Use imageio-ffmpeg for MP4 rendering instead of system ffmpeg

Adds imageio and imageio-ffmpeg as dependencies, which bundles a
self-contained ffmpeg binary. Replaces subprocess ffmpeg call and
Pillow GIF fallback with imageio's get_writer API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

toolchain/mfc/viz/renderer.py

@@ -7,7 +7,6 @@
 """
 
 import os
-import subprocess
 
 import numpy as np
 
@@ -198,49 +197,22 @@ def render_mp4(varname, steps, output, fps=10,  # pylint: disable=too-many-argum
         elif assembled.ndim == 3:
             render_3d_slice(assembled, varname, step, frame_path, **opts)
 
-    # Combine frames into MP4 using ffmpeg, or fall back to GIF via Pillow
-    frame_pattern = os.path.join(viz_dir, '%06d.png')
-    ffmpeg_cmd = [
-        'ffmpeg', '-y',
-        '-framerate', str(fps),
-        '-i', frame_pattern,
-        '-c:v', 'libx264',
-        '-pix_fmt', 'yuv420p',
-        '-vf', 'pad=ceil(iw/2)*2:ceil(ih/2)*2',
-        output,
-    ]
+    # Combine frames into MP4 using imageio + imageio-ffmpeg (bundled ffmpeg)
+    frame_files = sorted(f for f in os.listdir(viz_dir) if f.endswith('.png'))
 
     success = False
     try:
-        subprocess.run(ffmpeg_cmd, check=True, capture_output=True)
+        import imageio  # pylint: disable=import-outside-toplevel
+        writer = imageio.get_writer(output, fps=fps, codec='libx264',
+                                    pixelformat='yuv420p', macro_block_size=2)
+        for fname in frame_files:
+            writer.append_data(imageio.imread(os.path.join(viz_dir, fname)))
+        writer.close()
         success = True
-    except FileNotFoundError:
+    except ImportError:
         pass
-    except subprocess.CalledProcessError as e:
-        print(f"ffmpeg failed: {e.stderr.decode()}")
-
-    if not success:
-        # Fall back to GIF via Pillow
-        gif_output = output.rsplit('.', 1)[0] + '.gif'
-        try:
-            from PIL import Image  # pylint: disable=import-outside-toplevel
-            frames = []
-            frame_files = sorted(f for f in os.listdir(viz_dir) if f.endswith('.png'))
-            for fname in frame_files:
-                img = Image.open(os.path.join(viz_dir, fname))
-                frames.append(img.copy())
-                img.close()
-            if frames:
-                duration = max(int(1000 / fps), 1)
-                frames[0].save(gif_output, save_all=True, append_images=frames[1:],
-                               duration=duration, loop=0)
-                output = gif_output
-                success = True
-                print(f"ffmpeg not found; saved GIF to {gif_output}")
-        except ImportError:
-            print(f"Neither ffmpeg nor Pillow available. Frames saved to {viz_dir}/")
-            print(f"To create video: ffmpeg -framerate {fps} "
-                  f"-i {frame_pattern} -c:v libx264 -pix_fmt yuv420p {output}")
+    except Exception as exc:  # pylint: disable=broad-except
+        print(f"imageio MP4 write failed: {exc}")
 
     # Clean up frames
     if success:

toolchain/pyproject.toml

@@ -37,6 +37,10 @@ dependencies = [
     "seaborn",
     "matplotlib",
 
+    # Visualization (video rendering)
+    "imageio",
+    "imageio-ffmpeg",
+
     # Chemistry
     "cantera>=3.1.0",
     #"pyrometheus == 1.0.5",