(0, 0, (GLsizei) w, (GLsizei) h);
+ glMatrixMode(GL_PROJECTION);
+ glLoadIdentity();
+ if (w <= h)
+ gluOrtho2D (0.0, 1.0, 0.0, 1.0*(GLfloat)h/(GLfloat)w);
+ else
+ gluOrtho2D (0.0, 1.0*(GLfloat)w/(GLfloat)h, 0.0, 1.0);
+}
+
+/* ARGSUSED1 */
+void keyboard(unsigned char key, int x, int y)
+{
+ switch (key) {
+ case 't':
+ case 'T':
+ leftFirst = !leftFirst;
+ glutPostRedisplay();
+ break;
+ case 27: /* Escape key */
+ exit(0);
+ break;
+ default:
+ break;
+ }
+}
+
+/* Main Loop
+ * Open window with initial window size, title bar,
+ * RGBA display mode, and handle input events.
+ */
+int main(int argc, char** argv)
+{
+ glutInit(&argc, argv);
+ glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
+ glutInitWindowSize (200, 200);
+ glutCreateWindow (argv[0]);
+ init();
+ glutReshapeFunc (reshape);
+ glutKeyboardFunc (keyboard);
+ glutDisplayFunc (display);
+ glutMainLoop();
+ return 0;
+}