Viewing Direction is the central direction in which an observer, eye, camera, instrument or represented viewing system looks towards a spatial object or scene.
In the Dictionary of Perspective, the preferred technical term for this central direction is the Line of Sight.
Viewing Direction should be distinguished from Viewpoint. The Viewpoint identifies where the observer or imaging system is located, whereas the Viewing Direction identifies where it is looking.
In simplified form:
Viewpoint → position
Viewing Direction → direction
Field of View → angular extent
Together, these are among the fundamental conditions determining the Perspective Image or View produced from a spatial object or scene.
Viewing Direction within Perspective
A Perspective Image or View is normally related not only to a spatial position but also to a direction of observation.
From any particular Viewpoint, an observer or imaging system can look in many possible directions.
The chosen Viewing Direction determines which region of Object Space lies principally ahead of the observer and how the spatial object or scene is oriented relative to the act of viewing.
It therefore contributes to:
- the aspect of objects;
- the surfaces that are visible;
- apparent shape;
- foreshortening;
- projected line directions;
- vanishing relationships;
- the organisation of the Field of View; and
- the overall Perspective Form.
Viewing Direction and Line of Sight
Line of Sight is the preferred general term for the principal line associated with Viewing Direction.
It is a line extending from the eye or Centre of Projection towards a point in the spatial scene.
Thus:
Viewing Direction → directional relationship
Line of Sight → geometrical line expressing that direction
The Line of Sight may be:
- horizontal;
- inclined upwards;
- inclined downwards; or
- otherwise directed towards the relevant region of the spatial scene.
Changing the Line of Sight changes the central Viewing Direction even when the Viewpoint remains fixed.
Alternative Terms
Perspective terminology contains many names for closely related forms of the Viewing Direction or Line of Sight.
These include:
- Angle of View (Type 1);
- Angle of Vision (Type 1);
- Axis of Vision;
- Axis of the Cone of Vision;
- Central Axis of Vision;
- Central Visual Ray;
- Direction of Gaze;
- Direction of Vision;
- Direct Line of Vision;
- Line of Direction;
- Line of Vision;
- Looking Angle;
- Observation Angle;
- Optical Axis;
- Perspective Axis;
- Sight Line;
- Visual Axis; and
- Visual Direction.
Although these terms occur in different historical, optical, graphical and technical contexts, they frequently refer to the same basic concept of a principal direction of observation or projection.
Viewing Direction and Viewpoint
Viewpoint and Viewing Direction are two separate variables.
The Viewpoint specifies the location of the eye, observer, camera or imaging system.
The Viewing Direction specifies the direction from that position towards the spatial scene.
An observer can therefore:
- remain at one Viewpoint and change Viewing Direction;
- move to another Viewpoint while maintaining approximately the same Viewing Direction; or
- change both Viewpoint and Viewing Direction simultaneously.
These alternatives produce different perspective relationships and should not be treated as equivalent.
A Fixed Viewpoint can have many Viewing Directions
A single Viewpoint does not imply a single possible view.
An observer standing in one location can look:
- forwards;
- backwards;
- left or right;
- upwards;
- downwards; or
- in any intermediate direction.
The Viewpoint remains unchanged while each change of Viewing Direction reveals another part or aspect of the surrounding spatial scene.
This distinction becomes particularly important in panoramic, circular and spherical viewing systems.
Viewing Direction and Field of View
Viewing Direction is not Field of View.
Viewing Direction identifies the central direction in which the eye, camera or instrument is directed.
Field of View identifies the total angular extent of the spatial scene visible, captured, represented or displayed around that direction.
Thus:
Viewing Direction → central axis
Field of View → angular region surrounding that axis
A camera may therefore retain exactly the same Viewing Direction while changing its Field of View through a change of lens, focal length, image format or other imaging condition.
The Axis of the Cone of Vision
In classical visual and graphical perspective, Viewing Direction is often represented by the Axis of the Cone or Pyramid of Vision.
This is the principal central line extending from the eye or Viewpoint towards the spatial scene.
The surrounding rays define the broader visual or representational field, while the central axis provides the principal direction about which that field is organised.
Thus:
Central Viewing Direction → axis of the visual cone
Surrounding visual rays → extent of the visual field
This provides a geometrical link between Viewing Direction, Line of Sight and Field of View.
The Principal Visual Ray
The Dictionary also identifies the axis of the Cone of Vision with a Principal Visual Ray.
This should be distinguished from the many rays associated with individual visible points of an object or scene.
