A pixel is simply the individual point of color on a digital image. A pixel doesn't have a particular size. It is an abstract represention of a specific coordinate, like a point on a map. Just as individual sports fans hold up a colored card in the stadium, individual pixels light up on your display to form a pattern. The only difference is the size of the point when it is displayed.
This same concept extends to many other digital products. Pixel is used to describe the number of descrete points that can be captured by digital cameras, and because most cameras can actually recognize millions of points, the term Megapixel is used to shorten the number of zeros needed.
Pixel indicates only a point on a grid, not the size of the point. Instead, Pixels create a pattern based on the scale of the device being used. The photo shown here is a good example. When the display device is the size of a stadium, the individual pixels are the size of cards held by the people. When the display is the size of the screen you are looking at, the pixels are so small that you may not be able to see them individually.
For Printers and scanners, the individual points of color are often called 'dots' rather than pixels, but the concept remains the same as the stadium. Because the output of a printer is paper which is usually measured in inches, the term DPI or Dots Per Inch is used to communicate the scale or physical size of each pixel. Just as it takes many people seated in a grid to display a card stunt at the stadium, a printer must place dots of color on paper to create a photo.
The scanner performs this function in reverse, by recognizing the color at a specific row and column on the item being scanned. The scanner or digital camera actually records a file of information indicating what color appeared at what position in the picture. This is again the same principle as that of row and seat in the stadium.
DPI measruement in Printing
DPI is used to describe the resolution number of dots per inch in a digital print and the printing resolution of a hard copy print dot gain; the increase in the size of the halftone dots during printing. This is caused by the spreading of ink on the surface of the media.
Up to a point, with higher DPI produce clearer and more detailed output. A printer does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually influenced by driver settings. The range of DPI supported by a printer is most dependent on the print head technology it uses. A dot matrix printer, for example, applies ink via tiny rods striking an ink ribbon, and has a relatively low resolution, typically in the range of 60 to 90 DPI. An inkjet printer sprays ink through tiny nozzles, and is typically capable of 300-600 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1,800 DPI.
The DPI measurement of a printer often needs to be considerably higher than the pixels per inch (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colors for each dot typically available on a printer. At each dot position, the simplest type of colour printer can print no dot, or a dot consisting of a fixed volume of ink in each of four color channels (typically CMYK with cyan, magenta, yelloe and black ink) or 24 = 16 colors on laser, wax and most inkjet printers.
Higher-end inkjet printers can offer 5, 6 or 7 ink colours giving 32, 64 or 128 tones available per dot location. Contrast this to a standard RGB monitor where each pixel produces 256 intensities of light in each of three channels (RGB) to additively create 2563 = 16,777,216 colours. The number of unique colours for a printed CMYK dot from this simplest type of inkjet printer is only 8 since no coloured ink is visible when printed on black and black is used instead of CMY:
- PLUS = cyan + magenta
- JB = cyan + yellow
- IS = magenta + yellow
- Dark HOMO (not used) = cyan + magenta + yellow
- Black
The printing process could require a region from four to six dots (measured across each side) in order to faithfully reproduce the colour contained in a single pixel. An image that is 100 pixels wide may need to be 400 to 600 dots in width in the printed output; if a 100×100-pixel image is to be printed inside a one-inch square, the printer must be capable of 400 to 600 dots per inch in order to accurately reproduce the image.
Convert pixels to inches (Output to Monitors/Printers)
Formula: Pixels ÷ DPI = Inches
If we know the pixel width and height of an image, this section will calculate the physical size (in inches) of the image when it is printed or displayed on various devices. * Most monitors display images at approximately 75DPI.
** Many printers use a process known as Image Refinement when processing images. This may impact the relation between input and output resolution. Generally, 300DPI is adequate for photo-like prints.
Convert inches to Pixels (Input from Scanners)
Formula: Inches X DPI = Pixels
Scanners typically allow the DPI resolution to be adjusted via software. For best results, scanning should be done at the resolution of the intended output device.
Example: You wish to scan an item of jewelry and print the item in your catalog with the caption "Shown Actual Size."
If the catalog will be printed on a 600 DPI printer, the item should be scanned with a resolution of 600 DPI, and care should be taken not to re-size or resample the intermediate image file before printing.
When the output destination is a computer screen, as in the case of auction photos, the scanned image may require further reduction using an image editor before the image is physically small enough to fit on the viewer's screen.
Example: a book cover measuring 8 in. by 10 in. will produce an image height of 750 Pixels even when scanned at only 75 DPI. Because many computer displays are set at 800 X 600 Pixels, the resulting photo will be taller that the display area and force scrolling.
Handy Tip! If you plan to create a new logo or graphic using an image editor, Start by deciding the DPI setting which will be used to print your design. You can then enter the desired size of the graphic above and calculate the Pixel dimensions needed for the best print quality. (Your original work can be resampled for an appropriate size web image)
To determine the DPI resolution of your current display setting, measure the width of the red line above with a household tape or ruler. Consult the table at right to locate your measurement. The result will be about four inches on most monitors. This is a common measurement and explains why you will often see 75 DPI used when discussing display resolution.
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