In setup() I load the image data into the img global object, but I don't know to apply the image on the texture. This function wasn't created by me.

1. void drawFunction(){
2.   float x, y, z;
3.   int i = 0, j = 0;
4.   float in_steps = 1.0 / steps;
5.   float[][] matrix = new float[steps+1][steps+1];
6.   beginShape();
7.   texture(img);
8.   for (y = 0.0, j = 0; y <= 1.0; y+=in_steps, j++)
9.     for (x = 0.0, i = 0; x <= 1.0; x+=in_steps, i++)
10.       matrix[i][j] = function(x, y);
11.     for (j = 0, y = 0.0; j < steps; j++, y+=in_steps) {
12.       beginShape(QUAD_STRIP);
13.         for (i = 0, x = 0.0; i <= steps; i++, x+=in_steps) {
14.           vertex(x, y, matrix[i][j]);
15.           vertex(x, y + in_steps, matrix[i][j+1]);
16.         }
17.       endShape();
18.     }
19.   endShape();
20. }