Tanvir,

pre·view
   [font=courier new]/ˈpriˌvyu/
[pree-vyoo[/font] ]
–noun
1. an earlier or previous view.
2. an advance showing of a motion picture, play, etc., before its public opening.
3. an advance showing of brief scenes in a motion picture, television show, etc., for purposes of advertisement.
4. anything that gives an advance idea or impression of something to come.

@4: That means that a high quality rendering is meant to be high quality. A preview is a preview and may be low(er) quality. That includes jagged lines.

Hans