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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. Refer to Chapter 17 for the lowdown on attributes.

Everyone in a company or workgroup should aim for consistency as to 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.

Mastering the Raster

AutoCAD includes four more xref-like features: the ability to attach raster images, DWF files, PDF files, and DGN files to drawings. We cover DWF and PDF files in the next section and refer you to the online help if you should find some DGN files on your hands. The image 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 dots.

AutoCAD drawings are vector files. A vector drawing is a graphic file 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 from other programs, such as Photoshop. Raster images can also come into the computer from some kind of scanner that imports a blueline print, photograph, or other image.

Whether you’re doing your own scanning or having a service bureau do it for you, you need to know that AutoCAD handles most popular image file formats, including: the Windows BMP format; the popular Web graphics formats GIF and JPEG; common print formats such as PCX, PNG, and TIFF; the less popular DIB, FLC, FLI, GP4, MIL, RLE, RST, TGA formats, and several even more obscure ones.

Three scenarios in which you could incorporate raster images in your drawing include

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).

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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, and 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.)

Your raster image may be distorted a bit; for example, the aspect ratio (ratio of width to height) may not be correct, but AutoCAD won’t let you change it. No problem; simply attach the raster image, make a block that contains it, and then insert the block with different X and Y scale factors. We love cheating.

Using raster images is much like using external references. The raster image isn’t stored with your drawing file, though. Instead, 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 onscreen 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. Raster images are simply referenced from the drawing and aren’t part of it. If you don’t send the raster image file along with the drawing, the drawing displays a rectangle containing the name of the missing file in its place. The best way to make sure that you get all the required files when sending a drawing file to someone else is to use the ETRANSMIT command, which we cover in Chapter 20.

Some raster image files that you find on the Internet may not be appropriate

AutoCAD attachments.

Attaching a raster image

Follow these steps to bring a raster image into AutoCAD:

1.If the External References palette isn’t already open, click its icon on the Palettes panel of the View tab.

Use the drop-down list on the first toolbar button to attach a drawing, an image, or a DWF, PDF, or DGN file.

2.Click Attach Image and locate the image file you want to attach.

The Attach Image dialog box appears, as shown in Figure 18-6.

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

The Attach Image dialog box appears.

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

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Figure 18-6: The Attach Image dialog box.

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 onscreen, similar to what you can do with blocks and external references, as described earlier in this chapter.

The Attach Image dialog box includes the same Full Path, Relative Path, and No Path options as those for attaching xrefs. (See the “Forging an xref path” section, 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.

Maintaining your image

You manage the images in your drawing via the External References palette. 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, though; it always remains an external file.

You can clip images so that only part of the image is displayed in your drawing. Choose Clip from the Insert tab’s Reference panel and follow the prompts

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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’re working on your drawing. If you find raster attachments slowing down AutoCAD’s response, then follow these tips:

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.

Right-click an image in the External References palette and choose Unload from the menu that appears to temporarily hide an image without losing the attachment information.

In addition, raster files can also dramatically increase the time that AutoCAD takes to generate plots (and the plot file sizes). Before you settle on using large raster files in your AutoCAD drawing, do some testing on zooming, editing, and plotting.

You Say PDF, We Say DWF

The Adobe PDF format has been around for a very long time, and for a while, it seemed like Autodesk was trying to supplant it with its own universal file format — DWF (Design Web Format). That didn’t happen, but if you can’t beat them. . . . AutoCAD includes a very acceptable PDF printer driver, and both PDF and DWF are suitable candidates for external reference files.

You could think of a DWF as “DWG Lite” because it looks just like a drawing file and contains some of the actual drawing file data. We talk more about the web side of DWFs in Chapter 20; in this section, we explain how you can use DWFs as well as PDFs as reference files in your own drawings. (Some people call DWF files “dwiffs,” but we’re going to hold off on that one until we start hearing DWG files called “dwiggs”. And why is phonetic spelled that way?)

You create both DWFs and PDFs from within AutoCAD in one of two ways. Either choose Plot and select a DWF or PDF option in the Printer/Plotter name list, or choose Print Batch Plot from the Application Menu and then select DWF, DWFx, or PDF from the Publish To area of the Publish dialog box.

Both file types are compact and secure. Because you can’t edit either PDFs or DWFs in AutoCAD, both formats are ideal for two purposes:

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You can post DWFs or PDFs on the web.

You can send your drawings to consultants and clients in a form that they can’t mess up.

You can attach PDFs or DWFs to your drawing files in pretty much the same way you attach drawings as external references. DWFs and PDFs attached to drawing files are referred to as DWF underlays or PDF underlays.

As with DWF and DGN attachments, you can object-snap to entities in the PDF underlay by enabling the Snap to Underlays function in the Reference panel slideout on the Insert tab. Setting PDF parameters is virtually identical to setting DWF parameters, and you can attach a PDF to your drawing by using exactly the same steps as the ones that follow.

The previous sections show how to attach a DWG and a raster image. Follow these steps to attach a DWF (or a PDF!) file as an underlay:

1.If the External References palette isn’t already open, click its icon on the Palettes panel of the View tab to open it.

Use the toolbar at the top of the palette to attach an external file as an xref, a raster image file, or a DGN or DWF underlay. We cover attaching xrefs and images earlier in this chapter. See the online help for information on attaching DGN files.

2.Click Attach DWF or Attach PDF and then locate the file you want to attach.

The Select Reference File dialog box appears.

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

The Attach DWF Underlay or Attach PDF Underlay dialog box appears.

4.Specify the parameters for the DWF or the PDF in the dialog box.

Parameters include specifying a sheet, the insertion point, scaling factors, rotation angle, and path type (see Figure 18-7). You can set these parameters in the dialog box or specify them onscreen, just like you can do when inserting a block, attaching an xref, or attaching an image, as described earlier in this chapter.

5.Click OK.

The externally referenced DWF or PDF file appears in your drawing.

Neither PDF nor DWF files are as precise as DWGs, and that’s why they’re a lot smaller. When using object snaps to locate points in DWFs, you may see the word approximate on the Object Snap tooltip. If this is a problem, you can increase the precision of your DWF file when you create it.

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