urxvt-background(1) RXVT-UNICODE urxvt-background(1)NAME
background - manage terminal background
SYNOPSIS
urxvt --background-expr 'background expression'
--background-border
--background-interval seconds
QUICK AND DIRTY CHEAT SHEET
Just load a random jpeg image and tile the background with it without
scaling or anything else:
load "/path/to/img.jpg"
The same, but use mirroring/reflection instead of tiling:
mirror load "/path/to/img.jpg"
Load an image and scale it to exactly fill the terminal window:
scale keep { load "/path/to/img.jpg" }
Implement pseudo-transparency by using a suitably-aligned root pixmap
as window background:
rootalign root
Likewise, but keep a blurred copy:
rootalign keep { blur 10, root }
DESCRIPTION
This extension manages the terminal background by creating a picture
that is behind the text, replacing the normal background colour.
It does so by evaluating a Perl expression that calculates the image on
the fly, for example, by grabbing the root background or loading a
file.
While the full power of Perl is available, the operators have been
design to be as simple as possible.
For example, to load an image and scale it to the window size, you
would use:
urxvt --background-expr 'scale keep { load "/path/to/mybg.png" }'
Or specified as a X resource:
URxvt.background.expr: scale keep { load "/path/to/mybg.png" }
THEORY OF OPERATION
At startup, just before the window is mapped for the first time, the
expression is evaluated and must yield an image. The image is then
extended as necessary to cover the whole terminal window, and is set as
a background pixmap.
If the image contains an alpha channel, then it will be used as-is in
visuals that support alpha channels (for example, for a compositing
manager). In other visuals, the terminal background colour will be used
to replace any transparency.
When the expression relies, directly or indirectly, on the window size,
position, the root pixmap, or a timer, then it will be remembered. If
not, then it will be removed.
If any of the parameters that the expression relies on changes (when
the window is moved or resized, its position or size changes; when the
root pixmap is replaced by another one the root background changes; or
when the timer elapses), then the expression will be evaluated again.
For example, an expression such as "scale keep { load "$HOME/mybg.png"
}" scales the image to the window size, so it relies on the window size
and will be reevaluated each time it is changed, but not when it moves
for example. That ensures that the picture always fills the terminal,
even after its size changes.
EXPRESSIONS
Expressions are normal Perl expressions, in fact, they are Perl blocks
- which means you could use multiple lines and statements:
scale keep {
again 3600;
if (localtime now)[6]) {
return load "$HOME/weekday.png";
} else {
return load "$HOME/sunday.png";
}
}
This inner expression is evaluated once per hour (and whenever the
terminal window is resized). It sets sunday.png as background on
Sundays, and weekday.png on all other days.
Fortunately, we expect that most expressions will be much simpler, with
little Perl knowledge needed.
Basically, you always start with a function that "generates" an image
object, such as "load", which loads an image from disk, or "root",
which returns the root window background image:
load "$HOME/mypic.png"
The path is usually specified as a quoted string (the exact rules can
be found in the perlop manpage). The $HOME at the beginning of the
string is expanded to the home directory.
Then you prepend one or more modifiers or filtering expressions, such
as "scale":
scale load "$HOME/mypic.png"
Just like a mathematical expression with functions, you should read
these expressions from right to left, as the "load" is evaluated first,
and its result becomes the argument to the "scale" function.
Many operators also allow some parameters preceding the input image
that modify its behaviour. For example, "scale" without any additional
arguments scales the image to size of the terminal window. If you
specify an additional argument, it uses it as a scale factor (multiply
by 100 to get a percentage):
scale 2, load "$HOME/mypic.png"
This enlarges the image by a factor of 2 (200%). As you can see,
"scale" has now two arguments, the 200 and the "load" expression, while
"load" only has one argument. Arguments are separated from each other
by commas.
Scale also accepts two arguments, which are then separate factors for
both horizontal and vertical dimensions. For example, this halves the
image width and doubles the image height:
scale 0.5, 2, load "$HOME/mypic.png"
IF you try out these expressions, you might suffer from some
sluggishness, because each time the terminal is resized, it loads the
PNG image again and scales it. Scaling is usually fast (and
unavoidable), but loading the image can be quite time consuming. This
is where "keep" comes in handy:
scale 0.5, 2, keep { load "$HOME/mypic.png" }
The "keep" operator executes all the statements inside the braces only
once, or when it thinks the outcome might change. In other cases it
returns the last value computed by the brace block.
This means that the "load" is only executed once, which makes it much
faster, but also means that more memory is being used, because the
loaded image must be kept in memory at all times. In this expression,
the trade-off is likely worth it.
But back to effects: Other effects than scaling are also readily
available, for example, you can tile the image to fill the whole
window, instead of resizing it:
tile keep { load "$HOME/mypic.png" }
In fact, images returned by "load" are in "tile" mode by default, so
the "tile" operator is kind of superfluous.
