This file is indexed.

/usr/lib/urxvt/perl/background is in rxvt-unicode 9.20-1+b1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
#! 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

=head1 NAME

background - manage terminal background

=head1 SYNOPSIS

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

=head1 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 }

=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 keep { load "/path/to/mybg.png" }'

Or specified as a X resource:

   URxvt.background-expr: scale keep { 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 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.

=head2 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 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"

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 C<keep> comes in handy:

   scale 0.5, 2, keep { load "$HOME/mypic.png" }

The C<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 C<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 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 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.

=head2 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

=head2 CACHING AND SENSITIVITY

Since some operations (such as C<load> and C<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 C<load>, it could be shared by multiple terminal windows
running inside urxvtd.

=head3 C<keep { ... }> caching

The most important way to cache expensive operations is to use C<keep {
... }>. The C<keep> operator takes a block of multiple statements enclosed
by C<{}> 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 C<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 C<scale> or C<TW> will make the expression sensitive to the terminal
size, and thus to resizing events.

When such an event happens, C<keep> will automatically trigger a
reevaluation of the whole expression with the new value of the expression.

C<keep> is most useful for expensive operations, such as C<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 C<blur> is usually quite slow and
C<rootalign> is quite fast, this trades extra memory (for the cached
blurred pixmap) with speed (blur only needs to be redone when root
changes).

=head3 C<load> caching

The C<load> operator itself does not keep images in memory, but as long as
the image is still in memory, C<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
C<load> it can reuse that in-memory copy.

=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 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.

=back

=cut

our %_IMG_CACHE;
our $HOME;
our ($self, $frame);
our ($x, $y, $w, $h, $focus);

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

{
   package urxvt::bgdsl; # background language

   sub FR_PARENT() { 0 } # parent frame, if any - must be #0
   sub FR_CACHE () { 1 } # cached values
   sub FR_AGAIN () { 2 } # what this expr is sensitive to
   sub FR_STATE () { 3 } # watchers etc.

   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.

If the image is already in memory (e.g. because another terminal instance
uses it), then the in-memory copy is returned instead.

=item load_uc $path

Load uncached - same as load, but does not cache the image, which means it
is I<always> loaded from the filesystem again, even if another copy of it
is in memory at the time.

=cut

   sub load_uc($) {
      $self->new_img_from_file ($_[0])
   }

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

      $_IMG_CACHE{$path} || do {
         my $img = load_uc $path;
         Scalar::Util::weaken ($_IMG_CACHE{$path} = $img);
         $img
      }
   }

=item 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.

=cut

   sub root() {
      $frame->[FR_AGAIN]{rootpmap} = 1;
      $self->new_img_from_root
   }

=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, 0, $_[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
   }

=item 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.

=cut

   sub merge(@) {
      return $_[0] unless $#_;

      # rather annoyingly clumsy, but optimisation is for another time

      my $x0 = +1e9;
      my $y0 = +1e9;
      my $x1 = -1e9;
      my $y1 = -1e9;

      for (@_) {
         my ($x, $y, $w, $h) = $_->geometry;

         $x0 = $x if $x0 > $x;
         $y0 = $y if $y0 > $y;

         $x += $w;
         $y += $h;

         $x1 = $x if $x1 < $x;
         $y1 = $y if $y1 < $y;
      }

      my $base = $self->new_img (urxvt::PictStandardARGB32, $x0, $y0, $x1 - $x0, $y1 - $y0);
      $base->repeat_mode ($_[0]->repeat_mode);
      $base->fill ([0, 0, 0, 0]);

      $base->draw ($_)
         for @_;

      $base
   }

=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 whatever your compositor 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 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"

=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 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 C<rootalign> does btw.):

   move -TX, -TY, keep { load "mybg.png" }

=item TW

=item TH

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 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 }

=item 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 C<-fade> command line option. In fact,
there is a special function for just that use case: C<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" }

=cut

   sub TX   () { $frame->[FR_AGAIN]{position} = 1; $x     }
   sub TY   () { $frame->[FR_AGAIN]{position} = 1; $y     }
   sub TW   () { $frame->[FR_AGAIN]{size}     = 1; $w     }
   sub TH   () { $frame->[FR_AGAIN]{size}     = 1; $h     }
   sub FOCUS() { $frame->[FR_AGAIN]{focus}    = 1; $focus }

=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 50, 50, (now % 86400) * -72 / 8640, scale keep { 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($) {
      $frame->[FR_AGAIN]{time} = $_[0];
   }

   sub counter($) {
      $frame->[FR_AGAIN]{time} = $_[0];
      $frame->[FR_STATE]{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<$x> and C<$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 keep { 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 given, 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 keep { 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 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

=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]
   }

=item rotate $center_x, $center_y, $degrees, $img

Rotates the image clockwise by C<$degrees> degrees, around the point at
C<$center_x> and C<$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" }

=cut

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

=back

=head2 COLOUR MODIFICATIONS

The following operators change the pixels of the image.

=over 4

=item 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"

=cut

   sub tint($$) {
      $_[1]->tint ($_[0])
   }

=item shade $factor, $img

Shade the image by the given factor.

