Humminbird 531338-1_C Operation Manual page 21

Inverse is a method where weak returns are shown with dark
pixels and strong returns with lighter pixels. This has the
benefit of ensuring that weak signals will be clearly visible on
the display.
Structure ID® represents weak returns as light pixels and
strong returns as dark pixels. This has the benefit of
ensuring that strong returns will be clearly visible on the
display.
WhiteLine™ highlights the strongest sonar returns in white,
resulting in a distinctive outline. This has the benefit of
clearly defining the bottom on the display.
Bottom Black displays all pixels below the bottom contour as
black, regardless of signal strength. This has the benefit of
providing a high contrast between the bottom and other
sonar returns on the display. Any targets such as fish,
structure and thermoclines will be shown using the
Structure ID® method.
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