AutoCAD 2005 for Dummies

? Assemblies that are repeated in one or more drawings, especially if the assemblies are likely to change together (for example, repeated framing assemblies, bathroom layouts, modular furniture layouts).

? Pasting up several drawings (for example, details or a couple of plans) onto one plot sheet.

? Temporarily attaching a background drawing for reference or tracing.

On the other hand, blocks remain useful in simpler circumstances. Situations in which you might stick with a block are:

? Components that aren’t likely to change.

? Small components.

? A simple assembly that’s used repeatedly, but in only one drawing. (You can easily update a block in one drawing with the REDEFIT command.)

? When you want to include attributes (variable text fields) that you can fill in each time you insert a block. Blocks let you include attribute definitions; xrefs don’t.

Everyone in a company or workgroup should aim for consistency in when and how they use blocks and xrefs. Check whether guidelines exist for using blocks and xrefs in your office. If so, follow them; if not, it would be a good idea to develop some guidelines. Chapter 15 discusses how to start such guidelines.

Mastering the Raster

AutoCAD includes another xreflike feature: the ability to attach raster images to drawings. This feature is useful for adding a raster logo to a drawing title block or placing a photographed map or scene behind a drawing. A raster, or bitmapped, image is one that’s stored as a field of tiny points. 

Most AutoCAD drawings are vector images. A vector image is an image defined by storing geometrical definitions of a bunch of objects. Typical objects include a line, defined by its two endpoints, and a circle, defined by its center point and radius. Vector-based images are typically smaller (in terms of the disk space they occupy) and more flexible than raster images, but also are less capable of displaying visually rich images such as photographs.

Raster images often come from digital cameras or other programs, such as Photoshop. Raster images also can come into the computer from some kind of scanner that imports a blueline print, photograph, or other image.

Whether you’re doing your scanning yourself or having a service bureau do it for you, you need to know that AutoCAD handles most of the popular image file formats including the Windows BMP format, the popular Web graphics formats GIF and JPEG, the popular PCX and TIFF formats, as well as DIB, FLC, FLI, GP4, MIL, PNG, RLE, RST, and TGA.

Here are three scenarios to incorporate raster images in your drawing:

? Small stuff: You can add logos, special symbols, and other small images that you have in raster files.

? Photographs and maps: You can add photographs (such as a future building site) and maps (for example, showing the project location).

? Vectorization: To convert a raster image into a vector drawing by tracing lines in the raster image, you can attach the raster image in your drawing, trace the needed lines by using AutoCAD commands, then detach the raster image. (This procedure is okay for a simple raster image; add-on software is available, from Autodesk and others, to support automatic or semiautomatic vectorization of more complex images.)

Using raster images is much like using external references. The raster image isn’t stored with your drawing file; a reference to the raster image file is established from within your drawing, like an xref. You can clip the image and control its size, brightness, contrast, fade, and transparency. These controls fine-tune the appearance of the raster image on-screen and on a plot.

  When you attach raster images, you have to make sure that you send the raster files along when you send your drawing to someone else.

  AutoCAD LT can open, view, and plot drawings containing attached raster images, but LT can’t do the attaching. Raster masters require full AutoCAD.

Attaching an image

Follow these steps to bring a raster image into AutoCAD:

1. Choose Insert>Image Manager from the menu bar or type IMage at the command line to start the IMage command.

The Image Manager dialog box appears (see Figure 13-9).

Figure 13-9: The Image Manager dialog box. 

2. Click the Attach button.

The Select Image File dialog box appears. 

3. Browse to find the file you want to attach, select it, then click Open.

The Image dialog box appears.

  Click the Details button in the Image dialog box to see more information about the resolution and image size of the image you’re attaching. 

4. Specify the parameters for the attached image in the dialog box.

Parameters include the insertion point, scale factor, and rotation angle. You can set these parameters in the dialog box or specify them on-screen, similar to what you can do with blocks and external references, as described earlier in this chapter. Use the quick dialog box help (click the question mark in the dialog box’s title bar and then click the area in the dialog box for which you want help) or click the dialog box’s Help button to find out more about specific options.

  The AutoCAD 2005 IMage command includes the same Full path, Relative path, and No path options that debuted for xrefs in AutoCAD 2004. (See “Forging an xref path” earlier in this chapter.)

5. Click OK.

The image appears in your drawing.

6. If you need to ensure that the raster image floats behind other objects in the drawing, select the raster image, right-click, choose Draw Order, and then choose Send to Back.

The DRaworder command provides additional options for which objects appear on top of which other objects. If you need this kind of flexibility, look up “DRAWORDER command” in the AutoCAD online help system.

Managing images

You manage the images in your drawing with the Image Manager dialog box. It includes virtually the same options as the External References dialog box. You can view a list of image files that appear in the current drawing, detach (remove) image references, and unload and reload images when needed. You can’t bind an image to your drawing; it always remains an external file. You can clip images so that only part of the image is displayed in your drawing. Choose Modify>Clip>Image and follow the prompts to clip the image. You can have multiple overlapping or distinct pieces of any number of images in your drawing, and only the parts you need are loaded into memory when you have your drawing open.

  Raster image files often are larger than DWG files of corresponding complexity; raster file size can affect performance within AutoCAD, because the raster file loads into memory when you are working on your drawing. Some workarounds speed up operations:

? Attach raster images late in the production process.

? Create a lower-resolution version of the raster file, just large enough to create the desired effect in your drawing.

? Use the Unload button in the Image Manager dialog box to temporarily disable an image without losing