write obj for mass spring example

Author: liminchen

.gitignore

@@ -1,3 +1,4 @@
 .DS_Store
 test.py
-*.pyc
+*.pyc
+output/

mass_spring/simulator.py

@@ -37,6 +37,7 @@ def screen_projection(x):
     return [offset[0] + scale * x[0], resolution[1] - (offset[1] + scale * x[1])]
 
 time_step = 0
+square_mesh.write_to_file(time_step, x, n_seg)
 screen = pygame.display.set_mode(resolution)
 running = True
 while running:
@@ -60,5 +61,6 @@ def screen_projection(x):
     [x, v] = time_integrator.step_forward(x, e, v, m, l2, k, h, 1e-2)
     time_step += 1
     pygame.time.wait(int(h * 1000))
+    square_mesh.write_to_file(time_step, x, n_seg)
 
 pygame.quit()

mass_spring/square_mesh.py

@@ -1,4 +1,5 @@
 import numpy as np
+import os
 
 def generate(side_length, n_seg):
     # sample nodes uniformly on a square
@@ -24,4 +25,22 @@ def generate(side_length, n_seg):
             e.append([i * (n_seg + 1) + j, (i + 1) * (n_seg + 1) + j + 1])
             e.append([(i + 1) * (n_seg + 1) + j, i * (n_seg + 1) + j + 1])
 
-    return [x, e]
+    return [x, e]
+
+def write_to_file(frameNum, x, n_seg):
+    # Check if 'output' directory exists; if not, create it
+    if not os.path.exists('output'):
+        os.makedirs('output')
+
+    # create obj file
+    filename = f"output/{frameNum}.obj"
+    with open(filename, 'w') as f:
+        # write vertex coordinates
+        for row in x:
+            f.write(f"v {float(row[0]):.6f} {float(row[1]):.6f} 0.0\n") 
+        # write vertex indices for each triangle
+        for i in range(0, n_seg):
+            for j in range(0, n_seg):
+                #NOTE: each cell is exported as 2 triangles for rendering
+                f.write(f"f {i * (n_seg+1) + j + 1} {(i+1) * (n_seg+1) + j + 1} {(i+1) * (n_seg+1) + j+1 + 1}\n")
+                f.write(f"f {i * (n_seg+1) + j + 1} {(i+1) * (n_seg+1) + j+1 + 1} {i * (n_seg+1) + j+1 + 1}\n")