1 Descriptive geometry Introduction

The Method of Projection
In graphic language the shape is described by projection, which is the image of the object, formed by rays of light, taken in some particular direction, from the object into a picture plane, as it appears to an observer stationed at the point, from or towards which the projection is made.
Depending upon the orientation of the object, location of the point of sight, and the direction of lines of sight relative to the picture plane, different types of projections, e.g., central (perspective), parallel, orthographic, axonometric, oblique, etc., can be obtained.
The plane, on which the projection is taken, is called the plane of projection or picture plane. The point, from which the observer is assumed to view the object, is called the station point or the centre of projection.

The Method of Projection
Central Projection
Essentials of projection:
П1 – plane of projection (picture plane);
S – center of projection (station point ) (S∉Π1);
C – a point (original);
SC –projector (projecting ray);
C1 – projection (image) of a point C;
АВ – straight line segment (original);
SА, SB – projectors (projecting rays);
А1В1 – central projection of АВ line segment onto the plane Π1.
Algorithm how to construct central projection of a figure:
From the point S draw a projecting ray through the any point of a figure;
Find piercing point between projector and a plane of projection Π1. Obtained point is a projection (image) of an original to the plane of projection;
Find projections of the points А1 and В1, join them and obtain central projection of АВ line segment.

Properties of Projection
1. Point projects to a point (А→А1);
2. Straight line projects to a straight line: ([МК]→[М1К1]);
Exception to this property: if a straight line segment belongs to the projecting ray, then it projects to a point: ([ЕF]→E1=F1);
3. Plane projects to a plane;
Exception to this property: if a plane contains projectos, then it projects to a straight line.
4. If the point belongs to some geometrical object, then projection of that point belongs to the projection of that geometrical object (Nє[МК]→N1є[М1К1]);

Parallel Projection
2. If angle between projectors and a plane of projection is less than 90° α ≠ 90°, then it is parallel nonorthogonal projection.
All the properties of central projection preserved.
For this type of projection sizes of the image can exceed sizes of the original.
Parallel nonorthogonal projection.
1. When center of projection S is a point at infinity, all projectors (АА1, ВВ1, СС1) are parallel. Such a projection is called parallel projection.

Parallel Projection

2. Ratio between line segments of the same line or parallel lines preserves
3. Planar figure, parallel to the plane of projection, projects to a true size onto that plane.
4. For orthographical projection sizes of the image can’t exceed sizes of the original. If planar object is parallel to the plane of projection, then it projects to a true size.
Properties of Parallel Projection
Properties:
1. Parallel lines project to parallel lines ((CD) || (AB) → (C1D1) || (А1В1)).
If projecting rays are perpendicular to the plane of projection (α=90°), then such a projection is called orthogonal (orthographic) projection.

Orthographical projection of a point
Images on the drawings should be reversible to imagine real shape of an object.
It’s impossible to restore figure shape by only one orthographic projection.
Methods to obtain reversible images.
To obtain reversible images some additional terms have to be met. To meet these terms following methods can be used:
Orthographical projection on two planes (Monge’s method);
Axonometric projection method;
Method of projection with elevations.
Vector constructions.

Orthographical projection of a point
Reversibility in projection
A point projects onto two directions or two different planes. Points of original object are obtained from intersection of corresponding projectors.

Monge’s method
A point projects onto two mutually perpendicular principal planes (planes of projection). After that two principal planes are superposed by rotating one plane around the axes (which is a line of intersection between two planes of projection) up to coinciding to the other plane (rotating horizontal principal plane П1 around х-axis).

Monge’s method
Designations
П1 – horizontal principal plane (plane of projection) - H;
П2 – frontal (vertical) principal plane (plane of projection - F (V);
А1 – horizontal projection of a point А;
А2 – frontal projection of a point А;
AxA1 – depth of A-point (distance from П2 -plane);
AxA2 – height of A-point (distance from П1 -plane);
A1A2 – projectors (projecting lines).
Horizontal and frontal projections of a point lie on the same vertical projector, perpendicular to x-axis.

Concurrent points
А and В — horizontally concurrent points
C and D — frontally concurrent points
If projection of points are coinciding on the one principal plane, but different on the other plane, these points are called concurrent points.
A2
B2
B
A
C1
D1
C
D
Y
П2
B2
П1
C2 =D2
q2
q1
X
Z
D1
C1
A2
A1 =B1
X
Cx =Dx
Ax =Bx
C2 =D2
A1 =B1
Ax =Bx
Cx =Dx

