public boolean screenToWorldPosition(int theScreenX,
                                     int theScreenY,
                                     Plane3f thePlane,
                                     Vector3f theResult)

ClassCastException: processing.core.PGraphicsJava2D cannot be cast to processing.opengl.PGraphicsOpenGL

Always do the assignment/cast from g to pgl inside draw(), because 'g' may change from time to time. In addition, use the GL object returned by beginGL() for the same reason, that the GL context itself may have changed. In practice, I've not really seen this happen, but this is strongly encouraged to prevent things from crashing in a bad way (think blue screen/kernel panic kinds of bad ways). Processing

anchnk wrote :

The OpenGL glUnProject function is what you need.
The gelstat's class method you referred simply use that OpenGL function.

Great ! However how do I use this function ? Might be a dumb question but I am still a beginner  and I have no idea how to call a basic OpenGL functions...

1. //
2. // Grid creation with rows x cols points. Plane is set in an OpenGL 3D scene. Intersections can be moved.
3. //
4. import controlP5.*;
5. import processing.opengl.*;
6. import codeanticode.glgraphics.*;
7. import javax.media.opengl.*;
8. import javax.media.opengl.glu.*;
9. import java.nio.*;
10. import remixlab.proscene.*;
11. ControlP5 controlP5;
12. //PGraphicsOpenGL
13. int rows = 15; // number of rows
14. int cols = 8; // number of cols
15. int points = rows * cols ; // number of points
16. int x_size = 500; // width
17. int y_size = 500; // height
18. int margin = 50; // margin
19. int button_height = 100; // additional height for button zone
20. //int coords = 2; // number of coordinates per point (2D at the moment)
21. int bgColor = color(255, 255, 255); // background color
22. int squareColor = color(128, 0, 0); // square color upon mouse over point
23. boolean bover = false;
24. boolean locked = false;
25. int movingPoint = -1 ; // number of selected point
26. PVector[] mesPoints = new PVector[points];
27. // this is only a note.
28. // we will not use variable b in the code below.
29. // we have to use controlP5.Button here since there
30. // would be a conflict if we only use Button to declare button b.
31. controlP5.Button b;
32. // a button-controller with name buttonValue will change the
33. // value of this variable when pressed.
34. //int buttonValue = 0;
35. GL gl;
36. GLU glu;
37. //********** SETUP
38. void setup()
39. {
40.   // SEEKING REPLACEMENT FOR THE TWO NEXT LINES !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
41.   smooth();
42.   ellipseMode(CENTER);
43.   rectMode(CENTER);
44.   size(x_size, (y_size+button_height), OPENGL);
45.   background(bgColor);
46.   //Button
47.   controlP5 = new ControlP5(this);
48.   controlP5.addButton("Bouton1",0,50, (y_size+50),88,21);
49.   controlP5.setColorActive(color(255, 0, 0)); // color for mouse-over
50.   controlP5.setColorBackground(color(128, 0, 0)); // default background color
51.   controlP5.setColorLabel(color(255, 255, 255)); // text color
52.   controlP5.controller("Bouton1").setLabel("Regenerate points"); // button label
53.   //controlP5.setColorForeground(color(128, 0, 0)); // default color
54.   // Creates positions for points
55.   for (int i = 0; i < rows; i++) {
56.     for (int j = 0; j < cols; j++) {
57.       int current_point = (cols * i)+j;
58.       mesPoints[current_point] = new PVector( ((((x_size - (2*margin))/(cols-1)) * j)+margin), ((((y_size - (2*margin))/(rows-1)) * i)+margin), 0);
59.     }
60.   }
61. }
62. //********** DRAW
63. void draw()
64. {
65.  
66. PGraphicsOpenGL pgl = (PGraphicsOpenGL) g; // g may change
67. GL gl; // = pgl.beginGL(); // for mouse to world conversion
68. GLU glu = pgl.glu ; // same here
69.   // Erases everything  - redraws lines
70.   background(bgColor);
71.   //Open GL 3D stuff
72.   lights();
73.   //translate(width / 2, height / 2);
74.   rotateY(map(mouseX/10, 0, width, 0, -PI));
75.   rotateZ(map(mouseY/10, 0, height, 0, PI));
76.   // Draw lines
77.   strokeWeight(2);
78.   for (int i = 0; i < rows ; i++) {
79.     for (int j = 0; j < cols ; j++) {
80.       int current_point = (cols * i)+j;
81.       //Draw horizontal lines
82.       if (j < cols - 1) { // all but last column
83.         if ((i == 0) || (i == (rows - 1))) { // first and last rows
84.           stroke(0, 0, 0); // outer lines are black
85.         } else {
86.           stroke(0,192,255); // inner lines are blue
87.         }
88.         beginShape(LINES);
89.         vertex(mesPoints[current_point].x, mesPoints[current_point].y, 0);
90.         vertex(mesPoints[current_point+1].x, mesPoints[current_point+1].y, 0);
91.         endShape();
92.       }
93.       //Draw vertical lines
94.       if (i < rows - 1) { // all but last line
95.         if ((j == 0) || (j == (cols - 1))) { // first and last column
