Brightness and Contrast. A gimg has two options for controls of image brightness and contrast. The option -bright specifies a value between 0 and 1, which biases a display's dynamic range from black to white. A brightness value of 0.5 generates an image with an unbiased display's dynamic range. The option -contrast defines an index of dissplay contrast, which decreases or increases a display's dynamic range. A contrast index may vary between -1.0 and 1.0. A zero contrast index washhouts completely the image contents in display. A negative contrast index generates a negative image in display. A contrast index 1.0 defines a full dynamic range of display. 

The following examples show the familiar image of Lena displayed with different -bright and -contrast configurations. 
 

-contrast 1.0 -bright 0.5	-contrast -1.0 -bright 0.5
-contrast 1.0 -bright 0.3	-contrast 1.0 -bright 0.7