Orthographical projection of a point
Some problem can be solved easier, if two principle planes of projection would be supplemented by the third plane, perpendicular to them. Such a plane is called profile plane of projection (П3). This principal planes is superposed by rotating around the axes (which is a line of intersection between two planes of projection) up to coinciding to the combined planes (rotating profile principal plane П1 around z-axis).
Coordinates of the point А: XA, YA, ZA — numeric values of line segments along coordinate axes.
П1 – horizontal principal plane (plane of projection) - H;
П2 – frontal (vertical) principal plane (plane of projection - F (V);
П3 – profile (vertical) principal plane (plane of projection – P(W);
X, Y, Z – axes;
A1, A2, A3 – projections of А-point;
АА3=XA; АА2=YA; АА1=ZA;
AxA1 – depth;
AxA2 – height;
ОАx – width.
A2
II
III
x
IV
I
y
O
A
z
Ax
Az
A3
Ay
A1
П2
П1
П3
ZA
YA
XA

Principal planes of projection

Orthographical projection of a point А (ХА , YA , ZA)
x
y
z
y
П1
П2
П3
Drop ХА coordinate (width) from origin point along X-axis, designated АХ (ОАХ = ХА)
Construct vertical projector ⊥ X-axis
Construct А2 protracting ZА - coordinate (height) along z-axis (АХА2 = ZА)
Construct А1 protracting YА - coordinate (depth) along y-axis (АХА1 = YА)
Construct horizontal projector ⊥ Z-axis and find АZ.
Construct А3 protracting YА - coordinate (depth) along horizontal y-axis (АZА3 = YА).

Straight Lines. Classification.

Orthographic projection of a Straight Line.
Horizontal line
Horizontal (horizontally parallel) – straight line, parallel to П1.
x
O
Y'
Z
Y
Horizontal line projects onto П1 to a True Length.
Angles β and γ on that projection are true angles between a line and principal planes of projection П2 and П3.
Horizontal line projects onto П2 and П3 to horizontal line segments of a length, less then a true size.
A
β
γ
O
β
γ
X
z
A1
B1
A2
B2
B
B3
A3
y

Orthographic projection of a Straight Line.
Frontal
Frontal line projects onto П2 to a True Length.
Angles α and γ on that projection are true angles between a line and principal planes of projection П1 and П3.
Frontal line projects onto П1 and П3 to vertical line segments of a length, less then a true size.
Frontal (frontally parallel) – straight line, parallel to П2.
D2
γ
α
α
γ
y
z
x
O
f
C
D
C2
f2
D1
C1
D3
C3
f1
f3

Orthographic projection of a Straight Line.
Profile line
Profile (profily parallel) – straight line, parallel to П3.
Frontal line projects onto П3 to a True Length.
Angles α and β on that projection are true angles between a line and principal planes of projection П1 and П2.
Profile line projects onto П1 and П2 to vertical line segments of a length, less then a true size.
O

Orthographic projection of a Straight Line. Horizontally projecting (perpendicular) line
Line segment onto П1 projects to a point.
Horizontally-perpendicular line (AB⊥П1).

Orthographic projection of a Straight Line. Horizontally projecting (perpendicular) line
Line segment onto П1 projects to a point.
Projections on П2 and П3 are vertical lines of a true length.
Horizontally-perpendicular line (AB⊥П1).
Z
X
Y
Z
Х
О
О
A2
B2
B
A
A3
B3
A1=B1
A2B2 || OZ
A3B3 || OZ
Y
Y

Orthographic projection of a Straight Line. Frontally projecting (perpendicular) line
Frontally-perpendicular line (CD⊥П2).
Line segment onto П2 projects to a point.

Orthographic projection of a Straight Line. Frontally projecting (perpendicular) line
Frontally-perpendicular line (CD⊥П2).
Line segment onto П2 projects to a point.
Projections on П1 and П3 are lines of a true length.
X
Y
Z
O
Y
'
X
C
D
Z
Y
O
D3
C3
C1
D1
C2= D2
C1D1 || OY

Orthographic projection of a Straight Line.
Profily projecting (perpendicular) line
Profily-perpendicular line (MN⊥П3).
Line segment onto П3 projects to a point.

Orthographic projection of a Straight Line.
Profily projecting (perpendicular) line
Profily-perpendicular line (MN⊥П3).
Line segment onto П3 projects to a point.
Projections on П2 and П3 are lines of a true length.
E2F2
||
OX
E
O
Y
F
Z
X
X
Y
Y
'
Z
O
E2
F2
E1
F1
E3=F3

Oblique Line

Oblique Line neither parallel nor perpendicular to any principal plane of projection.
Oblique line projects onto principal planes П1, П2 and П3 to line segments of a length, less then a true size.
Oblique Line

Relative Position of a Line and a Plane
Relative position of a Line and a Plane
Two variants:
1) Point belongs to a Line.
From the property of a projection it is known: if a point belongs to a line, then projection of that point would belong to the projection of that line.
2) Point does not belong to a Line. In this case relating to the planes of projection a point can be: above, below, in front of, behind.

