The value of the
:ascent properties can be a special kind of expression that
is evaluated during redisplay. The result of the evaluation is used
as an absolute number of pixels.
The following expressions are supported:
expr ::= num | (num) | unit | elem | pos | image | xwidget | form num ::= integer | float | symbol unit ::= in | mm | cm | width | height
elem ::= left-fringe | right-fringe | left-margin | right-margin | scroll-bar | text pos ::= left | center | right form ::= (num . expr) | (op expr ...) op ::= + | -
The form num specifies a fraction of the default frame font
height or width. The form
(num) specifies an absolute
number of pixels. If num is a symbol, symbol, its
buffer-local variable binding is used; that binding can be either a
number or a cons cell of the forms shown above (including yet another
cons cell whose
car is a symbol that has a buffer-local
cm units specify the number of
pixels per inch, millimeter, and centimeter, respectively. The
height units correspond to the default width
and height of the current face. An image specification of the form
(image . props) (see Image Descriptors)
corresponds to the width or height of the specified image. Similarly,
an xwidget specification of the form
(xwidget . props)
stands for the width or height of the specified xwidget.
text specify the width of the corresponding area of the window.
When the window displays line numbers (see Size of Displayed Text), the width of the
text area is decreased by the screen
space taken by the line-number display.
right positions can be
:align-to to specify a position relative to the left
edge, center, or right edge of the text area. When the window
displays line numbers, the
left and the
are offset to account for the screen space taken by the line-number
Any of the above window elements (except
text) can also be
:align-to to specify that the position is relative to
the left edge of the given area. Once the base offset for a relative
position has been set (by the first occurrence of one of these
symbols), further occurrences of these symbols are interpreted as the
width of the specified area. For example, to align to the center of
the left-margin, use
:align-to (+ left-margin (0.5 . left-margin))
If no specific base offset is set for alignment, it is always relative to the left edge of the text area. For example, ‘:align-to 0’ in a header-line aligns with the first text column in the text area. When the window displays line numbers, the text is considered to start where the space used for line-number display ends.
A value of the form
(num . expr) stands for the
product of the values of num and expr. For example,
(2 . in) specifies a width of 2 inches, while
image) specifies half the width (or height) of the specified
image (which should be given by its image spec).
(+ expr ...) adds up the value of the
expressions. The form
(- expr ...) negates or subtracts
the value of the expressions.