Graphics Terminology
A graphic is a rectangular image stored digitally in a file. There are many file formats for graphics and there are some fundamental characteristics of graphics that you must be aware of.
Common Image File Attributes
- file type
- file size
- dimensions
- resolution
- bit depth
- compression
- representation
File type
The file type represents how the image is stored and its properties. Different file types have different properties. Common file type properties are bit depth, compression, and transparency. Common file types are bmp, jpg, gif, png, tiff, and svg. Most of these file types are raster images, meaning the image details are stored as pixels. Some graphical images are saved in a vector format, meaning the shapes are stored as mathematical equations.
- bmp, bitmap, raster format, uncompressed, up to 8 bits color depth
- jpg, "jay-peg", raster format, lossy compression, 24 bit color depth
- gif, Graphics Interchange Format, raster format, lossless compression, 8 bit color depth, transparent
- png, Portable Network Graphic, raster format, lossless compression, 24 or 48 bit color depth, transparent
- tiff, Tagged Image File Format, raster format, compression varies, up to 64 bit color depth
- svg, Scalable Vector Graphic, vector format, no compression, 24 bit color depth, transparent
File size
File size will vary greatly based on the image, the type, the compression, and the color depth.
Dimensions
The width and height of the image usually given in pixels, e.g., 640x480, 1920x1080.
Resolution
This is the number of pixels in a given length, e.g., pixels per inch (ppi) or dots per inch (dpi). Resolution is one of the trickier aspects with images on a computer. Screen resolution is different than print resolution. Where 72 dpi might look fine on a computer screen, it would likely look poor when printed. For example an image that is 1080 pixels in height would need to be 3.6 inches tall for a print resolution of 300 dpi.
Bit depth
The tells us how many colors are available.
| Bit Depth | Number of Colors |
|---|---|
| 1 | 2 |
| 2 | 4 |
| 4 | 8 |
| 8 | 256 |
| 24 | 224 = 16,777,216 |
Compression
Compression reduces the files size, but sometimes at a cost. There are lossless compression formats, meaning the file size can be reduced without the loss of image detail. There are lossy compression formats, meaning image details are lost when the file size is reduced.
Representation
Raster files are made with an array of dots, aka pixels.
Vector files are made with mathematical equations. Example Links to an external site..
This question Links to an external site. on a site called Stack Overflow has a great description of the difference between several image formats.