Scale and Resolution

CAD drawings are made in real-world scale, meaning they are drawn to actual size. Thus, drawing in CAD is easy because you don't have to worry about scale until it comes time to lay out the drawings on a sheet of paper. You choose a specific graphic scale (such as 1/8" = 1'-0") in a layout to fit the measured drawing on a sheet of paper.

When you convert drawings to images, the scale must relate to image resolution in a specific way in order to maintain the graphic scale on the printed page. For example, 1/8" scale means that 1/8" on paper represents 1' in the real world. Another way of saying this is that 1" on paper is equivalent to 8' in the real world. If you stay in the same units, that's like saying 1" on paper equals 96" (8 feet x 12 inches / foot) in the real world. In other words, AutoCAD must scale down your drawing 96 times to print it in 1/8" scale on paper.

If you use the ImagePrinter in AutoCAD to convert 1 square inch of vector space to 1 pixel, there is a direct equivalence between the units of graphic scale and resolution. Therefore, set the resolution in Photoshop to 96 pixels per inch (ppi) to represent 1/8" scale in the image. The following list shows this relationship.

Graphic Scale:

ImagePrinter in AutoCAD: Resolution in Photoshop:

1/8" on paper

1"

1"

1"

1" on paper

1'-0" in the real world

1 pixel 96 pixels

All this talk about scale and resolution can be confusing, so let's walk through a practical example.

1. If you have the Cottage.dwg file open in AutoCAD from an earlier section, you can continue here; if not, open this file from the companion CD.

Look back at Figure 5.1. The wall on the top was drawn exactly 24 feet in length in AutoCAD, as the dimension indicates. In 1/8" scale, this line should be exactly 3" long on paper. Let's try converting this data from vector to raster with the ImagePrinter and see if we can maintain the graphic scale inside Photoshop.

2. In AutoCAD, choose File > Plot, or press Ctrl+P. Click OK if a warning dialog box appears indicating a missing driver; you'll select the correct driver in a moment. The Plot dialog box opens, as shown in Figure 5.12. Click the Name drop-down under Printer/Plotter, select ImagePrinter.pc3, and click OK in the Paper Size Not Found dialog box if it appears.

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