A hidden line, also known as a hidden object line, is a medium-weight line composed of small dashes around 1/8" long separated by 1/16" spaces to indicate edges, surfaces, and corners that cannot be seen. They are sometimes used to make a drawing simpler to interpret. In an isometric perspective, they are frequently eliminated. They can be added in most CAD programs using the LINE function.
A visible line, also known as an object line, is a thick continuous line used to highlight an item's visible boundaries or outlines. It can be used to highlight changes in flooring and wall coverings, for example.
Object lines are used to identify features on drawings or models that would not be apparent from just looking at the overall image. They can also help show which parts of the drawing need to be included in the final product. There are several different types of object lines available, including pencil, watercolor, ink, colored marker, whiteout, and paint.
Pencil object lines are thin lines drawn with a sharp pencil. These lines are easy to erase if needed and don't leave a mark on your model. Watercolor object lines are thicker than pencil object lines and are made by simply gluing several strips of paper together to form a single object line. The glue used will allow you to remove and reuse these objects if necessary. Ink object lines are made up of fine, black lines drawn with a pen point. Colored marker object lines are similar to ink object lines but using color instead. Whiteout object lines are heavy lines used to reveal the underlying contours of a model or drawing surface.
Hidden lines are used to depict surfaces that are not immediately visible. In all perspectives, all surfaces must be visible. A concealed line is used to draw an edge or surface that is obscured from view by another feature. Figures 4-11 and 4-12 depict items in orthographic perspectives that need concealed lines.
In figures 4-11 and 4-12, the top of the bookcase is behind the lamp. Therefore, a concealed line is needed to show this edge. Conventional books have flat, even-thickness pages. So for every page in the book, there is a corresponding line in the orthographic perspective. But because the pages of this book are not uniform in thickness, some lines will go through more than one page.
A second example is shown in figure 4-13. Here, the top of the cabinet is behind the picture. Again, a concealed line is needed to show this edge. In this case, however, the edge is not part of the book's surface but instead represents the end of the shelf unit. Books can also conceal edges created by other objects such as lamps or tables. These would be considered secondary features and would be indicated by additional concealed lines.
Figure 4-13: Secondary features can create hidden edges that need to be depicted with concealed lines in an orthographic perspective.