Shouldn't be too hard to code, using a bit of math (trigonometry, probably).
Or can be as easy as making a Triangle class, instances being upside or downside, alternating them in rows, drawing rows after rows.

Hello thingo,

I found this, which comes from the forum, so not written by me (I guess)

Greetings, Chrisir    

If you need an answer, please send me a personal message since this forum doesn't notify.

1. //
2. ArrayList<TriangleClass> triangles;
3. //
4. // ---------------------------------------------------------------
5. //
6. void setup()
7. {
8.   // init
9.   size(800, 600);
10.   //  frameRate(3);
11.   int widthX= 50;
12.   int widthY= 50;
13.   int widthXhalf= widthX/2;
14.   int widthYhalf= widthY/2;
15.   triangles = new ArrayList<TriangleClass>();  // Create an empty ArrayList
16.   // the broad side up
17.   for (int y1=0; y1 < 12*widthY; y1+=widthY) {
18.     for (int x1=0; x1 < 17*widthX; x1+=widthX) {
19.       // for the Color :
20.       // color MyC1 = color( 22, (x1 % 2) == 0 ? 0 : 255, 22 );
21.       // color MyC1 = color (255, 0, 0);
22.       color MyC1 = color (0);
23.       // for the Life :
24.       int LifeValue=int (random(3)*(y1+1)*(x1+1));
25.       // int LifeValue = 16;     
26.       // int LifeValue = int(random(12));
27.       // A new TriangleClass object is added to the ArrayList (by default to the end)
28.       triangles.add(new TriangleClass(
29.       x1-widthXhalf, y1,
30.       x1+widthXhalf, y1,
31.       x1, y1+widthY,
32.       MyC1, LifeValue ));
33.     }
34.   }
35.   // 
36.   // the broad side down
37.   for (int y1=0; y1 < 12*widthY; y1+=widthY) {
38.     for (int x1=0; x1 < 17*widthX; x1+=widthX) {
39.       // for the Color :
40.       // color MyC1 = color(  (x1 % 2) == 0 ? 0 : 255, 22, 22);
41.       // color MyC1 = color (0, 255, 0);
42.       color MyC1 = color (0);
43.       // for the Life :
44.       int LifeValue=int (random(3)*(y1+1)*(x1+1)); 
45.       // int LifeValue = 16;
46.       // int LifeValue = int(random(12));     
47.       // A new TriangleClass object is added to the ArrayList (by default to the end)
48.       triangles.add(new TriangleClass(
49.       x1+2*widthXhalf, y1+widthY,
50.       x1+widthXhalf, y1,
51.       x1-0, y1+widthY,
52.       MyC1, LifeValue ));
53.     }
54.   }
55.   noStroke();
56. } // func
57. //
58. //
59. void draw()
60. {
61.   // runs again and again
62.   background(255);
63.   TriangleClass myCurrentTriangle ;
64.   for (int i=0; i < triangles.size(); i++) {
65.     // get object from ArrayList
66.     myCurrentTriangle = triangles.get(i);
67.     myCurrentTriangle.fade();
68.     myCurrentTriangle.mouseOver();
69.     myCurrentTriangle.display();
70.   } // for
71. } // func
72. //
73. void keyPressed() {
74.   // reset
75.   TriangleClass myCurrentTriangle ;
76.   for (int i=0; i < triangles.size(); i++) {
77.     myCurrentTriangle = triangles.get(i);
78.     myCurrentTriangle.life = 220;
79.   }
80. }
81. // =====================================================================
82. // Simple class
83. class TriangleClass {
84.   //
85.   float x1, y1, x2, y2, x3, y3;    // points
86.   color myColor;        // fill color
87.   float life = 255;     // for fill transparency / opacity
88.   float mouseSensorX, mouseSensorY;// check point for dist to mouse
89.   //
90.   // constructor
91.   TriangleClass(
92.   float tempX1, float tempY1,
93.   float tempX2, float tempY2,
94.   float tempX3, float tempY3,
95.   color tempmyColor1,
96.   int tempLife ) {
97.     x1 = tempX1;
98.     y1 = tempY1;
99.     x2 = tempX2;
100.     y2 = tempY2;
101.     x3 = tempX3;
102.     y3 = tempY3;   
103.     myColor=tempmyColor1;
104.     life= tempLife;
105.     mouseSensorX = (x1+x2+x3 )/ 3;
106.     mouseSensorY = (y1+y2+y3 )/ 3;
107.   }  // constructor
108.   //
109.   void fade() {
110.     life-=.5; // how fast fading? 0.5
111.   } // func
112.   //
113.   boolean mouseOver() {
114.     if (dist ( mouseX, mouseY, mouseSensorX, mouseSensorY ) < 60) {
115.       if (mouseButton==LEFT) {
116.         life=10;
117.       }
118.       else if (mouseButton==RIGHT) {
119.         life=75;
120.       }
121.       else {
122.         life=255;
123.       }
124.       return true;
125.     }
126.     return false;
127.   } // func
128.   //
129.   void display() {
130.     // show rectangle
131.     fill(myColor, life);
132.     triangle(x1, y1, x2, y2, x3, y3);
133.   } // func
134.   //
135. } // class
136. // =====================================================================

