Is video output in YUY2 color space lossless if the input is sRGB?
It's a bit of a semantics game.  The YUY2 video mode is not &quot;compressed,&quot; but it won't result in a bit-exact copy of RGB input.  The luminance is full resolution in that mode, but the chrominance is sampled at a lower resolution.  This is technically decimation rather than compression, but some information is indeed lost.
That said, if you are going to wind up posting your video online or something, it'll be even more decimated, and also subject to lossy compression.  So in most use cases, the loss of information from going into that YUY2 colorspace wouldn't be anything to really worry about one way or another.  You probably need to use some lossy compression to be able to reliably record it in real time unless you have very fast storage or are having the device output a low resolution.  If you really care about truly perfectly exact output for some sort of scientific image analysis process, you'd need more specialized hardware.