#! perl

#:META:X_RESOURCE:%.expr:string:background expression
#:META:X_RESOURCE:%.border:boolean:respect the terminal border
#:META:X_RESOURCE:%.interval:seconds:minimum time between updates

#TODO: once, rootalign

=head1 NAME

   background - manage terminal background

=head1 SYNOPSIS

   urxvt --background-expr 'background expression'
         --background-border
         --background-interval seconds

=head1 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 I<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 load "/path/to/mybg.png"'

Or specified as a X resource:

   URxvt.background-expr: scale load "/path/to/mybg.png"

=head1 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 C<scale 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 it's size changes.

=head2 EXPRESSIONS

Expressions are normal Perl expressions, in fact, they are Perl blocks -
which means you could use multiple lines and statements:

   again 3600;
   if (localtime now)[6]) {
      return scale load "$HOME/weekday.png";
   } else {
      return scale load "$HOME/sunday.png";
   }

This expression gets evaluated once per hour. It will set F<sunday.png> as
background on Sundays, and F<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 C<load>, which loads an image from disk, or C<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 L<perlop> manpage). The F<$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
C<scale>:

   scale load "$HOME/mypic.png"

Just like a mathematical expression with functions, you should read these
expressions from right to left, as the C<load> is evaluated first, and
its result becomes the argument to the C<scale> function.

Many operators also allow some parameters preceding the input image
that modify its behaviour. For example, C<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, C<scale>
has now two arguments, the C<200> and the C<load> expression, while
C<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"

Other effects than scalign are also readily available, for exmaple, you can
tile the image to fill the whole window, instead of resizing it:

   tile load "$HOME/mypic.png"

In fact, images returned by C<load> are in C<tile> mode by default, so the C<tile> operator
is kind of superfluous.

Another common effect is to mirror the image, so that the same edges touch:

   mirror load "$HOME/mypic.png"

This is also a typical background expression:

   rootalign root

It first takes a snapshot of the screen background image, and then
moves it to the upper left corner of the screen - the result is
pseudo-transparency, as the image seems to be static while the window is
moved around.

=head2 CYCLES AND CACHING

As has been mentioned before, the expression might be evaluated multiple
times. Each time the expression is reevaluated, a new cycle is said to
have begun. Many operators cache their results till the next cycle.

For example, the C<load> operator keeps a copy of the image. If it is
asked to load the same image on the next cycle it will not load it again,
but return the cached copy.

This only works for one cycle though, so as long as you load the same
image every time, it will always be cached, but when you load a different
image, it will forget about the first one.

This allows you to either speed things up by keeping multiple images in
memory, or comserve memory by loading images more often.

For example, you can keep two images in memory and use a random one like
this:

   my $img1 = load "img1.png";
   my $img2 = load "img2.png";
   (0.5 > rand) ? $img1 : $img2

Since both images are "loaded" every time the expression is evaluated,
they are always kept in memory. Contrast this version:

   my $path1 = "img1.png";
   my $path2 = "img2.png";
   load ((0.5 > rand) ? $path1 : $path2)

Here, a path is selected randomly, and load is only called for one image,
so keeps only one image in memory. If, on the next evaluation, luck
decides to use the other path, then it will have to load that image again.

=head1 REFERENCE

=head2 COMMAND LINE SWITCHES

=over 4

=item --background-expr perl-expression

Specifies the Perl expression to evaluate.

=item --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.

=item --background-interval seconds

Since some operations in the underlying XRender extension can effetively
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.

=back

=cut

our %_IMGCACHE;
our $HOME;
our ($self, $old, $new);
our ($x, $y, $w, $h);

# enforce at least this interval between updates
our $MIN_INTERVAL = 6/59.951;

{
   package urxvt::bgdsl; # background language

   use List::Util qw(min max sum shuffle);

=head2 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.

=over 4

=item load $path

Loads the image at the given C<$path>. The image is set to plane tiling
mode.

Loaded images will be cached for one cycle.