little simplified

1. //
2. ArrayList<TriangleClass> triangles;
3. //
4. // ---------------------------------------------------------------
5. //
6. void setup()
7. {
8.   // init
9.   size(800, 600);
10.   //  frameRate(3);
11.   int widthX= 50;
12.   int widthY= 50;
13.   int widthXhalf= widthX/2;
14.   int widthYhalf= widthY/2;
15.   triangles = new ArrayList<TriangleClass>();  // Create an empty ArrayList
16.   // the broad side up
17.   for (int y1=0; y1 < 12*widthY; y1+=widthY) {
18.     for (int x1=0; x1 < 17*widthX; x1+=widthX) {
19.       // for the Color :
20.       color MyC1 = color( 22, (x1 % 2) == 0 ? 0 : 255, 22 );
21.       // color MyC1 = color (255, 0, 0);
22.       // color MyC1 = color (0);
23.       // A new TriangleClass object is added to the ArrayList (by default to the end)
24.       triangles.add(new TriangleClass(
25.       x1-widthXhalf, y1,
26.       x1+widthXhalf, y1,
27.       x1, y1+widthY,
28.       MyC1 ));
29.     }
30.   }
31.   // 
32.   // the broad side down
33.   for (int y1=0; y1 < 12*widthY; y1+=widthY) {
34.     for (int x1=0; x1 < 17*widthX; x1+=widthX) {
35.       // for the Color :
36.       //color MyC1 = color(  (x1 % 2) == 0 ? 0 : 255, 22, 22);
37.       color MyC1 = color( random (255), random (255), random (255));  
38.       // color MyC1 = color (0, 255, 0);
39.       // color MyC1 = color (0);
40.       // A new TriangleClass object is added to the ArrayList (by default to the end)
41.       triangles.add(new TriangleClass(
42.       x1+2*widthXhalf, y1+widthY,
43.       x1+widthXhalf, y1,
44.       x1-0, y1+widthY,
45.       MyC1 ));
46.     }
47.   }
48.   noStroke();
49. } // func
50. //
51. //
52. void draw()
53. {
54.   // runs again and again
55.   background(255);
56.   TriangleClass myCurrentTriangle ;
57.   //
58.   for (int i=0; i < triangles.size(); i++) {
59.     // get object from ArrayList
60.     myCurrentTriangle = triangles.get(i);
61.     myCurrentTriangle.display();
62.   } // for
63. } // func
64. //
65. void mouseClicked () {
66.   TriangleClass myCurrentTriangle ;
67.   for (int i=0; i < triangles.size(); i++) {
68.     // get object from ArrayList
69.     myCurrentTriangle = triangles.get(i);
70.     myCurrentTriangle.mouseOver();
71.   }
72. }
73. void keyPressed() {
74.   // reset
75.   TriangleClass myCurrentTriangle ;
76.   for (int i=0; i < triangles.size(); i++) {
77.     myCurrentTriangle = triangles.get(i);
78.   }
79. }
80. // =====================================================================
81. // Simple class
82. class TriangleClass {
83.   //
84.   float x1, y1, x2, y2, x3, y3;    // points
85.   color myColor;                  // fill color
86.   float mouseSensorX, mouseSensorY;// check point for dist to mouse
87.   //
88.   // constructor
89.   TriangleClass(
90.   float tempX1, float tempY1,
91.   float tempX2, float tempY2,
92.   float tempX3, float tempY3,
93.   color tempmyColor1 ) {
94.     x1 = tempX1;
95.     y1 = tempY1;
96.     x2 = tempX2;
97.     y2 = tempY2;
98.     x3 = tempX3;
99.     y3 = tempY3;   
100.     myColor=tempmyColor1;
101.     mouseSensorX = (x1+x2+x3 )/ 3;
102.     mouseSensorY = (y1+y2+y3 )/ 3;
103.   }  // constructor
104.   //
105.   void mouseOver() {
106.     if (dist ( mouseX, mouseY, mouseSensorX, mouseSensorY ) < 10) {
107.       myColor = color( random (255), random (255), random (255));
108.     } // if
109.   } // func
110.   //
111.   void display() {
112.     // show rectangle
113.     fill(myColor);
114.     triangle(x1, y1, x2, y2, x3, y3);
115.   } // func
116.   //
117. } // class
118. // =====================================================================