The Principal Visual Ray represents the central direction of looking, whereas other visual rays relate individual object points to the eye or projection centre.
The central visual ray therefore provides a convenient geometrical representation of the overall Viewing Direction.
Viewing Direction in Single-View Perspective
Single-view, unified and many technical Perspective Systems employ a particular Viewpoint together with a fixed Viewing Direction.
The relationship can be represented as:
Fixed Viewpoint + Fixed Viewing Direction → Single Perspective View
This produces a stable geometrical relationship between:
- the observer or Centre of Projection;
- the spatial object or scene;
- the projection rays;
- the Picture or Image Plane; and
- the resulting Perspective Image.
Classical Linear Perspective is an important example of such a systematic single-view arrangement.
Viewing Direction in Linear Perspective
Linear Perspective normally represents a spatial scene from a fixed Station Point and a selected principal Viewing Direction.
The Viewing Direction helps establish the orientation of the complete construction relative to the Object Space and Picture Plane.
It influences the resulting:
- aspect of spatial Forms;
- foreshortening;
- principal image orientation;
- vanishing-point relationships;
- horizon and vanishing structures; and
- overall spatial organisation of the image.
The same object viewed from the same position but in a differently directed construction can consequently produce a different Perspective Image.
The Principal Point
In a standard central-perspective construction, the principal Line of Sight extends from the Station Point towards the Picture Plane.
Where the principal Line of Sight intersects the Picture Plane is commonly called the Centre of Vision or Principal Point.
Thus:
Station Point → principal Line of Sight → Principal Point on Picture Plane
The Station Point itself and the Principal Point on the Picture Plane must therefore not be confused.
One identifies the position of the observer or Centre of Projection; the other identifies where the principal Viewing Direction intersects the image surface.
Horizontal Viewing Direction
A horizontal Viewing Direction occurs when the principal Line of Sight is directed horizontally from the observer towards the scene.
This provides the familiar arrangement used in many conventional perspective diagrams.
However, a horizontal Line of Sight is only one possible viewing condition.
Perspective is not inherently restricted to looking horizontally or straight ahead.
Upward and Downward Viewing Directions
The Viewing Direction may be inclined.
An observer or camera can look:
- upwards towards a subject;
- downwards towards a subject; or
- at another oblique angle.
The Station Point need not change for this to occur.
Changing the Viewing Direction while keeping the same Viewpoint changes the orientation of the visual or projection system relative to the spatial scene and consequently alters the resulting Perspective Form.
Central Perspective and Viewing Direction
Central Perspective can be understood as a unidirectional form in which a spatial scene or object is viewed along one principal optical Line of Sight.
A Cone or Pyramid of Vision is directed towards the scene around this principal direction.
This unidirectional arrangement may include one-point, two-point or three-point perspective when the image remains organised as a unified forward-facing view.
Central Perspective should therefore not be equated simply with One-Point Perspective.
Its defining characteristic in this context is the organisation of the view around a single principal Viewing Direction.
Multidirectional Viewing
Not every Perspective System is organised around only one Viewing Direction.
A fixed observer may look successively or simultaneously in many directions.
This can produce:
- panoramic views;
- circular views;
- curvilinear representations;
- spherical images;
- 360-degree panoramas; and
- Sphere-of-Vision Perspective.
Such systems extend beyond the single central Viewing Direction characteristic of ordinary unidirectional perspective.
Viewing Direction and the Sphere of Vision
The Sphere of Vision provides a useful model for understanding multidirectional viewing from a fixed position.
The observer remains at one Viewpoint while looking in different directions throughout the surrounding spatial environment.
Thus:
Fixed Viewpoint + Changing Viewing Direction → Sphere of Vision
Potentially, all surrounding directions can be sampled from the same spatial position.
This is fundamentally different from travelling around an external object, where the Viewpoint itself changes.
Viewing Direction and the Sphere of Revolution
The distinction between Sphere of Vision and Sphere of Revolution can be clarified through Viewpoint and Viewing Direction.
Sphere of Vision:
Viewpoint remains fixed while Viewing Direction changes.
Sphere of Revolution:
the observer moves around the object, producing a series of different Viewpoints and associated Viewing Directions.
The two systems therefore obtain different kinds of spatial information despite both involving multiple views of spatial reality.
Viewing Direction and Object Aspect
The orientation of an object relative to the Viewing Direction determines its aspect.
A plane viewed frontally presents a different apparent Form from the same plane viewed obliquely.
