编程与软件工程nothings.org
角落并非如此：关于屏幕空间环境光遮蔽（2012）
Corners Don't Look Like That: Regarding Screenspace Ambient Occlusion (2012)
2015 — 2022，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
这是 2012 年一篇讨论屏幕空间环境光遮蔽（SSAO）的图形学文章：作者用实拍照片和图表指出，现实中的墙角不会像 SSAO 渲染的那样在凹角处明显变暗，SSAO 的明暗模型与真实光照并不一致。文章因此质疑把 SSAO 当作物理正确效果的做法，并解释墙角光照实际如何形成。
为什么经久不衰：它把“看起来对”和“物理正确”的差距讲得很直观，是理解 SSAO 这类实时近似为何存在、又错在哪里的经典材料，今天读仍有助于判断光追时代的取舍。
HN 评论精选
SSAO 本质是性能妥协；理解真实墙角光照能帮助开发者改进近似效果。
非常详尽地拆解了现实中墙角光照究竟如何发生，以及实现 SSAO 的人对此的普遍误解。老实说，SSAO 就是个性能取巧手段。但这没关系！游戏引擎里满是这种取巧：事情可以“不对”，但只要足够接近预期，我们一般注意不到。不过 shader 作者读读这类文章没坏处，也许即使 AO 仍是近似，这些问题也能处理得更好。
olliej查看英文原文
Very thorough break down of how lighting in corners actually happens in reality, and the general misunderstanding of it in people implementing SSAO. Honestly SSAO is just a performance hack. But that's ok! Game engines and the like are filled with performance hacks where things can be "wrong" but sufficiently close to what's expected that we don't (generally) notice it. Sometimes it's approximations (fast inverse sqrt for example), movement (strafe + forward having faster movement), or lighting (artificial glow, halos, SSAO, etc). That said it wouldn't hurt for shader writers to read something like this, because maybe these issues can be handled better even though the AO is still just being approximated.
文章自己的图表似乎与其论点矛盾，反而支持 SSAO 所模拟的凹角变暗现象。
这篇文章的论点大致是“表面在凹角附近并不会明显变暗，所以用 SSAO 不好”，但这页上又有那么多图表表明：表面确实会随着接近凹角而经验性地变暗，而且看起来相当像能用 SSAO 廉价模拟的样子……
jameshart查看英文原文
For an article whose thesis is broadly ‘surfaces don’t darken much towards concave corners, so using SSAO is bad’, there sure are a lot of graphs on this page that show that surfaces definitely, empirically darken as they approach concave corners, in a way that looks rather like it could be quite cheaply simulated using SSAO…
AO 假设均匀环境光，而真实房间的反弹光有方向性，因此差异在预期之内。
值得拿现实和“Ground Truth Ambient Occlusion”做对比来重温这篇文章。另外，AO 墙角与他的客厅不匹配并不奇怪：AO 建模的是光线从四面八方均匀射入的环境——就像你身处一个巨大发光的白色球体内部。他客厅的墙壁可能是被间接光照亮的，但反弹光仍有很强的方向性。
corysama查看英文原文
It would be interesting to revisit this comparing reality vs “Groung Truth Ambient Occlusion”. http://iryoku.com/downloads/Practical-Realtime-Strategies-for-Accurate-Indirect-Occlusion.pdf Also, it's not entirely surprising that AO wall corners don't match his living room. AO models an environment with light coming in from all directions uniformly --as if you were inside of a giant, glowing white sphere. His living room walls may be lit indirectly. But, there is still a lot of directionality to the bounced light.
SSAO 是被性能约束逼出的实用方案，并非开发者眼中的物理正确效果。
我在 3D 行业工作，可以很有把握地说开发者并不“热爱”SSAO。只是给动态模型做真正的环境光遮蔽既难设置，又会彻底耗尽大多数游戏的性能预算；而在多数 3D 引擎里，加 SSAO 只要几分钟，每帧约 1ms。希望支持光追的显卡普及后能改变。但在此之前，我们知道 SSAO 是个 hack，知道它物理不正确，只用它是因为（做得好的话）比没有 AO 或只给静态模型做 AO 好看。
esperent查看英文原文
> Game developers are in love with screenspace ambient occlusion (SSAO). I work in 3d and I think I can say with confidence that developers are not in love with SSAO. It's just that real ambient occlusion for moving models is both really hard to set up and would totally blow the performance budget for most games, while in most 3d engines you can add SSAO in couple of minutes and it costs ~1ms per frame. That will hopefully change as graphics cards that support raytracing become more common. But in the meantime, we know SSAO is a hack. We know it doesn't look physically correct, we only use it because (if done well) it looks better than the alternative, which is no ambient occlusion, or ambient occlusion only on static models.
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