Another common effect is to mirror the image, so that the same edges
touch:
mirror keep { load "$HOME/mypic.png" }
Another common background expression is:
rootalign root
This one first takes a snapshot of the screen background image, and
then moves it to the upper left corner of the screen (as opposed to the
upper left corner of the terminal window)- the result is pseudo-
transparency: the image seems to be static while the window is moved
around.
COLOUR SPECIFICATIONS
Whenever an operator expects a "colour", then this can be specified in
one of two ways: Either as string with an X11 colour specification,
such as:
"red" # named colour
"#f00" # simple rgb
"[50]red" # red with 50% alpha
"TekHVC:300/50/50" # anything goes
OR as an array reference with one, three or four components:
[0.5] # 50% gray, 100% alpha
[0.5, 0, 0] # dark red, no green or blur, 100% alpha
[0.5, 0, 0, 0.7] # same with explicit 70% alpha
CACHING AND SENSITIVITY
Since some operations (such as "load" and "blur") can take a long time,
caching results can be very important for a smooth operation. Caching
can also be useful to reduce memory usage, though, for example, when an
image is cached by "load", it could be shared by multiple terminal
windows running inside urxvtd.
"keep { ... }" caching
The most important way to cache expensive operations is to use "keep {
... }". The "keep" operator takes a block of multiple statements
enclosed by "{}" and keeps the return value in memory.
An expression can be "sensitive" to various external events, such as
scaling or moving the window, root background changes and timers.
Simply using an expression (such as "scale" without parameters) that
depends on certain changing values (called "variables"), or using those
variables directly, will make an expression sensitive to these events -
for example, using "scale" or "TW" will make the expression sensitive
to the terminal size, and thus to resizing events.
When such an event happens, "keep" will automatically trigger a
reevaluation of the whole expression with the new value of the
expression.
"keep" is most useful for expensive operations, such as "blur":
rootalign keep { blur 20, root }
This makes a blurred copy of the root background once, and on
subsequent calls, just root-aligns it. Since "blur" is usually quite
slow and "rootalign" is quite fast, this trades extra memory (for the
cached blurred pixmap) with speed (blur only needs to be redone when
root changes).
"load" caching
The "load" operator itself does not keep images in memory, but as long
as the image is still in memory, "load" will use the in-memory image
instead of loading it freshly from disk.
That means that this expression:
keep { load "$HOME/path..." }
Not only caches the image in memory, other terminal instances that try
to "load" it can reuse that in-memory copy.
REFERENCE
COMMAND LINE SWITCHES
--background-expr perl-expression
Specifies the Perl expression to evaluate.
--background-border
By default, the expression creates an image that fills the full
window, overwriting borders and any other areas, such as the
scrollbar.
Specifying this flag changes the behaviour, so that the image only
replaces the background of the character area.
--background-interval seconds
Since some operations in the underlying XRender extension can
effectively freeze your X-server for prolonged time, this extension
enforces a minimum time between updates, which is normally about
0.1 seconds.
If you want to do updates more often, you can decrease this safety
interval with this switch.
PROVIDERS/GENERATORS
These functions provide an image, by loading it from disk, grabbing it
from the root screen or by simply generating it. They are used as
starting points to get an image you can play with.
load $path
Loads the image at the given $path. The image is set to plane
tiling mode.
If the image is already in memory (e.g. because another terminal
instance uses it), then the in-memory copy is returned instead.
load_uc $path
Load uncached - same as load, but does not cache the image, which
means it is always loaded from the filesystem again, even if
another copy of it is in memory at the time.
root
Returns the root window pixmap, that is, hopefully, the background
image of your screen.
This function makes your expression root sensitive, that means it
will be reevaluated when the bg image changes.
solid $colour
solid $width, $height, $colour
Creates a new image and completely fills it with the given colour.
The image is set to tiling mode.
If $width and $height are omitted, it creates a 1x1 image, which is
useful for solid backgrounds or for use in filtering effects.
clone $img
Returns an exact copy of the image. This is useful if you want to
have multiple copies of the same image to apply different effects
to.
merge $img ...
Takes any number of images and merges them together, creating a
single image containing them all. The tiling mode of the first
image is used as the tiling mode of the resulting image.
This function is called automatically when an expression returns
multiple images.
TILING MODES
The following operators modify the tiling mode of an image, that is,
the way that pixels outside the image area are painted when the image
is used.
tile $img
Tiles the whole plane with the image and returns this new image -
or in other words, it returns a copy of the image in plane tiling
mode.
Example: load an image and tile it over the background, without
resizing. The "tile" call is superfluous because "load" already
defaults to tiling mode.
tile load "mybg.png"
mirror $img
Similar to tile, but reflects the image each time it uses a new
copy, so that top edges always touch top edges, right edges always
touch right edges and so on (with normal tiling, left edges always
touch right edges and top always touch bottom edges).