Changing Viewing Direction can therefore change:
- visible surfaces;
- apparent outline;
- apparent proportions;
- projected shape; and
- the degree of Aspect Foreshortening.
The object itself may remain physically unchanged while its appearance changes solely because the observer is looking at it from a different direction.
Viewing Direction and Aspect Foreshortening
Aspect Foreshortening is closely related to the orientation of a spatial feature relative to the Viewing Direction.
Dimensions directed substantially towards or away from the observer may appear compressed relative to dimensions extending more laterally across the field of view.
The amount of such apparent shape transformation depends upon the projection or viewing angle.
Aspect Foreshortening should therefore be distinguished from Perspectival or Optical Foreshortening, which is the distance-dependent component associated with diminution of size.
Viewing Direction in Camera Perspective
For an individual camera exposure or frame, the camera normally records a scene from a particular Viewpoint and central Viewing Direction.
The camera’s principal optical axis provides the main directional reference for the image.
The complete result also depends upon:
- Field of View;
- lens characteristics;
- image format;
- camera orientation;
- distance from the subject; and
- the geometry of the object or scene.
Changing the camera’s orientation while leaving its position unchanged changes the Viewing Direction without necessarily changing the Viewpoint.
Viewing Direction in Natural Vision
Natural human vision does not normally maintain one fixed Viewing Direction for long periods.
The eyes rotate, fixation shifts, the head turns and the body may move.
Volume 1 therefore treats Visual Direction as a dynamic problem involving the relationship between retinal information and the changing position and orientation of the eyes, head and body.
The visual system must continually determine where observed objects lie relative to the observer and to external space.
Natural visual experience is consequently much more dynamic than a fixed graphical perspective construction.
Perceived Visual Direction
Perceived Visual Direction should not be reduced simply to a point on the retina.
Volume 1 explains that visual direction also depends upon:
- eye position;
- head orientation;
- body orientation;
- visual transformations;
- vestibular information; and
- proprioceptive information.
The human visual system uses these relationships to register retinal information with external spatial reality.
This makes Visual Direction a physiological and perceptual concept as well as a purely geometrical one.
The Binocular Direction Problem
Binocular vision introduces a further problem because the two eyes occupy different positions and therefore receive slightly different views of the same scene.
Each eye has its own optical relationship to the object.
The visual system must nevertheless create a coherent experienced direction from these two separately angled views.
Thus:
Left-eye direction + Right-eye direction → integrated visual direction
The solution to this problem forms an important part of natural and Visual Perspective Type 2.
Viewing Direction in Virtual and New Media Systems
In CGI, CAD, computer games and Virtual Reality, Viewing Direction can be modelled and changed continuously.
A virtual camera may remain at one location while rotating towards different regions of the virtual environment.
Alternatively, both its Viewpoint and Viewing Direction may change as the user moves through the model.
This permits:
- single-direction views;
- multi-direction views;
- panoramic exploration;
- 360-degree viewing; and
- potentially unrestricted exploration of a modelled spatial reality.
New Media Perspective therefore makes the distinction between Viewpoint and Viewing Direction particularly explicit.
Viewing Direction is not Perspective Direction
Viewing Direction should not be confused with the broader directional classification of a Perspective Process.
Viewing Direction concerns the spatial direction in which an observer, camera or system looks.
The directional Perspective Classes instead distinguish whether a process operates principally:
- from object or scene towards an eye, sensor or imaging system — Viewing or Imaging; or
- from a source or representational system forwards into spatial reality — Projecting.
Thus one concept concerns spatial orientation, while the other concerns the directional organisation of the Perspective Process.
Why Viewing Direction matters
Viewing Direction is fundamental because an object’s perspective appearance depends not only upon where it is viewed from, but also upon the direction in which it is viewed.
Recognising Viewing Direction helps distinguish:
- Viewpoint from direction;
- Line of Sight from Field of View;
- fixed from changing Viewing Directions;
- unidirectional from multidirectional perspective;
- Sphere of Vision from Sphere of Revolution;
- object Form from view-dependent aspect;
- Aspect Foreshortening from distance-dependent foreshortening; and
- the spatial direction of looking from the broader directional Class of a Perspective Process.
Viewing Direction is therefore the central spatial direction in which an observer, eye, camera, instrument or represented viewing system is directed towards a Perspective Object / Scene, usually expressed geometrically by its principal Line of Sight.
Related Perspective Topics
Viewpoint →
Perspective Image / View →
Perspective Object / Scene →
Perspective Transformation →
Perspective Form →
Perspective Category Theory →