Fair Use: "Under the fair use clause of copyright law, limited copyrighted material can be used for a "transformative" purpose, such as commenting upon, criticizing or parodying the initial material." "The 4 key factors are
- the purpose of the derivative work
- the nature of the original work: was the original mostly factual or creative
- the amount of original work used
- the effect that the new work has on the potential or actual market value of the original"
Public Domain: "An image automatically enters the Public Domain when a copyright expires." "Public Domain images have no licensing restrictions."
Bitmap/Raster Graphics: "Bitmap or raster graphics are built from grids of pixels. Each tiny pixel contains a unit of color information. Bitmaps are used for digital photography and scanned images. Bitmap files are not as easily scalable as vector graphics."
Basically, raster graphics are composed of pixels, whereas vectors are made from mathematical lines.
