@cindex vertical scroll position
@dfn{Vertical fractional scrolling} means shifting text in a window
-up or down by a specified multiple or fraction of a line. Each window
-has a @dfn{vertical scroll position}, which is a number, never less than
-zero. It specifies how far to raise the contents of the window.
-Raising the window contents generally makes all or part of some lines
-disappear off the top, and all or part of some other lines appear at the
-bottom. The usual value is zero.
+up or down by a specified multiple or fraction of a line. Emacs uses
+it, for example, on images and screen lines which are taller than the
+window. Each window has a @dfn{vertical scroll position}, which is a
+number, never less than zero. It specifies how far to raise the
+contents of the window when displaying them. Raising the window
+contents generally makes all or part of some lines disappear off the
+top, and all or part of some other lines appear at the bottom. The
+usual value is zero.
The vertical scroll position is measured in units of the normal line
height, which is the height of the default font. Thus, if the value is
-.5, that means the window contents are scrolled up half the normal line
-height. If it is 3.3, that means the window contents are scrolled up
-somewhat over three times the normal line height.
+.5, that means the window contents will be scrolled up half the normal
+line height. If it is 3.3, that means the window contents are scrolled
+up somewhat over three times the normal line height.
What fraction of a line the vertical scrolling covers, or how many
lines, depends on what the lines contain. A value of .5 could scroll a
DEFUN ("window-vscroll", Fwindow_vscroll, Swindow_vscroll, 0, 2, 0,
doc: /* Return the amount by which WINDOW is scrolled vertically.
+This takes effect when displaying tall lines or images.
+
If WINDOW is omitted or nil, it defaults to the selected window.
Normally, value is a multiple of the canonical character height of WINDOW;
optional second arg PIXELS-P means value is measured in pixels. */)
DEFUN ("set-window-vscroll", Fset_window_vscroll, Sset_window_vscroll,
2, 3, 0,
doc: /* Set amount by which WINDOW should be scrolled vertically to VSCROLL.
+This takes effect when displaying tall lines or images.
+
WINDOW nil means use the selected window. Normally, VSCROLL is a
non-negative multiple of the canonical character height of WINDOW;
optional third arg PIXELS-P non-nil means that VSCROLL is in pixels.