3dRender.com : Glossary : DPI, PPI

DPI - Dots Per Inch, a measure of resolution counting dots (pixels) running across one inch of the image.  3D artists typically don't use DPI, and instead specify an image dimension in pixels to describe the resolution of a file.

If you are rendering an image to be printed, you can determine the pixel resolution to render by multiplying the DPI desired by the number of inches.  For example, if an image to be printed 8 inches by 10 inches in size, at 300 DPI, would need to be rendered at 2400 by 3000 pixels.  When only one number is specified for DPI, you can assume that the pixels are square, and use a 1.0 pixel aspect ratio.

DPI information is included in some image file formats, but not in others.  If an image is rendered or downloaded without a DPI asigned, it can be assigned in a program such as Adobe Photoshop, that allows you to assign any DPI without resampling the image or changing the number of pixels (although choosing a higher DPI will of course give you a smaller print if the number of pixels doesn't change.)

PPI - Pixels Per Inch.  Most users (and many computer programs) use DPI to mean PPI, and assume that the dots being counted are pixels. In reality, printers often use a halftone screen and dither many small dots of ink in order to reproduce the tone of each pixel from the original image, so the PPI that is actually reproduced is lower than the DPI of the printer.  This may sound like a technicality to a 3D artist, but it means that often you can get a good quality print by rendering an image as if it would be printed at a lower DPI, then scaling the file before printing. For example, many people would render an 8x10 print at 1200x1500, then scale the image size up to 8x10 at 300 DPI in Photoshop, and still produce an acceptably sharp print.

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