image-scroll-down
image-scroll-down is an interactive compiled Lisp function in `image-mode.el
'.
(image-scroll-down &optional N)
Scroll image in current window downward by N lines.
Stop if the top edge of the image is reached.
If ARG is omitted or nil, scroll downward by a near full screen.
A near full screen is `next-screen-context-lines' less than a full screen.
Negative ARG means scroll upward.
If ARG is the atom `-', scroll upward by nearly full screen.
When calling from a program, supply as argument a number, nil, or `-'.