Orthographic and Oblique Projection - Viewing Transformation
Orthographic projection is the easiest form of parallel projection that is commonly utilized for engineering drawings. They truly show the 'true' shape as well as size of a particular plane face of a specified object. The projection is said to be orthographic when the direction of projection i.e. d = (d1, d2, d3) which has the direction of view plane normal N (or d is perpendicular to view plane). However it is termed as Oblique projection. The orthographic and oblique projection shows in the following figure 6.
Figure: Orthographic and oblique projection
We can notice here that the orthographic or perpendicular projection demonstrates only front surface of an object that includes only two dimensions: length as well as width. The oblique projection, conversely, demonstrates the front surface and the top surface that includes three dimensions: width, height and length. Thus, an oblique projection is one manner to show all 3-dimensions of an object in a particular view.