Add ice_object.html: a faceted crystal shard frozen in the slab

Pass 1 draws one raymarched gem on sheet 1 instead of plasma; its alpha
carries coverage rather than depth, so pass 2 can light it as a solid
while every other sheet still absorbs by path length.

build_plasma.py

@@ -1,11 +1,17 @@
 #!/usr/bin/env python3
-"""Embed the two GLSL passes into the standalone ice_plasma.html page."""
+"""Embed two GLSL passes into a standalone page.
+
+    build_plasma.py [scene.frag] [ice.frag] [out.html]
+
+Defaults to the plasma wall: 6_ice_plasma.frag + 7_ice_over.frag -> ice_plasma.html."""
 import json
+import sys
 from pathlib import Path
 
 here = Path(__file__).resolve().parent
-scene = (here / '6_ice_plasma.frag').read_text()
-ice = (here / '7_ice_over.frag').read_text()
+args = (sys.argv[1:] + ['6_ice_plasma.frag', '7_ice_over.frag', 'ice_plasma.html'])[:3]
+scene = (here / args[0]).read_text()
+ice = (here / args[1]).read_text()
 html = r'''<!doctype html>
 <html lang="ru">
 <head>
@@ -119,5 +125,5 @@ if (shot) frame(0); else requestAnimationFrame(frame);
 </body>
 </html>
 '''
-(here / 'ice_plasma.html').write_text(html.replace('__SCENE__', json.dumps(scene)).replace('__ICE__', json.dumps(ice)))
-print('ice_plasma.html written')
+(here / args[2]).write_text(html.replace('__SCENE__', json.dumps(scene)).replace('__ICE__', json.dumps(ice)))
+print(args[2], 'written')