Line Illusions use surrounding colors/objects and our experience with perspective to use all our knowledge against us.
You may take a ruler or a piece of paper and put it on the screen to verify that all lines are perfect and not bent.
Cafe Wall Illusion
In this cafe wall illusion the parallel straight horizontal lines appear to be bent. Important is that each "brick" is surrounded by the grey line, so a colour in between the dark and light colour of the "bricks".
Cafe Wall Variation
Are the lines made out of the blue squares parallel? You bet they are.
This image suggests that the horizontal lines are bent, however, the distortion is caused by the background that simulates perspective and thus false depth perception is created. (by Ewald Hering)
Which line is a continuation of the black one - the blue or the red line? If you guess that it's the blue line then you are wrong. Take a paper or a ruler to verify that. Human brain very poorly interpretes the path of diagonals. (by Johann Christian Poggendorff)
Are the diagonal lines parallel? Of course, they are - angle of the short lines helps to create the impression that one end of the longer lines is nearer to us than the other end. (by Johann Karl Friedrich Zollner)
Slanted lines in the following illusions produce a great effect. Both rectangles are parallel - you can take a ruler to check it out.
Do you see some squares or rectangles? Really, this is nothing more than a bunch of lines going in every which direction, but the way our mind interprets these lines are totally different!
A perfect circle.
Ehrenstein's Perfect Square
Sides of a square placed inside a pattern of concentric circles seem curved although they are straight (by Walter Ehrenstein).
3 perfect squares.
There is no spiral on the following picture. It's just nicely put squares that our eyes incorrectly understand as spiral. Look closer.
A nice visual illusion using perfect squares. (by Akiyoshi Kitaoka)
Perfect squares one more time. (by Akiyoshi Kitaoka)
All the dice are perfect squares. Switching of black and white color is just tricking us into seeing bent lines although all the lines are straight.