Group: LibreDWG/DWG concepts

This page lists some concepts in AutoCAD which are part of the DWG file formats structure. This page is intended to collect questions and clarification about these concepts. Please keep the list in alphabetical order. If you have questions please add a title or bold text.

Entities

Entities are things that appear in the drawing. Things you see, can select and manipulate. Entities include lines, circles, arcs, text, etc. Those are called simple entities, but you'll find complex entities too, composed by a header entity, one or more sub-entities, and an End Sequence entity. Polylines or attributed blocks insertions are complex entities (but not Lightweight Polylines, or blocks insertions without attributes).

There are also non-graphic entities, grouped in dictionaries, tables and objects sections of the drawing (as an example, layers are stored in TABLES section, finding one entity for each layer).

Model Space / Paper Space

Autocad has different "views" of the same elements (database?). The principal view is called modell view. Most of the drawing work will be carried out in the modell and (almost) all of the drawing elements are contained in the modell view.

For example if you have a small architecture project, all of your floor plans (1. 2. 3. roof etc.) can be contained in only one drawing (file) and all the floor plans will be visible in the model view. However you might not want to print all the floor plans in only one sheet of paper, but print each floor plan in one piece of paper. Or you might want to print all your floor plans at a small scale and also each floor plan at a bigger scale. However, it would be a lot of work to draw the same floor plans twice just to achieve a different scale. Or it might be clumsy to have each floor plan in a different file (for a small project).

To achieve that you can create "layouts". The layouts have a paper size like A0, A1. The layouts allow you to present the contents of the model view in a sheet of paper. To show the model view in the layout you need to create a "viewport" which is basically a secondary view of the main model view. With a viewport you can crop, restyle, etc. the contents of the model view.

The important point is that you can also draw in the layout elements that are important for your final presentation. For example you can add the name of the project, the name of your company, logo etc. The elements that belong to the layout are in the Paper Space. Things that are in the paper space belong only to one layout. The elements of the layour will not be included in the model.

To make things more complex, autocad lets you edit the elements of the model through a viewport. So if you double click a viewport autocad will switch to Model space and you will be able to edit the elements of the model while being in the layout. Think about a piece of cardboard with a window. That window lets you see the model, but if you want, you can introduce your hand and move the model. Normally nobody edits the model from a layout. This functionality is intended to move, zoom in / out, the contents of a viewport (i.e. the model view) so that you can pick the section of the drawing that you need to be presented in that viewport. For example, if you want to show only one floor plan you can double click the viewport and move the contents of the model view until you find the right floor plan. When you are "inside" the viewport you are in the Model space. If you draw something there, you will draw it in the model.

Old description: Model Space and Paper Space are tabbed views in AutoCAD. When in Model Space a user is typically working on the drawing or model, concepts like scales and other print-related information are simply not relevant. When switched to the Paper Space tab the drawing / model should appear in a Viewport on a Sheet. Here concepts like scale, line styles and other print related information is set and in some cases seen. For example in Paper Space it is possible to see the results of line styles on the screen without making a print preview.

The powerful of the design is the possibility of showing in the Paper Space multiple views (Viewport entities) of the same area of the Model Space, at different scales and with layer visibility control for each view. This way, you can create a doc with a general view of a piece, and a detail view of an area of the piece, without having to duplicate drawing entities. So, changes made in Model Space to the drawing will instantly appear in all the Paper Space views to the modified drawing.

And last (but not least) Paper Space advantages are used to create simultaneous work views of Model Space, letting you control view (say plan, front and axonometric) and/or layer layout (say general view layers against detail view layers) for each Viewport.

There are limitations associated to Paper Space and Model Space: you cannot create Viewport entities in Model Space (still you can divide it in many views, with the VPORTS command, but without layer independent control), and there is a poor perspective and render support for Paper Space (but not for Paper Space's Viewport contents).

