-
Notifications
You must be signed in to change notification settings - Fork 790
Expand file tree
/
Copy pathOverview.src.html
More file actions
3026 lines (2460 loc) · 143 KB
/
Overview.src.html
File metadata and controls
3026 lines (2460 loc) · 143 KB
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
<h1>CSS Image Values and Replaced Content Module Level 4</h1>
<pre class='metadata'>
Status: ED
Shortname: css-images
Level: 4
Group: csswg
ED: http://dev.w3.org/csswg/css-images-4/
TR: http://www.w3.org/TR/css4-images/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Editor: fantasai, Mozilla, http://fantasai.inkedblade.net/contact
Abstract: This module contains the features of CSS level 4 relating to the <<image>> type and replaced elements. It includes and extends the functionality of CSS Image Values and Replaced Content Module Level 3. The main extensions compared to level 3 are several additions to the <<image>> type, additions to the <<gradient>> type, extentions to the ''image()'' function, definitions for interpolating several <<image>> types, and several properties controlling the interaction of replaced elements and CSS's layout models.
!Issue Tracking: <a href="http://www.w3.org/Style/CSS/Tracker/products/27">http://www.w3.org/Style/CSS/Tracker/products/27</a>
Previous Version: http://www.w3.org/TR/2012/WD-css4-images-20120911/
Ignored Terms: <offset>, background positioning area, border image area, <meetorslice>, <ending-shape>, Map
Link Defaults: css21 (property) display, css-values-3 (type) <position>
</pre>
<h2 id="intro">
Introduction</h2>
<em>This section is not normative.</em>
In CSS Levels 1 and 2,
image values, such as those used in the 'background-image' property,
could only be given by a single URL value.
This module introduces additional ways of representing 2D images,
for example as <a href="#image-notation">a list of URIs denoting fallbacks</a>,
as <a href="#element-image-type">a reference to an element</a>,
or as <a href="#gradients">a gradient</a>.
This module also defines several properties for <a href="#image-processing">manipulating raster images</a>
and for <a href="#object-fit">sizing</a>
or <a href="#object-position">positioning</a>
replaced elements such as images within the box determined by the CSS layout algorithms.
It also defines in a generic way CSS's <a href="#sizing">sizing algorithm</a> for images and other replaced elements.
<h3 id="placement">
Module Interactions</h3>
This module defines and extends the <<image>> value type defined in [[!CSS3VAL]].
Furthermore it replaces the <<url>> type
in the 'background-image' and 'list-style-image' definitions in CSS1 and CSS2
and adds <<image>> as an alternative to <<url>> in the 'content' property's value.
It is presumed that CSS specifications beyond CSS2.1 will use the <<image>> notation
in place of <<url>> where 2D images are expected. (See e.g. [[CSS3BG]].)
Of the properties defined in this module,
only 'image-resolution'
applies to <code>::first-line</code> and <code>::first-letter</code>.
<h3 id="values">
Values</h3>
This specification follows the <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> from [[!CSS21]].
Value types not defined in this specification
are defined in CSS Level 2 Revision 1 [[!CSS21]].
Other CSS modules may expand the definitions of these value types:
for example [[CSS3COLOR]],
when combined with this module,
expands the definition of the <<color>> value type as used in this specification.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification also accept the <a href="http://www.w3.org/TR/css3-values/">global keywords</a> as their property value.
For readability they has not been repeated explicitly.
<h2 id='planned-features'>
Planned Features</h2>
<ol>
<li>Resolution tagging in image()
<li>Logical keywords for specifying linear gradient direction.
<li>More interpolation modes for gradients: <a href="http://lists.w3.org/Archives/Public/www-style/2012Jan/0635.html">http://lists.w3.org/Archives/Public/www-style/2012Jan/0635.html</a>
<li>Ability to reference an SVG paint server via url (maybe as element(<<string>>)?)
