Push the eye distance to 30 and the two pictures converge — parallel projection is just perspective viewed from infinitely far away.
WhatA type of parallel projection where projection lines are PERPENDICULAR to the viewing plane.
WhereEngineering/architectural drawings (front view, top view, side view) — preserves exact measurements, no distortion.
📏 Parallel (Oblique) Projection
WhatProjection lines stay parallel to each other, but are NOT perpendicular to the viewing plane (unlike orthographic).
EffectPreserves relative proportions along each axis, but the result still doesn't look fully "realistic" the way perspective does.
🎯 Perspective Projection
WhatProjection lines converge toward a single point (the "eye" or camera position) — objects farther away appear smaller, exactly like real human vision.
WhereVideo games, movies, any rendering that needs to look visually realistic.
💡 One-liner: "Why do CAD/engineering drawings use orthographic (parallel) projection instead of perspective?" → Orthographic preserves exact measurements and proportions without size distortion from distance — essential for accurate technical drawings, where perspective's "farther = smaller" effect would be misleading.
The one formula behind perspective
Perspective is not a special drawing technique but a division. A point at depth z, viewed from distance d, is drawn at:
scale = 1 / (1 + z/d)
eye at d = 2, depth z = 2 → scale 0.500 far face HALF size
eye at d = 6, depth z = 2 → scale 0.750
eye at d = 20, depth z = 2 → scale 0.909
eye at d = ∞ → scale 1.000 no shrinking at all
That last row is the insight worth carrying: parallel projection is perspective with the eye at infinity. They are not two unrelated methods but the same formula at two ends of one range — which is why an engineering drawing, made to preserve measurements, is exactly what you get by moving the viewer infinitely far away.
💡 Note that depth and eye distance act only through their ratio z/d. A small object viewed closely is distorted exactly as much as a large one viewed proportionally further away — which is why a wide-angle photograph taken close up exaggerates so strongly, and why the same lens at a distance does not.
What perspective destroys
📐 What survives each projection
ParallelPreserves lengths along each axis and keeps parallel lines parallel. Measurements can be read straight off the drawing.
PerspectivePreserves neither. Equal lengths at different depths appear different, and parallel lines converge on a vanishing point.
So the choice is between measurable and realistic, and you cannot have both. A CAD drawing must be measurable, so it uses parallel projection even though it looks flat; a game must look right, so it uses perspective even though nothing in it can be measured.
💡 The number of vanishing points follows directly from how many axes are not parallel to the viewing plane: one-point perspective when only depth recedes, two-point when the object is rotated about the vertical, three-point when it is also tilted. It is a consequence of the geometry, not a stylistic choice.
Syllabus points
Orthographic projection
Parallel (oblique) projection
Perspective projection
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