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A Pictorial Drawing is an image of an object (actual or imagined) viewed as it would be seen by someone who looks at the object either in a chosen direction or from a selected point of view. Pictorial drawings or sketches often are more readily made and more clearly understood than are Orthogonal Drawings. (Front, top, and side views of an object). Pictorial drawings, either sketched freehand or made with drawing instruments, are frequently used by engineers and architects to convey ideas to their assistants and clients.
A Pictorial drawing provides a 3D image to help understand the shape of an object or to assist in interpreting a drawing. There are 3 main ways to draw a pictorial drawing,
1. Isometric,
2. Oblique,
3. Perspective.
(other methods of Pictorial drawings include planometric, dimetric drawing, trimetric drawing)
1. Isometric drawing
Definition – An Isometric drawing is a pictorial representation of an object in which all three dimensions are drawn at full scale. The term isometric means “equal measurement”.
Isometric drawing is the most commonly used method of pictorial drawing. Isometric drawings are built on three lines, called isometric axes. One is drawn vertically and the other two with the 30° set square either side of it. An Isometric drawing is a way of presenting designs in three dimensions (3D). They can be completed using the 30/60 set square or freehand. An Isometric drawing is one of a family of three-dimensional views called pictorial drawings. In an isometric drawing, the object’s vertical lines are drawn vertically, and the horizontal lines in the width and depth planes are shown at 30 degrees to the horizontal. The true dimension of the object is used to construct the drawing. These dimension can be taken from either drawings completed in orthographic or by measuring an object. Isometric drawings or images have become the industry standard for parts manuals, technical proposals, patent illustrations and maintenance publications due to their use of true length and the ability for untrained people to understand. An isometric projection results if the plane is oriented so that it makes equal angles with the three principal planes of the object. The representation of the object seen below is an isometric drawing of a cube. In this drawing the three visible faces appear as equilateral parallelograms; that is, while all of the parallel edges of the cube are projected as parallel lines, the horizontal edges are drawn at an angle (usually 30°) from the normal horizontal axes, and the vertical edges, which are parallel to the principal axes, appear in their true proportions.
2. Oblique projection
Oblique projection is a method of drawing objects in 3 dimensions. It is quite a simple technique compared to isometric or even perspective drawing. However, to draw accurately in oblique projection traditional drawing equipment is needed
Oblique projections illustrate the general 3 dimensional shape of the object. However only faces of the object parallel to the plane of projection are shown at there true size and shape, that is angles and lengths are preserved for these faces only. In fact ,the oblique projection of these faces is equivalent to an orthographic front view.
TYPES OF OBLIQUE PROJECTIONS-
1. Cavalier and Cabinet Projection
A cavalier projection is obtained when the angle between oblique projectors and the plane of projection is 45 degree. In a cavalier projection the foreshortening factors for all three principal direction are equal. The resulting figure appears too thick. A cabinet projection is used to correct this deficiency.
Cabinet projection- An oblique projection for which the foreshortening factor for edges perpendicular to the plane of projection is one half is called a cabinet projection.
3. Perspective drawings
A Perspective drawing is one of the best types of drawings to use to provide a pictorial representation of an object. They produce an image of an object in three dimensions that is very similar to what the human eye sees. Perspective drawings can be in one point, two point or three point perspective. One of the most common uses of perspective drawing is in architecture, where it is used to provide a client, or the public, an image that gives a good representation of how the project will look when it is built. One point perspective is often used to represent a room on paper. As well as architecture, perspective drawing is often used in the furniture industry to show clients the final appearance of a piece of furniture they might commission.
Other pictorial drawing types
Axonometric and Planometric drawing
A method of projection in which a three-dimensional object is represented by a drawing having all axes drawn to exact scale, resulting in the optical distortion of diagonals and curves. Axonometric (45 degree angle) or planometric (60/30 degree angles) as it is sometimes known is a method of drawing a plan view with a third dimension. It is used by interior designers, architects and landscape gardeners. Axonometric works by drawing a plan view at a 45 degree angle with the depth added vertically. All lengths are drawn as their true lengths unlike when you use oblique. This gives the impression that you are viewing the objects from above. One advantage of axonometric is that circles drawn on the top faces of objects can be drawn as a normal Isometric and Axonometric Projections are fairly simple ways to approach 3D drawing. These can be used for sketches or to draw to a predetermined scale. If a scale is used, every part of the drawing can be measured with accuracy. Unlike perspective drawing, lines in Isometric or Axonometric drawings do not converge. In fact they only go in 3 different directions. Vertical and 30 degrees left and right in Isometric Projections, vertical and 45 degrees left and right in Axonometric projections. Often, Axonometric projections may appear to be distorted but they are very useful to show as much as possible of, for example, the inside of a room.
The same scale is used on all three axes.
Follow this link for a more in-depth overview of Perspective Drawings.