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Raster Images

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1. Bringing in Raster Images

AutoCAD has the ability to bring in virtually every raster file out there. Well, that isn't exactly true, but it does support most all of the common file types. A raster image should reside as a file on your hard disk before being "imported" into AutoCAD. Pick Raster Image... from the Insert pulldown menu. That starts a dialog box in which you can pick and preview the image that you would like to bring into AutoCAD.

Once you have picked the Open button, you have some choices to make, as shown in Figure 1. AutoCAD is going to treat the raster file as a form of external reference; it is not actually going to paste the file in your drawing. Therefore, your drawing is not going to balloon up in size, and the actual raster file will remain on disk. AutoCAD also wants to know if you want to retain the path where the file is located. As you actually bring in the file, AutoCAD treats it as a block, and, as such, you will be asked for an insertion point, a rotation and a scale. One important point to remember is that since this works in a similar way as xref files, you must send the raster file along with the AutoCAD drawing file if you are sending the file to someone who does not share your hard drive.

Once the file is inserted it will have a border around the image, which you can turn on or off. It is this border that is the actual AutoCAD object.

2. How to Move and Scale the Raster Image

Unlike raster images brought into older versions of AutoCAD, this image is not a bitmap. What this means is that you can't just pick and drag the image. It is contained within a frame that is the actual AutoCAD object. Therefore, it has all the properties of a line or a circle. If you want to move the image, use the AutoCAD MOVE command. Remember, what you are actually moving is the frame. Now that is not to say that you can't drag and drop the frame the same as you can with any AutoCAD object. New in AutoCAD 2000 is the ability to pick any group of AutoCAD objects, then pick again on one of the selection set while holding the pick button down for about a second. This then gives you a dynamic move of the objects. But the point is that you are using the AutoCAD capabilities of an AutoCAD object.

You also have the ability to scale the image, and you can scale it precisely. Let's assume that it was a raster image of an aerial photograph. You want the photograph to be the same scale as the map of the area that you have drawn. You can use the AutoCAD scale-by-reference capability.

Begin by picking the AutoCAD SCALE command. Then you pick the frame on the raster image and confirm the selection. When AutoCAD asks for a scale factor or reference, type R for reference. AutoCAD will then ask you to specify the reference length. Pick two points within the raster image of which you know the real length. AutoCAD now asks you to specify the new length. This is the length that the two points are supposed to have in real life. AutoCAD will then scale the raster image to its new size, which will now correspond to the other vectors in your drawing.

3. Drawing on Top of the Raster Image

The beauty about the raster image that you have just brought into AutoCAD is that you can treat it as though it were an overlay to your existing drawing. What this means is that the image can be placed on its own layer and that layer turned on and off. You also have the ability to draw on top of the image.

Once the image is properly scaled, you can use your screen as a digitizer to digitize points. And since you can zoom in as you digitize, the results can be very accurate. This is a viable alternative to a large format digitizer. All you need to do is to have your paper drawing scanned then insert the image into your drawing (many service agencies will scan a drawing at a reasonable fee).

You can also leave the image where it is and red line proposed modifications or draw additions right on the image. If you are drawing on the image itself, the lines that you drew might disappear behind the image during a regen. If this occurs, you can change what is known as the display order. What is happening is that the vectors you drew are regenerating the pixels first and then the image is coming in on top of them. Thus, you can't see the part that you drew.

To solve this problem pick Display Order from the Tools pulldown menu, as shown in Figure 2. Here you have the ability to Bring to Front or Send to Back. If you pick Send to Back and then pick the frame of the object, it will always be in the background and what you draw will be in the foreground.

4. Hiding an Image

Even though the image is not saved as part of your drawing, that doesn't mean that the image is not taking up room and slowing down your drawing. While your drawing is in memory, a lot of overhead is being used to work with the raster image. This is the same as with an xref file. Even though the external reference does not make your drawing substantially larger when it is saved, that doesn't mean that while it is in memory it is not taking up space. But there is something that you can do about it if it is slowing your drawing down. Chances are you don't need to see the actual image all the time that you're working on your drawing. Therefore, you have the ability to hide the image.

Of course since the image is an AutoCAD object, you always have the ability to place it on a layer and then freeze the layer. This will help you during a regen, but a better way exists.

Remember that the image is really similar to an xref file. You can't get to it using the XREF command, but you can get to it using the IMAGE command. Take a look at Figure 3, and notice how it looks very similar to the Xref dialog box. In fact if you pick the attach button, it will take you to the same dialog box that brought the image into AutoCAD in the first place.

But what you want to concentrate on here is the Unload and Reload buttons. The effect of Unload is that the raster image is no longer part of the drawing. Therefore, no overhead or time is wasted while you are working on the rest of the drawing. But the frame of the image still exists, so that you can see where the image would be on your drawing.

When you want the image back, all you have to do is go back into the dialog box, pick Reload and the image will reappear. As you can see, it works just like the Xref dialog box except you don't have the ability to bind the image to the drawing.

Of course if you want to get rid of the image in its entirety, then you can pick the Detach button.

5. Getting Rid of the Frame

The frame around the image is your only connection to AutoCAD. If the frame is not visible, then you will not be able to move or scale the image or pick it in any way. The image itself is still visible, but you may not want the frame to be visible when you plot the drawing with the image.

