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What is a raster image file?

Raster vs vector

Before understanding what a raster file is we need to also know what the opposite of a raster is in the world of image files. Raster’s are images made of many individual pixels which put together, form one coherent image. Vectors however are not made out of pixels. They are constructed using mathematical formulas and are great when you frequently need to resize an image.

Raster files

Almost all images found on print and on the internet are raster images. Camera’s typically create raster images regardless of the quality. If you’re working with raster images its useful to know the ppi (pixels per inch). The higher the ppi, the better quality the image is. But note that for web a lower ppi is preferred in order to lower the loading time for the image to be displayed. We don’t want the file to be too heavy and so it its good to keep a lower ppi when posting images on your website. Here’s a quick guide:

Here’s a quick guide to ppi size and where to use them:

  • 72ppi – web
  • 100ppi – web. Great balance between load time and image quality
  • 200ppi – print at home
  • 300ppi – professional printing standard

Note that you can make a larger ppi image smaller without losing quality, but it doesn’t work the other way around which means that a lower ppi image cannot be made larger without compromising its quality.

4 most common types of raster files

JPG or JPEG

They are often used for photos and can be optimized so they can be produced at a higher or lower quality. When you save an image out of photoshop in this format it contains a white background. It cannot save with a transparent background which makes it less appealing for logo designs, mockups etc.

PNG

These typically have a higher quality that jpg’s but are also heavier. They can be saved with a transparent background which makes them ideal for images that have irregular shapes. They can be placed over other photos or colored backgrounds without affecting the layer behind it.

GIF

A GIF has considerably less colors to choose from when creating an image. To be exact it contains up to 256 colors from the RGB colorspace. This allows for the file to be smaller in size and is ideal for images that just use a few solid colors without many gradients. Only use for the web and when your image contains just a few colors. You wouldn’t want to use it for print.

TIF or TIFF

This is the largest file size among these 4 options. They are typically used for printing photos professionally because they have no loss in quality. They are most often used for print but not recommended to use for web because of a longer load time.

There are many other (less common) file types that are either raster files or have the capacity to produce raster files. You can find this list here: https://fileinfo.com/filetypes/raster_image

 

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