</ol>
<!-- ====================================================================== -->
<h2 id="image-values">
Image Values: the <<image>> type</h2>
The <<image>> value type denotes a 2D image. It can be a
<a href="#url-notation">url reference</a>,
<a href="#image-notation">image notation</a>,
<!-- <a href="#element-image">element reference</a>, -->
or <a href="#gradients">gradient notation</a>.
Its syntax is:
<pre class="prod"><dfn><image></dfn> = <<url>> | <<image()>> | <<image-set()>> | <<element()>> | <<cross-fade()>> | <<gradient>></pre>
An <<image>> can be used in many CSS properties,
including the 'background-image', 'list-style-image', 'cursor' properties [[!CSS21]]
(where it replaces the <<url>> component in the property's value).
In some cases, an image is invalid,
such as a <<url>> pointing to a resource that is not a valid image format.
An <dfn export>invalid image</dfn> is rendered as a solid-color ''transparent'' image with no intrinsic dimensions.
However, <a>invalid images</a> have special behavior in some contexts,
such as the ''image()'' notation.
<!--
UUUUUUUU UUUUUUUURRRRRRRRRRRRRRRRR LLLLLLLLLLL
U::::::U U::::::UR::::::::::::::::R L:::::::::L
U::::::U U::::::UR::::::RRRRRR:::::R L:::::::::L
UU:::::U U:::::UURR:::::R R:::::RLL:::::::LL
U:::::U U:::::U R::::R R:::::R L:::::L
U:::::D D:::::U R::::R R:::::R L:::::L
U:::::D D:::::U R::::RRRRRR:::::R L:::::L
U:::::D D:::::U R:::::::::::::RR L:::::L
U:::::D D:::::U R::::RRRRRR:::::R L:::::L
U:::::D D:::::U R::::R R:::::R L:::::L
U:::::D D:::::U R::::R R:::::R L:::::L
U::::::U U::::::U R::::R R:::::R L:::::L LLLLLL
U:::::::UUU:::::::U RR:::::R R:::::RLL:::::::LLLLLLLLL:::::L
UU:::::::::::::UU R::::::R R:::::RL::::::::::::::::::::::L
UU:::::::::UU R::::::R R:::::RL::::::::::::::::::::::L
UUUUUUUUU RRRRRRRR RRRRRRRLLLLLLLLLLLLLLLLLLLLLLLL
-->
<h3 id="url-notation">
Image References and Image Slices: the <<url>> type and ''url()'' notation</h3>
The simplest way to indicate an image is to reference an image file by URL.
This can be done with the <a href="http://www.w3.org/TR/CSS21/syndata.html#uri">>url()'' notation</a>, defined in [[!CSS21]].
<div class="example">
In the example below, a background image is specified with ''url()''syntax:
<pre>background-image: url(wavy.png);</pre>
</div>
If the UA cannot download, parse, or otherwise successfully display the contents at the URL as an image,
it must be treated as an <a>invalid image</a>.
<!--
IIIIIIIIII
I::::::::I
I::::::::I
II::::::II
I::::I mmmmmmm mmmmmmm aaaaaaaaaaaaa ggggggggg ggggg eeeeeeeeeeee
I::::I mm:::::::m m:::::::mm a::::::::::::a g:::::::::ggg::::g ee::::::::::::ee
I::::I m::::::::::mm::::::::::m aaaaaaaaa:::::a g:::::::::::::::::g e::::::eeeee:::::ee
I::::I m::::::::::::::::::::::m a::::ag::::::ggggg::::::gge::::::e e:::::e
I::::I m:::::mmm::::::mmm:::::m aaaaaaa:::::ag:::::g g:::::g e:::::::eeeee::::::e
I::::I m::::m m::::m m::::m aa::::::::::::ag:::::g g:::::g e:::::::::::::::::e
I::::I m::::m m::::m m::::m a::::aaaa::::::ag:::::g g:::::g e::::::eeeeeeeeeee
I::::I m::::m m::::m m::::ma::::a a:::::ag::::::g g:::::g e:::::::e
II::::::IIm::::m m::::m m::::ma::::a a:::::ag:::::::ggggg:::::g e::::::::e
I::::::::Im::::m m::::m m::::ma:::::aaaa::::::a g::::::::::::::::g e::::::::eeeeeeee
I::::::::Im::::m m::::m m::::m a::::::::::aa:::a gg::::::::::::::g ee:::::::::::::e
IIIIIIIIIImmmmmm mmmmmm mmmmmm aaaaaaaaaa aaaa gggggggg::::::g eeeeeeeeeeeeee
g:::::g
gggggg g:::::g
g:::::gg gg:::::g
g::::::ggg:::::::g
gg:::::::::::::g
ggg::::::ggg