Relative Position of Lines
K and L, M and N – concurrent points
Proper projections are parallel.
Lines are intersecting in a point at infinity
К1 and К2 are on the same projector
Intersecting
Parallel
Skew (Crossing)

Intersecting с∩d=K

Parallel a║b

Skew (Crossing) ―

Projecting of a Right Angle - Theorem.

Projecting of a Right Angle - Theorem.
Exception: If one side of an angle is parallel to a plane and the other is not perpendicular to that plane, then acute angle projects to acute angle, obtuse angle projects to obtuse angle and right angle projects to a right angle onto that plane.
Projections of angles depend on their location concerning the planes of projection. According to the properties of parallel projection their types and sizes are not preserved.
The last statement is known as a theorem about projecting of a right angle.
It can be formulated this way:
A
2
B
1
B
2
B
C
1
C
П
2
П
1
A
1
A
C
2
X
B1
А1
C1
C2
B2
А2
If one side of a right angle is parallel to a plane and the other is not perpendicular to that plane, then right angle projects to a true size (to a right angle) onto that plane.

Projecting of a Right Angle - Theorem.
If one side of a right angle is parallel to a plane and the other is not perpendicular to that plane, then right angle projects to a true size (to a right angle) onto that plane.

Orthographic projection of a Plane.
Representation of a Plane.
b
A Point and a Line
a
Three Points
c
Two Intersecting Lines
d
Two Parallel Lines
e
Plane Segment

Planes.
Classifications.

Oblique Plane
Z
Y'
Y
X
Oblique plane neither parallel nor perpendicular to any principal plane of projection.
B3
A
A2
C
C2
B
A3
C3
Z
Y
О
B2
A1
C1
B1
X

Principal Planes.
Horizontal (principal) plane ((ABC) || ∏1) is a plane, parallel to horizontal principal plane of projection.
Any planar figure, which lies in this plane, projects onto the horizontal principal plane П1 to a true size.
Frontal and Profile planes have similar properties.

Principal Planes.
Frontal (principal) plane ((ABC) || ∏2) is a plane, parallel to the frontal principal plane of projection.
Any planar figure, which lies in this plane, projects onto the frontal principal plane П2 to a true size.

Principal Planes.
Profile (principal) plane ((ABC) || ∏3) is a plane, parallel to the profile principal plane of projection.
Any planar figure, which lies in this plane, projects onto the profile principal plane П3 to a true size.

Planes.
Perpendicular (Projecting) Planes.
Horizontally-projecting (perpendicular) ((ABC) ⊥ ∏1) plane is a plane, perpendicular to the horizontal principal plane of projection. Any planar figure, which lies in this plane, projects onto the horizontal principal plane to a straight line. Horizontally-projecting plane can be represented by its one projection Σ1 only. Angles β and γ between Σ1 and orthogonal projectors are true angles between the plane Σ and principal planes П2 and П3.

Planes.
Perpendicular (Projecting) Planes.
Frontally-projecting (perpendicular) ((ABC) ⊥ ∏2) plane is a plane, perpendicular to the frontal principal plane of projection. Any planar figure, which lies in this plane, projects onto the frontal principal plane to a straight line. Frontally-projecting plane can be represented by its one projection Σ2 only. Angles α and γ between Σ2 and orthogonal projectors are true angles between the plane Σ and principal planes П1 and П3.

Planes.
Perpendicular (Projecting) Planes.
Profily-projecting (perpendicular) ((ABC) ⊥ ∏3) plane is a plane, perpendicular to the profile principal plane of projection. Any planar figure, which lies in this plane, projects onto the profile principal plane to a straight line. Profily-projecting plane can be represented by its one projection Σ3 only. Angles α and β between Σ3 and orthogonal projectors are true angles between the plane Σ and principal planes П1 and П2.

A Line in a Plane
A Straight Line belongs to a Plane, if it passes through two points, which lie in that plane.

A Point in a Plane.
Algorithm:
1.Through the frontal projection of the point М2(N2) draw any straight line, which belongs to the plane ((АВС) or (аIIb)).
2.Define points 3 and 4, which belong to the plane and the line simultaneously.
3.Construct horizontal projection of considered line.
4.On the obtained projection of the line find horizontal projections of the point М(N).
In the problem II according to the algorithm above, we can see, that point N doesn’t belong to the plane.
A2
I
II
A1
B2
C2
C1
B1
A Straight Line belongs to a Plane, if it passes through two points, which lie in that plane. A Point belongs to a Plane, if it belongs to the Line, which lies in that Plane.
a2
N2
N1
b2
b1
a1

Principal Lines in a Plane
Principal Lines in a Plane – Lines of a special location, which belong to that plane.
Principal Lines, parallel to the Planes of projection:
Horizontal h – line, parallel to П1,
Frontal f – line, parallel to П2,
Profile p – line, parallel to П3.
Horizontal
Frontal

