How to cut with Knife Tool parallel to the other edge in Ortho\Persp view? Blender 2.79b
1 Answer
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In this case Offset Edge Slide will suit the situation, although it will create additional geometry.
Select the edge you want the new one to be parallel to and run the tool from the left area of the viewport (depending on Blender version) or Shift+Ctrl+R.
The tool has the same properties as Knife which means you can make edge being created parallel to the edge you choose to. To make it parallel to the top one in this case enable Even in the tool options (press E while operator is running) and choose orientation by enabling Flipped (or with F) as desired.
Then select the other edge created on the other side of the main edge and dissolve it.
Note that with topology in question you might need to make a clean-up depending on the purpose of the model. For example, the shown topology isn't suitable for Subsurface modifier and might provide artifacts if bevelled.
Regarding the Knife tool for this case. It is not possible to cut with it as shown in your screenshot. Knife tool can only cut under certain angles (0, 45 and 90) and it can't use perspective as a guide for the straight cut.