96.           stroke(0, 0, 0); // outer lines are black
97.         } else {
98.           stroke(0,192,255); // inner lines are blue
99.         }
100.         beginShape(LINES);
101.         vertex(mesPoints[current_point].x, mesPoints[current_point].y, 0);
102.         vertex(mesPoints[current_point+cols].x, mesPoints[current_point+cols].y, 0);
103.         endShape();
104.       }
105.     }
106.   }
107.   // Draws points
108.   strokeWeight(1);
109.   for (int i = 0; i < (rows * cols) ; i++) {
110.     if ( (i % cols == 0) || (i % cols == (cols - 1)) || (((i+1) / cols)  == 0) || (((i+1) / cols)  == rows - 1 ) ) { //outer points
111.       stroke(0,0, 0);
112.       fill(0,0,0);
113.     } else { // inner points
114.       stroke(0,192, 255);
115.       fill(255,255,0);
116.     }
117.     ellipse(mesPoints[i].x, mesPoints[i].y, 10, 10);
118.   }
119.  
120.  
121.   //Tests whether mouse rolls over points and draws red squares
122.     strokeWeight(1);
123.     if (locked) {
124.       squareColor = color(255, 0, 0);
125.     } else {
126.       squareColor = color(128, 0, 0);
127.     }
128.     stroke(squareColor);
129.     fill(squareColor);
130.     for (int i = 0; i < points; i++) {
131.       // distance between mouse pointer and projection of underneath vertice on screen.
132.       if(dist(mouseX, mouseY, screenX(mesPoints[i].x, mesPoints[i].y, mesPoints[i].z), screenY(mesPoints[i].x, mesPoints[i].y, mesPoints[i].z)) <= 5) {
133.         movingPoint = i;
134.         rect(mesPoints[i].x, mesPoints[i].y, 10, 10);
135.         i = points ; //not necessary to go on with loop
136.       } else {
137.         movingPoint = -1;
138.       }
139.     }
140.     if (movingPoint != -1) {
141.       bover = true ;
142.     } else {
143.       bover = false;
144.     }
145.     //
146. }
147. //********** Detects whether mouse button is pressed
148. void mousePressed() {
149.   if(bover) {
150.     locked = true;
151.   } else {
152.     locked = false;
153.   }
154. }
155. //********** Moves selected point while button held
156. void mouseDragged() {
157.   if(locked) {
158.     strokeWeight(1);
159.     stroke(255,0,0);
160.     fill(255,0,0);
161.         mesPoints[movingPoint].x = getMouse3D()[0] ;
162.         mesPoints[movingPoint].y = getMouse3D()[1] ;
163.         mesPoints[movingPoint].z = getMouse3D()[2] ;  // mouse (screen) to world conversion
164.         //mesPoints[movingPoint].x = mouseX;
165.         //mesPoints[movingPoint].y = mouseY;
166.         //rect(mesPoints[movingPoint].x, mesPoints[movingPoint].y, 10, 10);
167.     }
168. }
169. //********** Unlocks upon button release
170. void mouseReleased() {
171.   locked = false;
172.   bover = false;
173. }
174. //**********function Bouton1 will receive changes from
175. //********** controller with name controlP5
176. public void Bouton1(int theValue) {
177.     for (int i = 0; i < rows; i++) {
178.       int randomX;
179.       int randomY;
180.       for (int j = 0; j < cols; j++) {
181.         int current_point = (cols * i)+j;
182.         if ( (current_point % cols == 0) || (current_point % cols == (cols - 1)) || (((current_point+1) / cols)  == 0) || (((current_point+1) / cols)  == rows - 1 )) { //outer points
183.           randomX = 0;
184.           randomY = 0;
185.       } else { // inner points
186.         randomX = int(random(-10, 10));
187.         randomY = int(random(-10, 10));
188.       }
189.       mesPoints[current_point] = new PVector( ((((x_size - (2*margin))/(cols-1)) * j)+margin+randomX), ((((y_size - (2*margin))/(rows-1)) * i)+margin+randomY), 0);
190.     }
191.   }
192. }
193. //************* SCREEN TO WORLD FUNCTION - found on old Processing forum ********************
194. float[] getMouse3D()
195. {
196.   ((PGraphicsOpenGL)g).beginGL();
197.   int viewport[] = new int[4];
198.   double[] proj=new double[16];
199.   double[] model=new double[16];
200.   ((PGraphicsOpenGL)g).gl.glGetIntegerv(GL.GL_VIEWPORT, viewport, 0);
201.   ((PGraphicsOpenGL)g).gl.glGetDoublev(GL.GL_PROJECTION_MATRIX,proj,0);
202.   ((PGraphicsOpenGL)g).gl.glGetDoublev(GL.GL_MODELVIEW_MATRIX,model,0);
203.   //java.nio.FloatBuffer fb = java.nio.ByteBuffer.allocateDirect(4).order(java.nio.ByteOrder.nativeOrder()).asFloatBuffer(); // replaces line below if java.nio not available
204.   FloatBuffer fb = ByteBuffer.allocateDirect(4).order(ByteOrder.nativeOrder()).asFloatBuffer();
205.   ((PGraphicsOpenGL)g).gl.glReadPixels(mouseX, height-mouseY, 1, 1, GL.GL_DEPTH_COMPONENT, GL.GL_FLOAT, fb);
206.   fb.rewind();
207.   double[] mousePosArr=new double[4];
208.   ((PGraphicsOpenGL)g).glu.gluUnProject((double)mouseX,height-(double)mouseY,(double)fb.get(0),model,0,proj,0,viewport,0,mousePosArr,0);
209.   ((PGraphicsOpenGL)g).endGL();
210.   return new float[]{(float)mousePosArr[0],(float)mousePosArr[1],(float)mousePosArr[2]};
211. }
212. //*********
213. // END
214. //*********