. introduction to descriptive-geometry

INTRODUCTION TO BASIC DRAWING :

THE ROLE OF DRAFTING & DESCRIPTIVE GEOMETRY - AS A LANGUAGE :

- Most of the objects in the world around us have three dimentions : length , width , and height .

- In order to draw a three dimentional object on a two dimentional (flat) sheet of paper - we need a system which will enable us to describe it , as accuratly as possible, on the flat surface of the paper , or even on the flat screen of our computer .

- There are several ways to do it - and in all of them , we get the views of all the edges which compose the object - on a chosen picture plane ( our paper , our screen , or any other flat surface we choose ).

- a View is the projected image of a point in space - on a given picture plane (our paper).

- the action itself is called P R O J E C T I O N .

- the image on the paper which is the result of the projection - is called a V I E W .

- The most common projection methodes are :

- P E R S P E C T I V E projections .

- I S O M E T R I C projections .

- O R T H O G E N I C projections - ( also called : the Monge method ) .

The three methodes differ from each other by :

- in the source of the projection rays .

- the direction of the rays .

- the relation between the projection rays and the picture plane (our paper ).

P E R S P E C T I V E - D R A W I N G :

perspective drawing

- In the perspective drawing , the center of projection is a point which is called the view point . From this point we project straight lines through all the edges of the object .

- When each of these rays hit our paper (which is our picture plane) , we get on the paper the graphic image of the object as it would have been seen by us , if we we stood at the view point which we chose , relative to the object .

The perspective drawing has a few dominant characteristics :

1 : Lines which are parallel to each other in the three dimentional world - will be seen in the perspective drawing as converging into one point . This point is an imaginary point and is called a vanishing point . There is an infinite number of such points .

2 : Objects whose size is identical in the real world will differ in size in the perspective drawing - the image of an object that is more distant from us will be seen smaller , and its drawn image will be smaller .

The A D V A N T A G E S of the P E R S P E C T I V E drawing :

The perspective drawing is easy to understand - because it shows the image of the object exactly as the human eye sees it , and one doesn't have to know to read drafting in order to understand it .

The D I S A D V A N T A G E S of the P E R S P E C T I V E drawing :

The perspective drawing is distorted and cannot be a source of any accurate dimensions .

P R O J E C T I O N S in P A R A L L E L L I N E S :

isometric view

I S O M E T R I C V I E W :

- When the center of projection (our eyes) is at an infinite distance from the viewed object - the lines of projection will be parallel to each other , and will never meet - as opposed to the perspective in which all the lines of projection converge into one point - into our eye - which is the center of projection .

- This method is called the - parallel projection - method and it is the basis to :

- the Isometric drawing method.
- the Orthogenic projection drawing method .

- Both methods enable us to know the exact measures of an object from such drawings.

- The isometric drawing shows the object in three dimentions - but its weakness is that the human eye sees in perspective - and because of that , the isometric view may seem a little distorted to the non proffesional viewer .

- The isometric view describes all three dimensions of an object - ( length , width , and hieght ) . The spatial direction of each of the three dimensions is represented by an axes which - in the real world is perpendicular to the other two - and whose graphic representation ( on the paper ) are three straight lines converging into one point with a 120 degrees angle separating between each two lines .

- The vertexes of objects are located on the isometric view according to their distances from each of the three axes .

- All the lines which are parallel to each other in the real world - will be drawn as parallel to each other in the isometric view too .

- Edges of an object which are equal in size in the real world - will be drawn as equal in length in the isometric view .

The A D V A N T A G E S of the I S O M E T R I C drawing :

- The isometric drawing enables us to see the drawn object without the perspective distortion - its dimensions everywhere in the drawing - are correct , and because of this , it is of great use to builders and all kind of other proffesionals .

- Another of its advantages is that the object is described three dimensionally - and that makes it easy to grasp .

The D I S A D V A N T A G E S of the I S O M E T R I C drawing :

- The main disadvantage of this method of projection is that the sizes of the drawn object are always true to its measurments in reality - while we are used to see it distorted as we see it in perspective in the real world , and sometimes this can be of a nuisanse .

the O R T H O G E N I C P R O J E C T I O N :

orthogenic views

- While In the first two methodes - the objects are described as they appear - on one picture-plane , in this method - the object is described by projections on as many picture-planes as we need , and that enables us to produce a more detailed and accurate technical description of the object from any angle we wish .

- In this method , different views of the object are drawn separatly on each picture-plane by projecting parallel lines through the object on to the- picture plane and perpendicular to it . All the different views are connected to each other through a system of agreed rules .

- The basic system of the Monge method consists of two picture-planes which are situated perpendicular to each other - a vertical picture plane & a perpendicular one . The line where they cut each other is the X axes .

- The viewer is located always at an infinite distance from the object and from the picture-planes .

- In the time of the projection on font the font horizontal picturefont - plane the viewer is located at an infinite distance above it - and the resulting view is called the : T O P V I E W .

- In the time of the projection on the perpendicular picture - plane the viewer is located at an infinite distance away from it - and theresulting view is called a : F R O N T V I E W .

The A D V A N T A G E S of the O R T H O G E N I C drawing :

- This drafting method was invented by a french mathematician called Monge - and it enables us to describe an accurate image of an object & its exact dimensions . It does so by enabling us to locate as many picture-planes as we need , in any angle and in any position we desire , in front of the object , and this helps us to solve complicated spatial problems .

The D I S A D V A N T A G E S of the O R T H O G E N I C drawing :

- The main disadvantage of this method is that it is understood only by people who studied it - and it is not an easy language to be understood and to master . However , it is a language that each proffesional has to know to read , and to use .

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