Ice slab with layered plasma, two-pass refraction

Pass 1 renders eight plasma sheets into an atlas with linear depth in alpha;
pass 2 marches the refracted ray to each sheet and absorbs it by real path
length, so near and far layers separate under pointer parallax. Plasma cycle
repeats exactly every 30 s; - / = change speed.

render.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""Render a GLSL fragment shader to PNG via headless firefox (readPixels -> 2D canvas).
+
+usage: ./render.py shader.frag out.png [time] [width] [height] [mouseX] [mouseY]
+"""
+import base64, os, subprocess, sys, tempfile
+
+HTML = """<!doctype html><html><body style="margin:0;background:#000">
+<canvas id=c width=%(w)d height=%(h)d></canvas><script>
+const W=%(w)d,H=%(h)d,FRAG=%(frag)s,VS='attribute vec2 p;void main(){gl_Position=vec4(p,0,1);}';
+const c=document.getElementById('c'),gl=c.getContext('webgl',{antialias:false,preserveDrawingBuffer:true});
+function sh(t,s){const o=gl.createShader(t);gl.shaderSource(o,s);gl.compileShader(o);
+ if(!gl.getShaderParameter(o,gl.COMPILE_STATUS)){document.body.insertAdjacentHTML('beforeend','<pre style="color:red;font:14px monospace">'+gl.getShaderInfoLog(o)+'</pre>');return o;}return o;}
+const pr=gl.createProgram();gl.attachShader(pr,sh(gl.VERTEX_SHADER,VS));gl.attachShader(pr,sh(gl.FRAGMENT_SHADER,FRAG));
+gl.linkProgram(pr);gl.useProgram(pr);
+const b=gl.createBuffer();gl.bindBuffer(gl.ARRAY_BUFFER,b);
+gl.bufferData(gl.ARRAY_BUFFER,new Float32Array([-1,-1,1,-1,-1,1,1,1]),gl.STATIC_DRAW);
+const l=gl.getAttribLocation(pr,'p');gl.enableVertexAttribArray(l);gl.vertexAttribPointer(l,2,gl.FLOAT,false,0,0);
+const U=n=>gl.getUniformLocation(pr,n);
+gl.uniform2f(U('u_res'),W,H); gl.uniform1f(U('u_time'),%(t)s);
+gl.uniform2f(U('u_mouse'),%(mx)s,%(my)s);
+gl.viewport(0,0,W,H); gl.drawArrays(gl.TRIANGLE_STRIP,0,4);
+const px=new Uint8Array(W*H*4); gl.readPixels(0,0,W,H,gl.RGBA,gl.UNSIGNED_BYTE,px);
+const out=document.createElement('canvas'); out.width=W; out.height=H; out.style.display='block';
+const ctx=out.getContext('2d'); const img=ctx.createImageData(W,H);
+for(let y=0;y<H;y++) img.data.set(px.subarray((H-1-y)*W*4,(H-y)*W*4), y*W*4);
+ctx.putImageData(img,0,0);
+c.remove(); document.body.appendChild(out);
+</script></body></html>"""
+
+def render(frag_path, out, t=0.0, w=640, h=640, mx=0.5, my=0.5):
+    src = open(frag_path).read().encode()
+    b64 = base64.b64encode(src).decode()
+    html = HTML % dict(w=w, h=h, t=t, mx=mx, my=my,
+                        frag="atob('%s')" % b64)
+    tmp = tempfile.NamedTemporaryFile("w", suffix=".html", delete=False)
+    tmp.write(html); tmp.close()
+    subprocess.run(["firefox", "--headless", "--new-instance", "--profile", "/tmp/ff2",
+                    "--window-size=%d,%d" % (w, h), "--screenshot=" + os.path.abspath(out),
+                    "file://" + tmp.name],
+                   env={**os.environ, "MOZ_NO_REMOTE": "1"},
+                   stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, timeout=120)
+    os.unlink(tmp.name)
+
+if __name__ == "__main__":
+    a = sys.argv[1:]
+    render(a[0], a[1], *(float(x) for x in a[2:]))