=cut

   sub load($) {
      my ($path) = @_;

      $new->{load}{$path} = $old->{load}{$path} || $self->new_img_from_file ($path);
   }

=item root

Returns the root window pixmap, that is, hopefully, the background image
of your screen. The image is set to extend mode.

This function makes your expression root sensitive, that means it will be
reevaluated when the bg image changes.

=cut

   sub root() {
      $new->{rootpmap_sensitive} = 1;
      die "root op not supported, exg, we need you";
   }

=item solid $colour

=item solid $width, $height, $colour

Creates a new image and completely fills it with the given colour. The
image is set to tiling mode.

If C<$width> and C<$height> are omitted, it creates a 1x1 image, which is
useful for solid backgrounds or for use in filtering effects.

=cut

   sub solid($;$$) {
      my $colour = pop;

      my $img = $self->new_img (urxvt::PictStandardARGB32, $_[0] || 1, $_[1] || 1);
      $img->fill ($colour);
      $img
   }

=item 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.

=cut

   sub clone($) {
      $_[0]->clone
   }

=back

=head2 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.

=over 4

=item 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 C<tile> call is superfluous because C<load> already defaults
to tiling mode.

   tile load "mybg.png"

=item 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"

=item 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 wahtever your compisotr does
in alpha mode, else background colour).

   pad load "mybg.png"

=item 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 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"

=cut

   sub pad($) {
      my $img = $_[0]->clone;
      $img->repeat_mode (urxvt::RepeatNone);
      $img
   }

   sub tile($) {
      my $img = $_[0]->clone;
      $img->repeat_mode (urxvt::RepeatNormal);
      $img
   }

   sub mirror($) {
      my $img = $_[0]->clone;
      $img->repeat_mode (urxvt::RepeatReflect);
      $img
   }

   sub extend($) {
      my $img = $_[0]->clone;
      $img->repeat_mode (urxvt::RepeatPad);
      $img
   }

=back

=head2 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 C<TW> (terminal width) means your expression is evaluated
again when the terminal is resized.

=over 4

=item TX

=item 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 make 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.

   move -TX, -TY, load "mybg.png"

=item TW

Return the width (C<TW>) and height (C<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 make 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, blur 5, root

=cut

   sub TX() { $new->{position_sensitive} = 1; $x }
   sub TY() { $new->{position_sensitive} = 1; $y }
   sub TW() { $new->{size_sensitive}     = 1; $w }
   sub TH() { $new->{size_sensitive}     = 1; $h }

=item now

Returns the current time as (fractional) seconds since the epoch.

Using this expression does I<not> make your expression sensitive to time,
but the next two functions do.

=item again $seconds

When this function is used the expression will be reevaluated again in
C<$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 TW, TH, 50, 50, (now % 86400) * -720 / 86400, scale load "myclock.png"

=item counter $seconds

Like C<again>, but also returns an increasing counter value, starting at
0, which might be useful for some simple animation effects.

=cut

   sub now() { urxvt::NOW }

   sub again($) {
      $new->{again} = $_[0];
   }

   sub counter($) {
      $new->{again} = $_[0];
      $self->{counter} + 0
   }

=back

=head2 SHAPE CHANGING OPERATORS

The following operators modify the shape, size or position of the image.

=over 4

=item clip $img

=item clip $width, $height, $img

=item clip $x, $y, $width, $height, $img

Clips an image to the given rectangle. If the rectangle is outside the
image area (e.g. when C<$x> or C<$y> are negative) or the rectangle is
larger than the image, then the tiling mode defines how the extra pixels
will be filled.

If C<$x> an C<$y> are missing, then C<0> is assumed for both.

If C<$width> and C<$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 blur 10, load "mybg.png"

=cut

   sub clip($;$$;$$) {
      my $img = pop;
      my $h = pop || TH;
      my $w = pop || TW;
      $img->sub_rect ($_[0], $_[1], $w, $h)
   }

=item scale $img

=item scale $size_factor, $img

=item scale $width_factor, $height_factor, $img

Scales the image by the given factors in horizontal
(C<$width>) and vertical (C<$height>) direction.

