颜色深度位？
（快速版本：跳到最后一个段落旁边的段落 - 以“但是”开头的段落）
我很高兴我的无知相信 PVRTC 图像是每个通道 4 或 2 位。这听起来似乎有道理。它将给出 4+4+4+4（16 位）或 2+2+2+2（8 位）纹理，分别具有 2^16 (65536) 和 2^8 (256) 颜色深度。但是读了一些关于PVRTC的文档，我突然意识到它说的是4 bpp（和2 bpp），即每像素4位。混乱和疯狂进入了我的世界。
什么？！ 4位？每个像素？但这只是每个通道 1 位！ （甚至不要让我开始研究 2 位的，那个对我的大脑来说太奇怪了，目前无法理解。）在这个令人痛苦的现实中的一些时刻，我开始明白这毕竟不是那么真实。显然，当说 4 bpp 时，它指的是压缩，而不是颜色深度。唷，我毕竟没有生气。
但后来我开始想：这些图像解压后的颜色深度是多少？我试图查找这些信息，但显然它并不被认为是重要的提及（或者我只是不擅长查找信息）。
事实上，PVRTC 压缩图像在像素格式 RGBA4444 的 OpenGLES 中似乎没有产生任何可见的伪影，这表明它们是 16 位（在 iPhone 上的 OpenGLES 中使用像素格式 RGBA4444 的 32 位 png 图像会产生非常明显的伪影）。
(Quick version: jump to paragraph next to the last one - the one beginning with "But")
I was happy in my ignorance believing that PVRTC images were 4 or 2 bits per channel. That sounded plausible. It would give 4+4+4+4 (16 bit) or 2+2+2+2 (8 bit) textures, that would have 2^16 (65536) and 2^8 (256) color depth respectively. But reading through some documents about PVRTC, I suddenly realized that it said 4 bpp (and 2 bpp), i.e. 4 bits per pixel. Confusion and madness entered my world.
What?! 4 bits? Per pixel? But that's just 1 bit per channel! (And don't even get me started on the 2 bit one, that one was far too weird for my brain to grasp at the moment.) Some moments into this agonizing reality, I came to understand this wasn't so real after all. Apparently, when saying 4 bpp, it's referring to the compression, and not the color depth. Phew, I wasn't not mad, after all.
But then I started to wonder: what color depth do these images have then, after decompression? I tried to look this information up, but apparently it's not considered important to mention (or I'm just bad at finding info).
The fact that PVRTC compressed images don't seem give any visible artifacts in OpenGLES with the pixel format RGBA4444 would suggest they're 16 bit (using 32 bit png images with the pixel format RGBA4444 in OpenGLES on the iPhone gives very visible artifacts).
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根据论文 http://web.onetel.net.uk/~ simonnihal/assorted3d/fenney03texcomp.pdf 解压缩器的最终输出是每通道 8 位。
According to the paper http://web.onetel.net.uk/~simonnihal/assorted3d/fenney03texcomp.pdf the final output of the decompressor is 8 bits per channel.