What is projection
SŠ6 minParallel vs central projection, projection rays and the image plane.
What is happening
The rays are mutually parallel – the centre is at infinity. Moving the object perpendicular to π does not change the size of the image, which is why technical drawings can be measured directly.
Scene
Spatial view
Theory
Projection is a rule that assigns a point of the plane to a point of space. Through a point A we send a projecting ray, and where it meets the picture plane π we get the image A′. All of descriptive drawing rests on this single step.
A projection is always one dimension poorer: a three-dimensional solid becomes a two-dimensional picture. Some information is necessarily lost – which is exactly why in practice we draw two views, not one.
The image is neither a shadow nor a photograph. It is an exact geometric construction: once the ray and the picture plane are known, the image is uniquely determined and can be built with ruler and compass.
Step-by-step construction
Practice
Build the construction yourself
What is the next step?
1 / 5Choose the step that comes next. The drawing only advances on a correct choice.
Check your understanding
What is the image of a point A?
How many dimensions does the image of a 3D solid have?
Why do we draw two views in practice, not one?
Downloadable material
A PDF for this lesson is in preparation.
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