How to Extract a Raster Image from AutoCAD - dummies

How to Extract a Raster Image from AutoCAD

By Bill Fane

You can extract a raster image file from AutoCAD. The most basic raster image file is a simple bitmap, usually with the file extension BMP (usually pronounced “bump”). The file consists mostly of a whole bunch of numbers that correspond to a color number for every individual pixel. One problem with BMP files is that they are not compressed, so as the resolution (the number of rows and columns) goes up, the file size goes up exponentially. If you, say, double the resolution, the file size quadruples.

The file size problem can be greatly reduced by using any one of several file compression techniques. The basic principle of file compression is that the software looks for repetitions and then describes them appropriately. For example, if green is color number 3, a horizontal green line in a BMP file would be stored as “3 3 3 3 3 3 3 3 3 3 3 3.” A compressed file would store the same information as “12×3,” which obviously requires less space.

The bad news is that some compression formats achieve higher compression rates by filtering finer details, so that “3 3 3 3 3 3 4 3 3 3 3 3” might still be stored as “12×3.” The false information is called an artifact. Artifacts may not be a problem in busy photographs but are a problem when details are lost from CAD images.

The most common compressed raster file formats are JPG (“jay-peg”), which is used by digital cameras, PNG (“ping”), which is used on websites, and TIF (“tiff”), which is used in desktop publishing.

You can extract a raster image from AutoCAD in several ways, ranging from convenient to flexible:

  • Press Alt+PrtSc on your keyboard. This method captures the current screen view of the current application to the Windows clipboard. You can now paste it directly (with Ctrl+V) into many other applications or into a graphics program such as Paint for further editing, cropping, and saving to a variety of raster formats. This keystroke combination works equally well for any Windows application, by the way, and is convenient because there are no options to choose.
  • Use the EXPORT command. This method saves drawings to a file on the hard drive in a variety of raster and vector formats. For use in non-CAD applications, the two formats to consider are WMF (Windows metafile) and BMP (bitmap). This command allows you to select which entities to export; press Ctrl+A to select all visible ones. Other than that, there are no options to consider.
  • Use the PLOT command. The Printer/Plotter Name drop-down list includes Publish to web JPG and Publish to web PNG. When you select either, the Paper Size drop-down list displays a variety of image resolutions. You may need to do a bit of playing with the Plot Area and Plot Scale values to get what you want. Extents and Fit to Paper are usually good starting points.

Doing a screen capture or a plot to a raster file is strictly a case of WYGIWYS (what you get is what you see). If a detail is small enough that it isn’t visible on-screen at the time the image was captured, it isn’t in the raster file and no amount of zooming in will bring it back. The only way a raster image can be resized is by dropping or adding pixels.

Resizing smaller can result in loss of detail, which can sometimes mess up a drawing image. In addition, resizing bigger usually doesn’t restore the lost detail. You should definitely play with the capture settings so that the image can be inserted 1:1 in the target document, without the need to resize it.

The three panels show possible side effects of resizing a raster image.

raster image
How to mess up a raster image.