Parallel Projection

Perspective projection

Coordinate position are transformed to view plane along parallel lines. The vector defining the direction of lines is called projection vector. 

Coordinate position are transformed

to view plane along lines that converge to a point called projection reference point (PRP). 

Object distance from the view plane is not significant.

Object's distance from the PRP and view plane is important. It plays a role in determining size of the image.

Farther the object, smaller is the image. 

Parallel lines remain parallel. 

Parallel lines may not remain parallel. 

Ratios are preserved in general. 

Ratio are not preserved in general.

Useful in civil, architectural design, machine design, sectional view design, etc.

Useful in virtual reality, gaming and other such applications. 

Less realistic.

More realistic.