Example: load an image and mirror it over the background, avoiding
sharp edges at the image borders at the expense of mirroring the
image itself
mirror load "mybg.png"
pad $img
Takes an image and modifies it so that all pixels outside the image
area become transparent. This mode is most useful when you want to
place an image over another image or the background colour while
leaving all background pixels outside the image unchanged.
Example: load an image and display it in the upper left corner. The
rest of the space is left "empty" (transparent or whatever your
compositor does in alpha mode, else background colour).
pad load "mybg.png"
extend $img
Extends the image over the whole plane, using the closest pixel in
the area outside the image. This mode is mostly useful when you use
more complex filtering operations and want the pixels outside the
image to have the same values as the pixels near the edge.
Example: just for curiosity, how does this pixel extension stuff
work?
extend move 50, 50, load "mybg.png"
VARIABLE VALUES
The following functions provide variable data such as the terminal
window dimensions. They are not (Perl-) variables, they just return
stuff that varies. Most of them make your expression sensitive to some
events, for example using "TW" (terminal width) means your expression
is evaluated again when the terminal is resized.
TX
TY Return the X and Y coordinates of the terminal window (the terminal
window is the full window by default, and the character area only
when in border-respect mode).
Using these functions makes your expression sensitive to window
moves.
These functions are mainly useful to align images to the root
window.
Example: load an image and align it so it looks as if anchored to
the background (that's exactly what "rootalign" does btw.):
move -TX, -TY, keep { load "mybg.png" }
TW
TH Return the width ("TW") and height ("TH") of the terminal window
(the terminal window is the full window by default, and the
character area only when in border-respect mode).
Using these functions makes your expression sensitive to window
resizes.
These functions are mainly useful to scale images, or to clip
images to the window size to conserve memory.
Example: take the screen background, clip it to the window size,
blur it a bit, align it to the window position and use it as
background.
clip move -TX, -TY, keep { blur 5, root }
FOCUS
Returns a boolean indicating whether the terminal window has
keyboard focus, in which case it returns true.
Using this function makes your expression sensitive to focus
changes.
A common use case is to fade the background image when the terminal
loses focus, often together with the "-fade" command line option.
In fact, there is a special function for just that use case:
"focus_fade".
Example: use two entirely different background images, depending on
whether the window has focus.
FOCUS ? keep { load "has_focus.jpg" } : keep { load "no_focus.jpg" }
now Returns the current time as (fractional) seconds since the epoch.
Using this expression does not make your expression sensitive to
time, but the next two functions do.
again $seconds
When this function is used the expression will be reevaluated again
in $seconds seconds.
Example: load some image and rotate it according to the time of day
(as if it were the hour pointer of a clock). Update this image
every minute.
again 60;
rotate 50, 50, (now % 86400) * -72 / 8640, scale keep { load "myclock.png" }
counter $seconds
Like "again", but also returns an increasing counter value,
starting at 0, which might be useful for some simple animation
effects.
SHAPE CHANGING OPERATORS
The following operators modify the shape, size or position of the
image.
clip $img
clip $width, $height, $img
clip $x, $y, $width, $height, $img
Clips an image to the given rectangle. If the rectangle is outside
the image area (e.g. when $x or $y are negative) or the rectangle
is larger than the image, then the tiling mode defines how the
extra pixels will be filled.
If $x and $y are missing, then 0 is assumed for both.
If $width and $height are missing, then the window size will be
assumed.
Example: load an image, blur it, and clip it to the window size to
save memory.
clip keep { blur 10, load "mybg.png" }
scale $img
scale $size_factor, $img
scale $width_factor, $height_factor, $img
Scales the image by the given factors in horizontal ($width) and
vertical ($height) direction.
If only one factor is given, it is used for both directions.
If no factors are given, scales the image to the window size
without keeping aspect.
resize $width, $height, $img
Resizes the image to exactly $width times $height pixels.
fit $img
fit $width, $height, $img
Fits the image into the given $width and $height without changing
aspect, or the terminal size. That means it will be shrunk or grown
until the whole image fits into the given area, possibly leaving
borders.
cover $img
cover $width, $height, $img
Similar to "fit", but shrinks or grows until all of the area is
covered by the image, so instead of potentially leaving borders, it
will cut off image data that doesn't fit.
move $dx, $dy, $img
Moves the image by $dx pixels in the horizontal, and $dy pixels in
the vertical.
Example: move the image right by 20 pixels and down by 30.
move 20, 30, ...
align $xalign, $yalign, $img
Aligns the image according to a factor - 0 means the image is moved
to the left or top edge (for $xalign or $yalign), 0.5 means it is
exactly centered and 1 means it touches the right or bottom edge.