Therefore, it is possible to hide the frame. Type IMAGEFRAME. You have two options: on or off. No border is created around the image when you turn the frame off, but there is another repercussion. If the frame is off and you then Unload the image, you will not be able to see the outline of where the image would be because the frame is invisible.

Sometimes you might receive a drawing that contains what looks like AutoCAD vectors, but you can't seem to pick them in any way. The images that AutoCAD brings in are so good that a drawing that has been scanned and brought in as an image might look similar to an original AutoCAD drawing.

The reason you might not be able to grab any part of it is that you don't recognize it as an image-especially if the frame is off. Try turning imageframe on so you can see if a border is around it. In this way you will know what it is and how to work with it.

6. Image Adjustment Controls

You also have the ability to make adjustments to the quality of the image using the IMAGEADJUST command, which is found under the Modify, Object, Image pulldown menu.

As you can see from Figure 4, you can make changes to the brightness and contrast as well as lightening the image with the fade option. Other adjustments can be made to the image as well. For instance, you can vary the quality with the IMAGEQUALITY command. If you set it to low quality, then it will look grainier but will regenerate much faster than if set to high quality (this might be a compromise with having to unload the image). With the TRANSPARENCY command you can set some images to transparent so that you can see through them to other objects behind them. You can also control many aspects of an image through the regular AutoCAD PROPERTIES command, as shown in Figure 5.

7. Create a Raster Image of Your Drawing

AutoCAD provides you with numerous ways to create a raster image from your drawing. If you want the entire drawing that you have visible on your graphics screen to be turned into a raster output file, the easiest way is with the command SAVEIMG. Simply type SAVEIMG at the AutoCAD command line and pick a file format. You may choose from a bitmap (BMP), targa (TGA) or tiff (TIF) file.

Although you technically have the ability to crop the screen from the dialog box, it is not graphical, and, unless you know the exact coordinates, you're doing little more than guessing. Once you pick the OK button, the next dialog box permits you to name the file and pick its path destination. With this method the entire graphical screen including the UCS icon is made into a raster file. The crosshairs are not included, so you don't have to worry about moving them out of the way. If you don't want the UCS icon visible on the output file, then simply turn it off using the UCSICON command before the file is created. If you don't want the entire screen to be included as a raster file, the export options permit you to select just the objects that you want written out to a file.

The EXPORT command gives you the ability to choose a wide variety of file formats-some raster, some not. The true raster format that you can choose is bitmap (BMP). The others are really just to export to other programs such as ACIS (SAT) or stereo lithography (STL). But one that should not be ignored for its usefulness is the Windows metafile (WMF). This is the best format for moving an AutoCAD drawing into a word-processing document. However, it is not a pure raster file format because it maintains vector information. Therefore, as you scale it up or down in the word processor, it does not distort the image. It is truly scalable.

The Windows metafile can work in both directions. You also can bring a metafile into AutoCAD. You do this with the IMPORT command. When the metafile comes into AutoCAD, it comes in as a block. If you were to explode the block, the vectors inside the metafile become AutoCAD objects. What this means is that you can save parts of a drawing as a metafile, then if you want to, you can bring that metafile into another drawing at any scale as though it were a raster file. This is a quick alternative to WBLOCKs for bringing in details, especially if you have used those same details in word-processing specifications.

8. Creating Raster Plotters

Not every raster file that you want to create can be neatly summed up as a zoomed view of your screen or selected as objects. This is especially true if you need a 3D view with hidden lines removed. Sometimes the best alternative is to turn what would have been a plot into a raster file. AutoCAD gives you the ability to do this by setting up what are called raster plotters.

A raster plotter is set up just like any other plotter, but it must be set up from within AutoCAD. Pick Tools, Options, Plotting, and then pick Add or Configure Plotters, and choose Add a Plotter Wizard. Among the plotters that you can choose from is a group called Raster File Formats. Here there are numerous options such as BMP, JPG, TIFF, TGA, PCX, EPS and so on. Once the plotter is set up, creating the raster file is easy. All you have to do is plot. But when you are in the plot dialog box, be sure to change the plotter to the raster plotter that you have chosen. Plotting to a file is the only option. Obviously the physical plotter doesn't really exist. Therefore, the resulting plot is a raster file.

However, you must concern yourself with one more thing when using a raster plotter: the resolution of the output. When you go into the plot dialog box and pick the raster plotter, you will have an opportunity to pick the properties button next to the name of the plotter. Take a look at Figure 6. Here you can see that you have the ability to pick a resolution, but remember that the higher the resolution the larger the file.

Figure 1. You have several choices to make when bringing in a raster image, including insertion point, scale and rotation

Figure 2. Picking Send to Back from this menu will move the selected item to the back of the drawing.

Figure 3. The Image Manager is the xref equivalent for raster images.

Figure 4. The Image Adjust dialog box lets you modify the brightness, contrast and fade (darkness and lightness) of an image.

Figure 5. Using the PROPERTIES command for images brings up the Properties dialog box.

Figure 6. You can set the resolution of the raster plotter in the Plotter Configuration Editor.

Important note: The author is not or may not be a natural English speaker and there is a high chance of mistakes in every way. Corrections and comments are welcome.
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