Epson 660 - Manual (Page 51)

The x-axis in the Tonal Map represents the input brightness level of the pixels in an
image. The y-axis represents the output brightness level of the image. By clicking and
dragging the curve to change its shape, you can adjust the difference between input and
output brightness level of an image without extensively modifying the dark and bright
areas. If we compare the following images and their Tonal Maps:
The original image and its Tonal
Map.
The curve is straight and diago-
Drag the curve to the left, and
the mid-tone area of the image
becomes brighter.
Drag the curve to the right, and
the mid-tone area of the image
becomes darker.