Oblique projection is a type of technical drawing and is primarily used for producing two-dimensional images of three-dimensional objects.
Technically, oblique projection is a type of parallel projection where it projects an image by intersecting parallel rays (projectors), and from the three-dimensional source object with the drawing surface (projection plane).
In both oblique projection and orthographic projection, parallel lines of the source object produce parallel lines in the projected image. The projectors in oblique projection intersect the projection plane at an oblique angle to produce the projected image, as opposed to the perpendicular angle used in orthographic projection.
Oblique drawing is also the crudest “3D” drawing method but the easiest to master. One way to draw using an oblique view is to draw the side of the object you are looking at in two dimensions, i.e. flat, and then draw the other sides at an angle of 45°, but instead of drawing the sides full size they are only drawn with half the depth creating ‘forced depth’ – adding an element of realism to the object. Even with this ‘forced depth’, oblique drawings look very unconvincing to the eye. For this reason, oblique is rarely used by professional designers and engineers.
Three main oblique projections:
1 cavalier projection | the two axes, x, and z in the figure, are perpendicular and the length of these axes are drawn on a 1:1 scale; it is thus similar to the dimetric projections, although it is not an axonometric projection, as the third axis, here y, is drawn in diagonal, making an arbitrary angle with the x″ axis, usually 30 or 45°. The length of the third axis is not scaled.
2 cabinet projection | one face of the projected object is parallel to the viewing plane, and the third axis is projected as going off at an angle. Unlike cavalier projection, where the third axis keeps its length, with cabinet projection the length of the receding lines is cut in half.
3 military projection | the angles of the x– and z-axes are at 45°, meaning that the angle between the x-axis and the z-axis is 90°. That is, the xz-plane is not skewed. It is rotated over 45°.
Oblique projection was used almost universally by Chinese artists from the first or second centuries to the 18th century, especially when depicting rectilinear objects such as houses.
Besides technical drawing and illustrations, video games (especially those preceding the advent of 3D games) also often use a form of oblique projection.