DPI - Printer Resolution

Measuring Resolution in Desktop Publishing

DPI (dots per inch) is probably the most familiar and most misused measure of resolution. For desktop publishing, it is probably the least important measure as well. It is not the resolution of scanned images. It is not the resolution of images on screen. It is the measure of how many dots of ink or toner a printer can place within an inch (or centimeter).

Most printers print the same number of dots horizontally and vertically, though some may have differing numbers. Basically, 600 dpi printers print 600 tiny little dots across one inch and 600 dots vertically for one inch.

How SPI, PPI, DPI, and LPI relate
The lower the DPI of a printer, the less fine detail it can print and the fewer shades of gray it can simulate. Because monitor resolution is so much lower than printers, low resolution images that look fine on-screen often print poorly.

DPI, Scanning, & Digital Images: DPI is only relevant in that the combination of your printer's DPI and the required LPI determine how much resolution your image must have. If you will be printing to a 300 DPI printer, resolution in excess of 300 SPI is rarely necessary.

DPI and Printing: In general, the higher the resolution of your printer or imagesetter, the greater detail you can print and the better appearance of your output. See Part 5: LPI for more on printing images.

DPI and the Web: On the Web, DPI is irrelevant because images display on-screen in pixels (PPI).

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).

(sumber: about.com)

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PPI - Display Resolution

Measuring Resolution in Desktop Publishing

PPI (pixels per inch) is how a monitor displays an image. How an image looks on screen is determined by the resolution of the monitor — the number of pixels it can display in a given area. PPI and SPI are frequently used interchangeably as a measure of image resolution, as most Adobe Photoshop users will know.

Although Real World Scanning and Halftones and some other sources make a distinct separation between samples and pixels, in practice it is probably not reasonable to expect most users to trade in one term for the other. However, for the purposes of this article, for the purists, and for the benefit of the curious, PPI does differ from other terms of resolution. It may also be the failure to differentiate the terms that leads to the greatest confusion over how an image looks on screen and how it looks when printed.

How SPI, PPI, DPI, and LPI relate
PPI is a function of your monitor. A pixel or "picture element" is a specific x,y coordinate (dot) on your screen. A high-resolution setting displays more pixels per inch. In practical terms, the same picture on a low resolution monitor looks larger than it does on a higher resolution monitor because the pixels are spread out over a larger area.

One of the biggest challenges new (and even seasoned) desktop publishers face is reconciling image display size to the size of the printed image. Throw in resampling and resizing and it really gets confusing. Here's a little exercise I performed and the results. I freely admit that it confuses the heck out of me too but at least you know you're not alone. With practice and patience you'll be able to discern a pattern and won't be quite so surprised when the image you see on screen prints at a radically different size some of the time.

The following exercise refers to the illustration in the sidebar:

1. Scanned a 1" x 1" (physical size of the photograph) image at 300 SPI
2. Downsampled the 300 SPI image to 96 SPI at 1" x 1" (96px x 96px)
3. Downsampled the 300 SPI image to 96 SPI at 3" x 3" (300px x 300px)

The first thing you might notice is the difference in size of the images. Why do image one and two appear so different in physical size when I only changed the resolution? Because the monitor displays in pixels and 300 pixels per inch takes up more screen space than 96 pixels per inch. (And they would take up differing amounts of screen real estate if your monitor is set to 640 x 480 or 800 x 600, etc.) So what happened with image 3? It's only 96 SPI but its physical size is 3" x 3" (300 pixels square) so it displays at the larger size.

So, do you want to guess what the printed images will look like? The first two (large 300 SPI and small 96 SPI) printed at the same 1" x 1" size. (The 300 SPI image looked better though). The large 96 SPI image with a physical size of 3" x 3" printed at 3" square.

As you can see, how it looks on screen can be quite different than how it prints. Check the actual dimensions, as well as resolution of your image, to know how it will print.

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).

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SPI - Image Resolution

Measuring Resolution in Desktop Publishing

SPI (samples per inch) is a measurement of image resolution. To reproduce an image the scanner takes a sampling of portions of the image measuring the value at a specific place in a scanned image. The more samples it takes per inch, the closer the scan is to the original image because there is more information available about how the image should look.

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).

(sumber: about.com)

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Measuring Resolution Inch by Inch

SPI, PPI, DPI, LPI Demystified

One of the most confusing aspects of desktop publishing is resolution and the measurement of resolution: SPI, PPI, DPI, and LPI. Often DPI is used in place of SPI and PPI although they aren't really the same. That only makes it more confusing. But it need not be. Here are the short definitions for each term. See Pages 2-5 of this feature for further explanation, examples, charts, and formulas for each measurement of resolution.

SPI (samples per inch) is scanner and digital image resolution. To scan an image the scanner takes a sampling of portions of the image. The more samples it takes per inch, the closer the scan is to the original image. The higher the resolution, the higher the SPI.

PPI (pixels per inch) is the number of pixels displayed in an image. A digital image is composed of samples that your screen displays in pixels. The PPI is the display resolution not the image resolution. (Adobe Photoshop uses PPI and Corel Photo-Paint uses DPI for image resolution so it's no wonder everyone is confused.)

DPI (dots per inch) is a measure of the resolution of a printer. It properly refers to the dots of ink or toner used by an imagesetter, laser printer, or other printing device to print your text and graphics. In general, the more dots, the better and sharper the image. DPI is printer resolution.

LPI (lines per inch) refers to the way printers reproduce images, simulating continuous tone images by printing lines of halftone spots. The number of lines per inch is the LPI, sometimes also called line frequency. You can think of LPI as the halftone resolution.

To fully understand all these terms we need to know more than the proper definitions. We need to know how this translates to real world usage. The next four pages explain each term more fully and provide examples of how the terms relate to each other, to onscreen display and to printing.

It is important to remember that many of the tips, tutorials, and even software that you'll encounter (including some of the links from this feature) use SPI/PPI/DPI interchangeably. Look at the terms in context to help you decipher whether it means samples, pixels, dots, or spots.

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