If only one factor is give, it is used for both directions.

If no factors are given, scales the image to the window size without
keeping aspect.

=item resize $width, $height, $img

Resizes the image to exactly C<$width> times C<$height> pixels.

=item fit $img

=item fit $width, $height, $img

Fits the image into the given C<$width> and C<$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.

=item cover $img

=item cover $width, $height, $img

Similar to C<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.

=cut

   sub scale($;$;$) {
      my $img = pop;

      @_ == 2 ? $img->scale ($_[0] * $img->w, $_[1] * $img->h)
      : @_    ? $img->scale ($_[0] * $img->w, $_[0] * $img->h)
              : $img->scale (TW, TH)
   }

   sub resize($$$) {
      my $img = pop;
      $img->scale ($_[0], $_[1])
   }

   sub fit($;$$) {
      my $img = pop;
      my $w = ($_[0] || TW) / $img->w;
      my $h = ($_[1] || TH) / $img->h;
      scale +(min $w, $h), $img
   }

   sub cover($;$$) {
      my $img = pop;
      my $w = ($_[0] || TW) / $img->w;
      my $h = ($_[1] || TH) / $img->h;
      scale +(max $w, $h), $img
   }

=item move $dx, $dy, $img

Moves the image by C<$dx> pixels in the horizontal, and C<$dy> pixels in
the vertical.

Example: move the image right by 20 pixels and down by 30.

   move 20, 30, ...

=item align $xalign, $yalign, $img

Aligns the image according to a factor - C<0> means the image is moved to
the left or top edge (for C<$xalign> or C<$yalign>), C<0.5> means it is
exactly centered and C<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

=item center $img

=item 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 C<$width> and C<$height> if
given).

Example: load an image and center it.

  center pad load "mybg.png"

=item 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 C<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 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

=cut

   sub move($$;$) {
      my $img = pop->clone;
      $img->move ($_[0], $_[1]);
      $img
   }

   sub align($;$$) {
      my $img = pop;

      move $_[0] * (TW - $img->w),
           $_[1] * (TH - $img->h),
           $img
   }

   sub center($;$$) {
      my $img = pop;
      my $w = $_[0] || TW;
      my $h = $_[1] || TH;

      move 0.5 * ($w - $img->w), 0.5 * ($h - $img->h), $img
   }

   sub rootalign($) {
      move -TX, -TY, $_[0]
   }

=back

=head2 COLOUR MODIFICATIONS

The following operators change the pixels of the image.

=over 4

=item contrast $factor, $img

=item contrast $r, $g, $b, $img

=item contrast $r, $g, $b, $a, $img

Adjusts the I<contrast> of an image.

The first form applies a single C<$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.

=item brightness $bias, $img

=item brightness $r, $g, $b, $img

=item brightness $r, $g, $b, $a, $img

Adjusts the brightness of an image.

The first form applies a single C<$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 idiosynchrasies in the underlying XRender extension, biases less
than zero can be I<very> slow.

=cut

   sub contrast($$;$$;$) {
      my $img = pop;
      my ($r, $g, $b, $a) = @_;

      ($g, $b) = ($r, $r) if @_ < 3;
      $a       = 1        if @_ < 4;

      $img = $img->clone;
      $img->contrast ($r, $g, $b, $a);
      $img
   }

   sub brightness($$;$$;$) {
      my $img = pop;
      my ($r, $g, $b, $a) = @_;

      ($g, $b) = ($r, $r) if @_ < 3;
      $a       = 1        if @_ < 4;

      $img = $img->clone;
      $img->brightness ($r, $g, $b, $a);
      $img
   }

=item blur $radius, $img

=item blur $radius_horz, $radius_vert, $img

Gaussian-blurs the image with (roughly) C<$radius> pixel radius. The radii
can also be specified separately.

Blurring is often I<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).

