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Images: Image Resolution

Image Resolution

What are PPI & DPI?

These two acronyms are often used interchangeably although they do have different meanings.

  • PPI (Pixels Per Inch) refers display resolution, or, how many individual pixels are displayed in one inch of a digital image.
  • DPI (Dots Per Inch) refers to printer resolution, or, the number of dots of ink on a printed image.

This guide will use only PPI (Pixels Per Inch) to describe resolution. 

Image

Compression

Image compression is minimizing the size in bytes of a graphics file without degrading the quality of the image to an unacceptable level

DPI

Dots per inch. It is similar to PPI, but the pixels (virtual drive) are replaced by the number of points (physical drive) in a printed inch. The more dots the image has the higher the quality of the print (more sharpness and detail).

Lossless Compression

Lossless compression refers to compression in which the image is reduced without any quality loss. Image file types that are lossless are GIF, PNG and TIFF

Lossy Compression

Lossy compression refers to compression in which some of the data from the original file is lost. JPEG image files are lossy files.

Pixel

The smallest controllable element of a picture represented on the screen.

PPI

Pixels per inch. It is associated with screens of digital devices. Each pixel is equivalent to a point of light coming from any monitor, then the utility of PPI is to report the quantity of pixels on a screen inch, with exact 2.54cm.

Raster images

Composed of a fixed set of tiny dots or pixels. Some of the basic file types include JPG, GIF, PNG and TIFF.

Resolution

Resolution refers to the number of pixels in an image. Resolution is sometimes identified by the width and height of the image as well as the total number of pixels in the image.

Transparency

The ability to eliminate the background in an image.

Vector images

Composed of mathematical equations where lines and curves (or paths) make up graphic shapes, images, and text in XML format. SVG and EPS are vector file types.

The greater the number of pixels, the higher the resolution, providing more details and better definition to the image. Logically, an image with low resolution will have less pixels, less details and definition.

The greater the number of pixels, the higher the resolution, providing more details and better definition to the image. Logically, an image with low resolution will have less pixels, less details and definition.

How do I resize?

Not all of our images are the exact size we need them to be, so it's important to understand how to properly resize an image and how resizing works. When an image is resized, its pixel information is changed. For example, an image is reduced in size, any unneeded pixel information will be discarded by the photo editor (Photoshop). When an image is enlarged, the photo editor must create and add new pixel information -- based on its best guesses -- to achieve a larger size which typically results in either a very pixelated or very soft and blurry looking image.

This is why it is much easier to downsize an image than it is to enlarge an image. If an image is needed for high-quality (publishing) or large format (poster) prints, be sure that it is captured using the highest resolution and quality possible because of the difficulty in enlarging.
 
 
Are you sure you need to resize or just change resolution? If you are unsure, check out the What is Resolution tab to learn about how resolution affects images. We recommend that you adjust your image's resolution first to the quality needed before resizing the image.

 

 

 

Scaling vs. Resizing

When working with raster images (pixel-based) it is important to understand that scaling an image in programs, such as Word, Powerpoint, InDesign, or Dreamweaver, does not actually resize the image, but rather stretches images larger or scales them smaller. When scaling, the resolution is not adjusted to best suit the new size, rather the pixels are stretched and can appear pixelated.

The most common side effect of scaling an image larger than its original dimensions is that the image may appear to be very fuzzy or pixelated. Scaling images smaller than the original dimensions does not affect quality as much, but can have other side effects. For example, if you upload a very large image to a website and scale it down to a smaller size, the website still must load the full size version of that image and could cause the web page to load more slowly.

 

Exceptions:

  • A very small amount of scaling can be OK, specifically when scaling down. However, we strongly recommended resizing images in photo editors, such as Photoshop or GIMP, to achieve maximum photo quality.
     
  • Vector graphics (such as clipart and charts in Word or Powerpoint) are not comprised of pixels and therefore can be stretched to any size without loss of quality.

NO! Please, don’t confuse “image size” with “file size”. Image size refers to the dimensions of the image (height x length) while file size is about how much space the image takes up on a hard drive (kilobytes or megabytes).