A number of options that govern the size of your final image and you will find them all under the "Raytrace" tab in the "Settings" dialog. The first is whether you will render your image in the AutoCAD viewport, or in the Walkabout window. If you choose the former, the image will appear in the current AutoCAD viewport, showing the same view. This is one way to get orthographic images such as plans and axonometric views, but there are disadvantages (see question 5). Although you can render in multiples of the size of the viewport (1X, 2X etc) it is not easy to control the actual resolution because you don’t know its real size.
Rendering to the walkabout window is a whole different ballgame. Firstly, you can choose the exact size of the rendering from within the dialog box. This makes it easier to calculate the resolution of the final image. Other advantages are that you can use 2-point perspective and radiosity. Once you have set the image size, you will need to think about the resolution, and how big you want the image to appear on the page.
DPI means "Dots per inch". Simple as that. It’s a measure of resolution, or, put another way, level of image detail. That means if you have 150 dots (or pixels) along the top edge of an image, and you print your image is at 150 DPI, then it measures 1 inch along the top when you print it. All very well for Americans and English people, but what about the rest of the world ? Simply divide by 25 (approximately) to get the "Dots per millimeter". (150 DPI is 6 "Dots per Millimeter", 300DPI is 12)
Your image is made of differently colored dots, or pixels. When you print your beautifully crafted rendering onto a printer, it will be translated into tiny dots – again. This time, though, the dots will be different because a printed image is made of different stuff to an image on a screen. Even if your printer is capable of 600 DPI, it may be that a 150 DPI original image is enough because of the "dithering" (or color mixing).
In general, 150 DPI is good for most purposes. Increasing the resolution may actually make the picture worse because the image will have to be re-sized. If in doubt, it is best to read the printer’s documentation or ask an expert at the service bureau you use. As a guide, though, you should always use a simple fraction of the printer’s resolution as your image resolution. If the printer’s quoted resolution is 600DPI, you should consider using 600DPI, 300DPI, 150DPI or 75DPI depending on the quality you need.
Remember that increasing the number of dots in your image makes rendering times much longer. For every doubling of the resolution, the rendering time will increase by 4 times. So be careful, and render only what you need.
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