Understanding the PPI, DPI and Resolution Dilemma

By: Shiv Verma

 

This article is designed to address PPI and DPI.  LPI and SPI will not be covered in detail.

 

Definitions

PPI (pixels per inch) is the number of pixels displayed in an image. A digital image is information that your screen displays in pixels. The PPI is the display resolution not the image resolution.

DPI (dots per inch) is a measure of resolution counting dots (pixels) running across one inch of the image. It is a measure of the resolution of a printer. It refers to the dots of ink or toner used by an ink jet printer, 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) also called screen frequency or ruling, is how many rows of halftone spots make up a halftone screen that is used to convert a continuous tone image (photograph) into a series of dots. How fine or coarse the screen is.

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.

  • Image Size - There are three measures that are used to define the size of an image:

  • Pixel count - e.g 3000x2000 pixels

  • Physical size - e.g. 8" x 10"

  • Resolution  - e.g. 240 pixels per inch (PPI)

Lets start with a the digital image file each image is tagged with a lot of information beyond the pixel data captured for the image.  This is the EXIF data - the width and height in pixels, the resolution (PPI), camera data, lens data, exposure (speed and aperture) data, focus data, flash data. EXIF stands for Exchangeable Image File Format, and is a standard for storing interchange information in image files.

To resolve some of the confusion and to understand the relationship of these terms lets start with the Display or Monitor or LCD/DLP Projector.  Monitors are measured in pixels (e.g. a 1024x768 display), and visual screen width typically in inches 15inch monitor 17 inch monitor, etc. (confusion - this is the diagonal measure not the true width).  If image display and monitor display had a one to one correlation it would not be possible to view an image in its entirety if the image has a higher pixel count than the display in the visual portion of the screen. Although monitors do have a measure for PPI (pixels per inch), the PPI information embedded in an image is discarded when it is displayed on a monitor.

When you display a digitally captured image on a monitor, the only determination of the size of the image is its width and height in pixels. All the other data embedded in the image file including resolution (PPI data) is ignored. If your image is a 480Kbyte file which is 1024 pixels wide by 768 pixels wide, it will display as a full screen image if you are using an 1024 x 768 display or higher. It doesn't matter if your PPI is set to 75 or 300 or 1000  This is as far as any monitor display goes software intervenes and interpolates the image to resize it to fit on the screen. In most cases the embedded PPI information is ignored.

 

DETAILS:

PPI Pixels per Inch
When using an image editor if you change the PPI information you will see that the width and height in the image size boxes will change.  "Pixels per inch" is almost exclusively used for printing, not for display using a monitor or projector. If you take an image that is 1024 pixels wide and 768 pixels high, and you print it with a PPI setting of 200 pixels per you will get a print that has dimensions of 5.12 inches by 3.84 inches. If you resample the image down to 512 pixels wide by 384 pixels wide and you keep the PPI setting at 200, the print will now be 2.778 inches wide by 1.92 inches high.  To still get an image of 5.12 inches by 3.84 inches you can decrease the PPI to 100 you will however, deteriorate the quality.

DPI - Dots per Inch
DPI is associated with printers and printing. DPI is a property of a printer and printer driver software, not the digital image. DPI is a means of defining the spacing of  droplets of ink that are pot on the paper to produce a print.  Printer have settings of 360DPI, 720DPI, 1440DPI and 2880DPI but the difference between these setting is subtle nad not really perceptible o the naked eye. The key point here is that changing the DPI setting has no effect on the size of the print. PPI controls the size and DPI controls print quality.  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.

In conclusion DPI describes the resolution of printers. PPI describes the density of pixels in an image file. In simplistic terms to produce a print the system (computer, printer, software and settings) map the PPI of the image file to the DPI of the print

 

 

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