How SPI, PPI, DPI, and LPI relate
A digital image is composed of samples, information about how to display that image. When you scan an image the SPI is its scanning resolution or the amount and type of information stored for that image. Through resizing and resampling you can change the stored information so that the image resolution is different from the original scanning resolution. However, both are expressed as SPI.
Some scanner manufacturers use DPI in place of SPI in listing their scanners resolution capabilities; however, there are no dots in an image until it is printed. The term SPI is not as common as DPI or even PPI. In most cases the terms are used as if they were the same.
SPI and Scanning: When scanning photographic images you need to know the final output method in order to insure that you scan at the proper resolution (SPI). Insufficent resolution will result in lower quality printed images. Too much resolution results in wasted information and unnecessarily large file sizes.
SPI and Digital Images: As with scans, images acquired through digital photography, from the Web, or from CD collections require the right amount of resolution for the final output method.
- Tip: If you will be manipulating an image in your graphics software applying filters, for example you may want to start with an image at a higher resolution (SPI) than what you will eventually need. Your file will contain more image data and you have more margin for error and experimentation. Once you achieve your desired effect, downsample the image to the required SPI.
SPI and Printing: When printing photographic images you need to know the final output method in order to insure that your digital images (scans, digital photos, images from CD, etc.) have sufficient resolution (SPI) for printing or for screen display. Use the LPI formulas in Part 5 to determine if an acquired image has sufficient resolution for printing.
SPI and the Web: For display on the Web, monitors typically display images at 72 or 96 PPI. Much less sampling (SPI) is needed for on-screen display and the information in images beyond 96 SPI is wasted, won't enhance the display, and unnecessarily increases the file size and download time. You may wish to scan images at a higher SPI in order to have 'pixels to play with' but you will need to downsample the image before using on the Web.
In practice, SPI and PPI are often used interchangeably. DPI is frequently used in place of one or both terms. However, even if you call it DPI, remember that each dot or "unit of measure" behaves differently depending on whether it is a scanner (or scanned image), a monitor (or on-screen image), or a printer (or printed image).
Next > PPI: Display Resolution
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