=cut

   sub shade($$) {
      $_[1]->shade ($_[0])
   }

=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 idiosyncrasies in the underlying XRender extension, biases less
than zero can be I<very> slow.

You can also try the experimental(!) C<muladd> operator.

=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 muladd $mul, $add, $img # EXPERIMENTAL

First multiplies the pixels by C<$mul>, then adds C<$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<$c> without changing image
brightness too much.

   muladd $c, (1 - $c) * 0.5, $img

=cut

   sub muladd($$$) {
      $_[2]->muladd ($_[0], $_[1])
   }

=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 focus_fade $img

=item focus_fade $factor, $img

=item focus_fade $factor, $color, $img

Fades the image by the given factor (and colour) when focus is lost (the
same as the C<-fade>/C<-fadecolor> command line options, which also supply
the default values for C<factor> and C<$color>. Unlike with C<-fade>, the
C<$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";

=cut

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

      return $img
         if FOCUS;

      my $fade   = @_ >= 1 ? $_[0] : defined $self->resource ("fade") ? $self->resource ("fade") * 0.01 : 0;
      my $color  = @_ >= 2 ? $_[1] : $self->resource ("color+" . urxvt::Color_fade);

      $img = $img->tint ($color)         if $color ne "rgb:00/00/00";
      $img = $img->muladd (1 - $fade, 0) if $fade;

      $img
   }

=back

=head2 OTHER STUFF

Anything that didn't fit any of the other categories, even after applying
force and closing our eyes.

=over 4

=item 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 C<keep> simply returns the image it computed
previously (yes, it should only be used with images). Or in other words,
C<keep> I<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 C<load>, which can be quite slow.

In fact, urxvt itself encloses the whole expression in some kind of
C<keep> block so it only is reevaluated as required.

Putting the blur into a C<keep> block will make sure the blur is only done
once, while the C<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.

=cut

   sub keep(&) {
      my $id = $_[0]+0;

      local $frame = $self->{frame_cache}{$id} ||= [$frame];

      unless ($frame->[FR_CACHE]) {
         $frame->[FR_CACHE] = [ $_[0]() ];

         my $self  = $self;
         my $frame = $frame;
         Scalar::Util::weaken $frame;
         $self->compile_frame ($frame, sub {
            # clear this frame cache, also for all parents
            for (my $frame = $frame; $frame; $frame = $frame->[0]) {
               undef $frame->[FR_CACHE];
            }

            $self->recalculate;
         });
      };

      # in scalar context we always return the first original result, which
      # is not quite how perl works.
      wantarray
         ? @{ $frame->[FR_CACHE] }
         : $frame->[FR_CACHE][0]
   }

#   sub keep_clear() {
#      delete $self->{frame_cache};
#   }

=back

=cut

}

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

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

   $self->{root} = []; # the outermost frame
   $self->{expr} = $expr;
   $self->recalculate;
}

# takes a hash of sensitivity indicators and installs watchers
sub compile_frame {
   my ($self, $frame, $cb) = @_;

   my $state = $frame->[urxvt::bgdsl::FR_STATE] ||= {};
   my $again = $frame->[urxvt::bgdsl::FR_AGAIN];

   # don't keep stuff alive
   Scalar::Util::weaken $state;

   if ($again->{nested}) {
      $state->{nested} = 1;
   } else {
      delete $state->{nested};
   }

   if (my $interval = $again->{time}) {
      $state->{time} = [$interval, urxvt::timer->new->after ($interval)->interval ($interval)]
         if $state->{time}[0] != $interval;

      # callback *might* have changed, although we could just rule that out
      $state->{time}[1]->cb (sub {
         ++$state->{counter};
         $cb->();
      });
   } else {
      delete $state->{time};
   }

   if ($again->{position}) {
      $state->{position} = $self->on (position_change => $cb);
   } else {
      delete $state->{position};
   }

   if ($again->{size}) {
      $state->{size}     = $self->on (size_change     => $cb);
   } else {
      delete $state->{size};
   }

   if ($again->{rootpmap}) {
      $state->{rootpmap} = $self->on (rootpmap_change => $cb);
   } else {
      delete $state->{rootpmap};
   }

   if ($again->{focus}) {
      $state->{focus} = $self->on (focus_in => $cb, focus_out => $cb);
   } else {
      delete $state->{focus};
   }
}

# 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 $frame = $self->{root};

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

   # evaluate user expression

   my @img = eval { $self->{expr}->() };
   die $@ if $@;
   die "background-expr did not return anything.\n" unless @img;
   die "background-expr: expected image(s), got something else.\n"
      if grep { !UNIVERSAL::isa $_, "urxvt::img" } @img;

   my $img = urxvt::bgdsl::merge @img;

   $frame->[FR_AGAIN]{size} = 1
      if $img->repeat_mode != urxvt::RepeatNormal;

   # if the expression is sensitive to external events, prepare reevaluation then
   $self->compile_frame ($frame, sub { $arg_self->recalculate });

   # clear stuff we no longer need

#   unless (%{ $frame->[FR_STATE] }) {
#      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");

   ()
}