This page defines some important words and expressions used in QCAD sources and documentation.
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Block - A block (or block definition) is a group of entities with a name. A block can be inserted into the same drawing more than once with different attributes and different locations, scaling factors, angles, ...
Block Insert - See Block Reference.
Block Reference - A block reference is an entity that refers to a block definition. A block reference acts like a pointer to a certain block definition. Multiple block references in a drawing might refer to the same block.
Bulge - Measurement for the geometry of an arc segment in a polyline. The bulge is the tangent of 1/4 the included angle for an arc segment, made negative if the arc goes clockwise from the start point to the endpoint; a bulge of 0 indicates a straight segment, and a bulge of 1 is a semicircle. See also DXF Reference.
BYBLOCK - Special attribute value for colors, line types and line widths. If an entity has attributes set to this value, the attribute value is inherited from the block reference the entity belongs to. Please note that block definitions don't have attributes assigned to them while block references have. The name "BYBLOCK" might be misleading.
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DXF - DXFTM is the Drawing Exchange Format defined by Autodesk. See also DXF Reference.
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Entity - An entity is a graphical object. I.e. an object with a graphical representation. For example a line entity is a line in your drawing. A layer is not an entity, but an object without its own graphical representation.
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Insert - See Block Reference.
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Layer - A layer usually contains entities of a similar nature or with the same attributes . Layers allow the user to quickly change the attributes of all entities on them. Layers can be made invisible (see Freeze).