Example: remove any visible border around an image, center it
vertically but move it to the right hand side.
align 1, 0.5, pad $img
center $img
center $width, $height, $img
Centers the image, i.e. the center of the image is moved to the
center of the terminal window (or the box specified by $width and
$height if given).
Example: load an image and center it.
center keep { pad load "mybg.png" }
rootalign $img
Moves the image so that it appears glued to the screen as opposed
to the window. This gives the illusion of a larger area behind the
window. It is exactly equivalent to "move -TX, -TY", that is, it
moves the image to the top left of the screen.
Example: load a background image, put it in mirror mode and root
align it.
rootalign keep { mirror load "mybg.png" }
Example: take the screen background and align it, giving the
illusion of transparency as long as the window isn't in front of
other windows.
rootalign root
rotate $center_x, $center_y, $degrees, $img
Rotates the image clockwise by $degrees degrees, around the point
at $center_x and $center_y (specified as factor of image
width/height).
Example: rotate the image by 90 degrees around its center.
rotate 0.5, 0.5, 90, keep { load "$HOME/mybg.png" }
COLOUR MODIFICATIONS
The following operators change the pixels of the image.
tint $color, $img
Tints the image in the given colour.
Example: tint the image red.
tint "red", load "rgb.png"
Example: the same, but specify the colour by component.
tint [1, 0, 0], load "rgb.png"
shade $factor, $img
Shade the image by the given factor.
contrast $factor, $img
contrast $r, $g, $b, $img
contrast $r, $g, $b, $a, $img
Adjusts the contrast of an image.
The first form applies a single $factor to red, green and blue, the
second form applies separate factors to each colour channel, and
the last form includes the alpha channel.
Values from 0 to 1 lower the contrast, values higher than 1
increase the contrast.
Due to limitations in the underlying XRender extension, lowering
contrast also reduces brightness, while increasing contrast
currently also increases brightness.
brightness $bias, $img
brightness $r, $g, $b, $img
brightness $r, $g, $b, $a, $img
Adjusts the brightness of an image.
The first form applies a single $bias to red, green and blue, the
second form applies separate biases to each colour channel, and the
last form includes the alpha channel.
Values less than 0 reduce brightness, while values larger than 0
increase it. Useful range is from -1 to 1 - the former results in a
black, the latter in a white picture.
Due to idiosyncrasies in the underlying XRender extension, biases
less than zero can be very slow.
You can also try the experimental(!) "muladd" operator.
muladd $mul, $add, $img # EXPERIMENTAL
First multiplies the pixels by $mul, then adds $add. This can be
used to implement brightness and contrast at the same time, with a
wider value range than contrast and brightness operators.
Due to numerous bugs in XRender implementations, it can also
introduce a number of visual artifacts.
Example: increase contrast by a factor of $c without changing image
brightness too much.
muladd $c, (1 - $c) * 0.5, $img
blur $radius, $img
blur $radius_horz, $radius_vert, $img
Gaussian-blurs the image with (roughly) $radius pixel radius. The
radii can also be specified separately.
Blurring is often very slow, at least compared or other operators.
Larger blur radii are slower than smaller ones, too, so if you
don't want to freeze your screen for long times, start
experimenting with low values for radius (<5).
focus_fade $img
focus_fade $factor, $img
focus_fade $factor, $color, $img
Fades the image by the given factor (and colour) when focus is lost
(the same as the "-fade"/"-fadecolor" command line options, which
also supply the default values for "factor" and $color. Unlike with
"-fade", the $factor is a real value, not a percentage value (that
is, 0..1, not 0..100).
Example: do the right thing when focus fading is requested.
focus_fade load "mybg.jpg";
OTHER STUFF
Anything that didn't fit any of the other categories, even after
applying force and closing our eyes.
keep { ... }
This operator takes a code block as argument, that is, one or more
statements enclosed by braces.
The trick is that this code block is only evaluated when the
outcome changes - on other calls the "keep" simply returns the
image it computed previously (yes, it should only be used with
images). Or in other words, "keep" caches the result of the code
block so it doesn't need to be computed again.
This can be extremely useful to avoid redoing slow operations - for
example, if your background expression takes the root background,
blurs it and then root-aligns it it would have to blur the root
background on every window move or resize.
Another example is "load", which can be quite slow.
In fact, urxvt itself encloses the whole expression in some kind of
"keep" block so it only is reevaluated as required.
Putting the blur into a "keep" block will make sure the blur is
only done once, while the "rootalign" is still done each time the
window moves.
rootalign keep { blur 10, root }
This leaves the question of how to force reevaluation of the block,
in case the root background changes: If expression inside the block
is sensitive to some event (root background changes, window
geometry changes), then it will be reevaluated automatically as
needed.
9.22 2016-02-18 urxvt-background(1)