Viewports

When in Paper Space the drawing is conceptually placed behind the Sheet of paper. By creating Viewports we can grab the drawing, scale it and place it on the Sheet. By this method we can show different parts of a drawing on the same Sheet, or the same part of the drawing at different scales.

A viewport is a "window"

Blocks

The following is a description of 'Blocks' from a CAD user's perspective. It is important to understand that when a CAD user says "block", they usually are talking about an insert of a block definition.

A block definition is almost a secondary drawing that can be inserted many times within the drawing. A block has an own origin point. A block definition contains also entities (lines, polylines, circles and also blocks, etc.), which have all the properties of the normal entities (layer, color, line style etc.)

An important feature of the "Blocks" is that you can modify the block definition once and the all "Blocks" will be updated automatically. For example think about a hotel project. The same room will be repeated many times across the project. Instead of drawing the whole room many times you can define the room as a block and insert it many times in your project. In this way, you can change the block definition once and all the rooms will be updated automatically.

Blocks can be tricky. An "insert of a block" has also a layer, and if that layer is frozen all the block's entities will be not visible. The same block definition can be inserted into two different layers. If one of the layers is frozen only the elements of that block insertion will disappear.

As the block's entities have also layers, if you turn off those layers, then the entities of the block will be also turned off. It is really messy, but is the way it works.

If you copy/paste a block from one drawing to another, the layers of the block's entities will be also added to the target drawing. If a block definition with the same name is already present in the target drawing the "auto cad" will not insert the pasted block.

A block can be in a block and all the mentioned problems will be reproduced in nested blocks.

There is an important property of the cad elements called "by block". This means that the entities contained in the block will inherit those properties from the block. For instance if you have a block definition of a plant you can set the line color to "by block". Then, you can insert that block into a layer called "plants" and set its color to "by layer". Then you can set the color of the layout "plants" to green.

Line >>> by block ||| Block >>> by layer ||| Layer >>> Green ||| (to make things complex, this can be further changed in the layout...)

Old description: Blocks are a way of grouping lines/curves, many programs do similar things and call the result a group (a group is something different in AutoCAD. A Block in AutoCAD is a collection of objects (parametric lines) 'glued' together. When a block is Exploded all the contained lines lose their connections to each other. This means for example that a polyline is broken into its constituent pieces when exploded. Blocks can contain other blocks, when a block is exploded sub blocks are not and need to be exploded separately.

There is a strong trade in Blocks and Dynamic Blocks, some manufacturers provide such Blocks for free on their websites.

There are additional explanations at the autocadcentral.com website in Lesson 12 of 12 - Blocks.

Block (definition)

A block in the dwg file is a grouping of entities. It is really a block definition. As an example, a user may draw a chair with lines and arcs and then create a block definition called chair. This creates a block in the dwg file. Then the user "inserts a block" several times into his drawing. This creates insert entities. Each insert may have different location, rotation and various other properties. Each insert always references a block definition that is to be drawn at the insert location.

External Reference (definition)

An External Reference is nearly the same as a block definition, with one big difference: instead of taking that definition from the BLOCKS section in the current drawing, it's taken from an external drawing, so creating a local block definition in current drawing that is updated with the ENTITIES section (and all non-graphical info required) from that external drawing.

This way, changes made to the Model Space entities in the external drawing will be updated in the local block definition, and reflected in all block INSERTs in the current drawing.

Dynamic Blocks

Dynamic Blocks are a special type of block which has some parametric variables. For example a window which can be stretched to fit one of several pre determined sizes 1200x1200, 900x1200 or 600x1200. These block can be quite complex with a lot of variables.

There is a strong trade in Blocks and Dynamic Blocks, some manufacturers provide such Blocks for free on their websites.

There are more resources to understanding Dynamic Blocks on the resources.autodesk.com website at AutoCAD_2006_Dynamic_Blocks_Part_3 (PDF).

Inserts

An insert is a block reference. When an insert is displayed, the block that it references is drawn using the insert's location, rotation, scale and other properties.