# Question about archsynthesis tutorial, Perspective Projection / Camera Perspective

Hi,

This may seem like a trivial question, but I want to understand everything completely.

In the section of the tutorial mentioned in the title, in the seventh paragraph, it says that “…the offset from the projection plane to the eye, is always -1”. I don’t understand why it is -1, instead of +1. Since we’re talking about camera space, where Z grows (gets more positive) when coming closer to the viewer, shouldn’t the offset be +1, that is, one unit closer to the viewer than where the plane is? I probably have something mixed up, but I’m not sure exactly what.

I really enjoy the tutorial so far, so if the author reads this, thank you!

It says that camera space defines positive Z to be forward, which means towards the viewer. Therefore, negative Z is away from the viewer. So if you want the projection plane to be in front of the camera, it would have to be at a negative offset.

Ah, I get it now, thanks!

One more thing: I think I found a typo in Introduction / Vector Math / Equation 4 and 5: the components in the vector are named ax,az,az. If I’m not mistaken, that should be ax,ay,az.

Thanks, I’ll fix those the next time it gets updated.