=cut

   sub blur($$;$) {
      my $img = pop;
      $img->blur ($_[0], @_ >= 2 ? $_[1] : $_[0])
   }

=item rotate $new_width, $new_height, $center_x, $center_y, $degrees

Rotates the image by C<$degrees> degrees, counter-clockwise, around the
pointer at C<$center_x> and C<$center_y> (specified as factor of image
width/height), generating a new image with width C<$new_width> and height
C<$new_height>.

#TODO# new width, height, maybe more operators?

Example: rotate the image by 90 degrees

=cut

   sub rotate($$$$$$) {
      my $img = pop;
      $img->rotate (
         $_[0],
         $_[1],
         $_[2] * $img->w,
         $_[3] * $img->h,
         $_[4] * (3.14159265 / 180),
      )
   }

=back

=cut

}

sub parse_expr {
   my $expr = eval "sub {\npackage urxvt::bgdsl;\n#line 0 'background expression'\n$_[0]\n}";
   die if $@;
   $expr
}

# compiles a parsed expression
sub set_expr {
   my ($self, $expr) = @_;

   $self->{expr} = $expr;
   $self->recalculate;
}

# evaluate the current bg expression
sub recalculate {
   my ($arg_self) = @_;

   # rate limit evaluation

   if ($arg_self->{next_refresh} > urxvt::NOW) {
      $arg_self->{next_refresh_timer} = urxvt::timer->new->after ($arg_self->{next_refresh} - urxvt::NOW)->cb (sub {
         $arg_self->recalculate;
      });
      return;
   }

   $arg_self->{next_refresh} = urxvt::NOW + $MIN_INTERVAL;

   # set environment to evaluate user expression

   local $self = $arg_self;

   local $HOME = $ENV{HOME};
   local $old = $self->{state};
   local $new = my $state = $self->{state} = {};

   ($x, $y, $w, $h) =
      $self->background_geometry ($self->{border});

   # evaluate user expression

   my $img = eval { $self->{expr}->() };
   warn $@ if $@;#d#
   die "background-expr did not return an image.\n" if !UNIVERSAL::isa $img, "urxvt::img";

   $state->{size_sensitive} = 1
      if $img->repeat_mode != urxvt::RepeatNormal;

   # if the expression is sensitive to external events, prepare reevaluation then

   my $repeat;

   if (my $again = $state->{again}) {
      $repeat = 1;
      my $self = $self;
      $state->{timer} = $again == $old->{again}
                        ? $old->{timer}
                        : urxvt::timer->new->after ($again)->interval ($again)->cb (sub {
                             ++$self->{counter};
                             $self->recalculate
                          });
   }

   if (delete $state->{position_sensitive}) {
      $repeat = 1;
      $self->enable (position_change => sub { $_[0]->recalculate });
   } else {
      $self->disable ("position_change");
   }

   if (delete $state->{size_sensitive}) {
      $repeat = 1;
      $self->enable (size_change => sub { $_[0]->recalculate });
   } else {
      $self->disable ("size_change");
   }

   if (delete $state->{rootpmap_sensitive}) {
      $repeat = 1;
      $self->enable (rootpmap_change => sub { $_[0]->recalculate });
   } else {
      $self->disable ("rootpmap_change");
   }

   # clear stuff we no longer need

   %$old = ();

   unless ($repeat) {
      delete $self->{state};
      delete $self->{expr};
   }

   # set background pixmap

   $self->set_background ($img, $self->{border});
   $self->scr_recolour (0);
   $self->want_refresh;
}

sub on_start {
   my ($self) = @_;

   my $expr = $self->x_resource ("%.expr")
      or return;

   $self->has_render
      or die "background extension needs RENDER extension 0.10 or higher, ignoring background-expr.\n";

   $self->set_expr (parse_expr $expr);
   $self->{border} = $self->x_resource_boolean ("%.border");

   $MIN_INTERVAL = $self->x_resource ("%.interval");

   ()
}