About resolution in bitmap images

Resolution is the number of dots or pixels per linear unit used to reproduce artwork and images. Output devices display images as groups of pixels. The resolution of vector graphics, such as Illustrator artwork, depends on the device used to display the artwork. The resolution of bitmap images, such as digital photographs, depends on both the display device and the inherent resolution of the bitmap image.

Pixel dimensions

The number of pixels along the height and width of a bitmap image. The display size of an image on-screen is determined by the pixel dimensions of the image plus the size and setting of the monitor. The file size of an image is proportional to its pixel dimensions. A typical 21-inch monitor displays 1152 pixels horizontally and 870 vertically. An image with pixel dimensions of 1152 by 870 would fill this small screen.

Image resolution

The number of pixels displayed per unit of printed length in an image, usually measured in pixels per inch (ppi). An image with a high resolution contains more, and therefore smaller, pixels than an image of the same printed dimensions with a lower solution. For example, a 1-inch-by-1-inch image with a resolution of 72 ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1-inch-by-1-inchimage with a resolution of 300 ppi would contain a total of 90,000 pixels.

Because they use more pixels to represent each unit of area, higher-resolution images usually reproduce more detail and subtler color transitions than lower-resolution images when printed. However, increasing the resolution of an image scanned or created at a lower resolution only spreads the original pixel information across a greater number of pixels and rarely improves image quality

To determine the image resolution to use, consider the medium of final distribution for the image. If you’re producing an image for online display, the image resolution only needs to match the typical monitor resolution (72 or 96 ppi). However, using too low are solution for a printed image results in pix elation output with large, coarse-looking pixels. Using too high a resolution (pixels smaller than what the output device can produce) increases the file size and slows the printing of the image.

Monitor resolution

The number of pixels or dots displayed per unit of length on the monitor, usually measured in dots per inch (dpi). Monitor resolution depends on the size of the monitor plus its pixel setting. A PC or Mac OS monitor can range from 60 to 133 dpi. Understanding monitor resolution helps explain why the display size of an image on-screen often differs from its printed size.

Printer resolution

The number of ink dots per inch (dpi) produced by an image setter or laser printer. For best results, use an image resolution that is proportional to, but not the same as, printer resolution. Most laser printers have output resolutions of 600 dpi to1200 dpi and produce good results with images from 72 ppi to 185 ppi. High-end image setters can print at 1200 dpi or higher and produce good results with images from 200 ppi to 350 ppi.

Screen frequency

The number of printer dots or halftone cells per inch used to print gray scale images or color separations. Also known as screen ruling or line screen, screen frequency is measured in lines per inch (lpi) – or lines of cells per inch in a halftone screen. The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency .But with some images and output devices, a lower resolution can produce good results.

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