You've filled the whole sketch window (which is a square (which is also a rectangle)) with diagonal lines.
So... what's the problem? Do you want to fill a rectangle less than the whole window?
Do you want to do lines at any angle?

Fun example:

That's pretty cool. Yes my bad, I tried to simplify the embedded code.

Maybe something like this helps you out, is makes a texture with lines and puts that on a square. To make it work copy paste the code, save it and put any 512x512 image in the scene folder, rename that  image to "image512.jpg" (it uses the image as a placeholder)
press "1" and "2" for making the lines more/less diagonal and "+" and "-" to add and substract lines. If you want to call the function frequently it is advisible to use a smaller placeholder image.

1. //made by jan jacobs
   public PImage textureImg;
   public boolean texChange;
   public int lineCount;
   public float rotVal;
   
   void setup() {
     size(300, 300, P3D);
     textureImg = loadImage("image512.jpg"); // dummy image
     texChange = true;
     lineCount = 22;
     rotVal =0;
   }
   
   void draw() {
   
     float sinX = 0;
     if (texChange) {
       println(lineCount);
       for (int i=0; i<512; i++) {
         for (int j=0; j<512; j++) {
           //use sine of i and j for diagonals
           sinX = sin(lineCount*2*PI*((float(i)/512)+rotVal*(float(j)/512)));
           textureImg.set(i, j, color(sinX*255, sinX*255, sinX*255));
         }
       }
       texChange = false;
     }
   
     image(textureImg, 0, 0);
     textureMode(NORMALIZED);
     beginShape(QUADS);
     texture(textureImg);
     vertex(20, 20, 0, 0);
     vertex(250, 25, 0, 1);
     vertex(260, 260, 1, 1);
     vertex(20, 250, 1, 0);
     endShape();
   }
   
   void keyReleased() {
     if (key == '=') {
       lineCount = lineCount +1;
       texChange = true;
     }
     if (key == '+') {
       lineCount = lineCount +1;
       texChange = true;
     }
     if (key == '-') {
       lineCount = lineCount -1;
       texChange = true;
     }
     if (key == '1') {
       rotVal = rotVal -(0.1);
       texChange = true;
     }
     if (key == '2') {
       rotVal = rotVal +(0.1);
       texChange = true;
     }
   }