The rule

One sentence of the official instructions governs every line you will read in this section: "Lines that cannot be seen from the viewing surface are DOTTED."

That gives you two kinds of line and a single test that sorts them:

A solid line is an edge of the object in direct sight from that direction. Stand where the panel says and nothing is between you and it.
A dotted line is an edge with material in front of it. The edge is really there on the object; you simply could not see it from where the panel puts you.

A schematic cutaway, not one of the item's views: the near wall is in direct sight, the edge behind it is not.

Notice what the test is not. It has nothing to do with how important an edge is, how far away it is, or whether it belongs to a hole rather than a bump. Only one question is ever asked: is there material between this edge and the viewer? Yes makes it dotted, no makes it solid.

Dashes are computed, not decorated

Each panel is produced by projecting the actual solid and running a real hidden-line calculation over it, edge by edge. No one draws these dashes by hand and no one adds them for looks. That makes a dash evidence about the object, on exactly the same footing as a solid line — and it means you can trust the style of every line you see.

See it for yourself

Below is one object in all three of its views. The buttons restyle the two line sets the engine produced — they change nothing about the drawing itself.

TOP VIEW

FRONT VIEW

END VIEW

One object in all three views. The panels are normalized to one scale here so the same object reads the same size in each; a live item does not normalize the views it gives you.

Both line sets as an item presents them: solid dark edges for what is in direct sight, lighter dashes for what is not.

The same edge can change style with the view

Because the test asks only what lies between an edge and the viewer, and because the viewer stands somewhere different for each panel, one edge can be solid in one view and dotted in another. Nothing about the object changed. You moved.

The object in the demo shows this plainly. It carries two upright posts of different heights, one behind the other. In the front view the shorter post is drawn entirely in dashes: the taller post stands directly in front of it and is wider, so it hides the shorter one completely. Step around to the end view and the two posts stand side by side with nothing between either of them and you — so the same shorter post is drawn solid.

Fade the dotted lines in the front panel and you do not lose a detail. You lose a whole post.

There is no item without dashes

Hidden lines are not an advanced feature that shows up once the questions get hard. Across a large sample of generated items, the view being asked for never once came out with zero dotted lines, and on most items the dotted runs outnumbered the solid ones — including on the easiest ones. Reading only the solid lines is not a shortcut that costs you a little accuracy. It is a habit that discards most of the drawing.

On the top-front-end practice generator, hidden edges are drawn dashed and in a lighter grey, visible edges solid and dark, exactly as in the figures above. Get used to reading both weights at once.

The takeaway

Solid means in direct sight from that direction; dotted means real, but with material in the way. The style is computed from the solid itself, so it is evidence, not decoration. Because it depends only on where you are standing, the same edge can switch style between panels — and since every item carries dashes, and usually more of them than solid lines, the dotted lines are half of what you were given to work with.

Solid Lines and Dotted Lines

Summary

Practice Questions

AI Tutor