Principal Lines in a Plane
B2
A2
C2
B3
A3
C1
B1
A1
Principal lines in a plane, which are parallel to one plane of projection, parallel to each other.
To construct horizontal principal line of ABC-plane first draw its frontal projection, parallel to x-axis, and then draw its frontal and (if necessary) profile projections.
Execute similar constructions to obtain frontal or
profile principal lines in ABC-plane.
Principal Lines in a Plane – Lines, which belong to that plane and parallel to the Planes of projection. Horizontal h – line, parallel to П1;
Frontal f – line, parallel to П2;
Profile p – line, parallel to П3.
C3

Steepest Lines in a Plane
The Steepest Lines in a plane are lines, lying in this plane and perpendicular to its principal lines.
The Steepest line to the plane П1 is called Steepest Ascent or Steepest Descent Line.
It may serve to determine the angle between the plane (ABC) and horizontal principal plane П1.
х
A2
B2
С2
A1
B1
С1
h (h2, h1) – horizontal
t (t1, t2) – steepest line
a
b
c

Parallelism of a Line and a Plane!
A Line, parallel to a Plane, is a Line, which doesn’t belong to the plane and doesn’t intersect it. If a Line is parallel to a Plane, then it is parallel at list to the one line, which lies in that plane.
Line L passes through the point M and parallel to ABC-plane.
Line L’ is parallel to the line L and passes through the point A, which belongs to the line m.

Two Planes are Parallel, if two intersecting lines of one plane are parallel to two intersecting lines of the other plane.
Parallelism of Planes

Polyhedrons. Terms and Definitions
Polyhedron is a solid figure, bounded by plane polygons.
These polygons are called Faces of a polyhedron.
Faces of a polyhedron intersect in straight line segments called Edges.
Edges meet in points called Vertexes (Vertices) of a polyhedron. Multiple of faces form polyhedral surface of a polyhedron.
All the vertices and edges of a polyhedron form the mesh of the solid. To construct orthographic view of a polyhedron means to construct orthographic view of the mesh of the polyhedron.
The Mesh completely defines the polyhedron and is called Determinant of a polyhedron.

Regular Polyhedrons.
A Regular polyhedron is a polyhedron whose faces are all regular polygons which are identical in both shape and size.
There are two particular spheres associated with any regular polyhedron.
First is the circum-sphere This is the sphere which fits around the outside of the polyhedron so as to touch all its vertices.
Second is the in-sphere This is the sphere which fits inside the polyhedron so as to touch all its faces.
If no face of a polyhedron can cut it on extension, then polyhedron is convex, otherwise it is concave.
For every convex polyhedron ratio between faces, edges and vertexes can be defined by Euler formula
F – E + V = 2
F - number of faces
E - number of edges
V - number of vertexes

Regular Polyhedrons.
tetrahedron 4 triangles
cube 6squares
octahedron 8 triangles
dodecahedron 12 pentagons
icosahedron 20 triangles

Prism and Pyramid

Pyramid and Prism.
Pyramid is a polyhedron formed by connecting a polygonal base and a point, which doesn’t lie in the base plane. This point is called the apex of a pyramid. Each base edge and apex form a triangle, which called lateral face.
Prism is a polyhedron made of polygonal bases and lateral faces, joining corresponding sides. Thus these lateral faces are parallelograms. All cross-sections parallel to the base faces are of the same shape and size.
Pyramid Prism

Polyhedrons. Visibility definition
Prism (Lateral edges intersect in ∞).
A1
D1
C2
C1
y
y
z
F1
Е1
Е2
F2
D2
x
A2
B2
B1
Outline is always visible.
If two edges intersect inside outline, then one is visible, and the other is invisible (hidden).
If three edges meet in one point (vertex) inside the contour, then they are all visible or they are all hidden.
If sequence of vertexes for the polyhedron’s face is the same on orthographic views, then visibility is the same, otherwise it is different.
How to define Visibility of edges and faces in orthographic views of a polyhedron.
Four rules for visibility definition in polyhedrons.
ADFB-
visible
ADFB-
visible
ADFB-
hidden

Polyhedrons. Pyramid
Pyramid - Lateral edges meet in one common point (Apex).
B1
y
A2
A1
x
B2
C1
C2
y
z
S2
S1
Example:
Point M belongs to SBC-face Мє(SBC). M2 is given.
Find M1.
a) Through the projection of the point М2 draw straight line segment S212;
б) Find its projection S111 on the plane П1;
в) Protract vertical projector from the М2 and find projection М1.
Criteria of representation.
If it’s possible to complete projection of the point, which belongs to polyhedral surface, by its one projection, then polyhedron is represented on the orthographic drawing.

B1
y
A2
A1
x
B2
C1
C2
y
z
S2
S1
Pyramid - Lateral edges meet in one common point (Apex).
Polyhedrons. Pyramid