I have an object representing a room. I want to cut a door on it. I planed to use knife tool, because I do not know any better for this - any other (or "proper") idea would be good too BTW. I want to move X units from the corner on the bottom edge and make a cut from there (green dot) to the top. How can I do this?
Use the knife tool or loop cuts to build your mesh, and then here is a trick to move a vertex or edge to a precise position relative to the corner:
- select the vertex or edge you want to move
- move it all the way to the corner, validate
- move it back (for example with GX), enter the distance on the keyboard.
There is no direct way, sadly, afaik. One different way is to cut anywhere on the edge, and then move the vertices where you need precisely (in your image, along the x axis). Or you could also just create a vertex in the middle of the edge (by subdividing) then move it on x axis and then cut with the knife tool using snapping, to that precisely positioned vertex.
Note, you can also position a vertex on the edge by using "vertex slide" SHIFTV, and see its precise position on the edge in the viewport activating, in the properies panel (press N) mesh display > edge info > length