gggggg
-->
<h3 id="image-notation">
Image Fallbacks and Annotations: the ''image()'' notation</h3>
The ''image()'' function allows an author to:
<ul>
<li>
use <a href="http://www.w3.org/TR/media-frags/">media fragments</a> to clip out a portion of an image
<li>
specify fallback images in case the preferred image can't be decoded or is a type that the browser doesn't recognize
<li>
use a solid color as an image
</ul>
The ''image()'' notation is defined as:
<pre class='prod'>
<dfn>image()</dfn> = image( <<image-tags>>? [ <<image-src>> , ]* [ <<image-src>> | <<color>> ] )
<dfn><image-tags></dfn> = [ ltr | rtl ]
<dfn><image-src></dfn> = [ <<url>> | <<string>> ]
</pre>
Each <<string>> or <<url>> inside ''image()'' represents an image,
just as if the <a href="#url-notation">>url()'' notation</a> had been used.
As usual for URLs in CSS,
relative URLs are resolved to an absolute URL
(as described in Values & Units [[!CSS3VAL]])
when a specified ''image()'' value is computed.
<h4 id="image-fragments">
Image Fragments</h4>
When a URL specified in ''image()'' represents a portion of a resource
(e.g. by the use of <a href="http://www.w3.org/TR/media-frags/#naming-space">media fragment identifiers</a>)
that portion is clipped out of its context and used as a standalone image.
<div class="example">
For example, given the following image and CSS:
<a href="images/sprites.svg">
<img src="images/sprites.svg" height="20" width="180" alt="[9 circles, with 0 to 8 eighths filled in]">
</a>
<pre>background-image: image('sprites.svg#xywh=40,0,20,20')</pre>
...the background of the element will be the portion of the image that starts at (40px,0px) and is 20px wide and tall,
which is just the circle with a quarter filled in.
</div>
So that authors can take advantage of CSS's forwards-compatible parsing rules to provide a fallback for image slices,
implementations that support the ''image()'' notation
<em>must</em> support the <code>xywh=#,#,#,#</code> form of media fragment identifiers
for images specified via ''image()''. [[!MEDIA-FRAGS]]
<div class='example'>
Note that image fragments can also be used with the ''url()'' notation.
However, a legacy UA that doesn't understand the media fragments notation
will ignore the fragment and simply display the entirety of the image.
Since the ''image()'' notation requires UAs to support media fragments,
authors can take advantage of CSS's forward-compatible parsing rules
to provide a fallback when using an image fragment URL:
<pre>
background-image: url('swirl.png'); /* old UAs */
background-image: image('sprites.png#xywh=10,30,60,20'); /* new UAs */
</pre>
</div>
<h4 id="image-fallbacks">
Image Fallbacks</h4>
Multiple <<image-src>>s can be given separated by commas,
in which case the function represents the first image that's not an <a>invalid image</a>.
The final argument can specify a <<color>> to serve as an ultimate fallback;
this can be used, e.g. for 'background-image',
to ensure adequate contrast if none of the preceding <<image-src>>s can be used.
If the final argument is a <<color>>,
it represents a solid-color image of the given color with no <a>intrinsic dimensions</a>.
If all of the provided <<image-src>>s are <a>invalid images</a>
and a fallback color was not provided as the last argument,
the entire ''image()'' function must be treated as an <a>invalid image</a>.
<div class="example">
The rule below would tell the UA to load "wavy.svg" if it can;
failing that to load "wavy.png";
and failing that to display "wavy.gif".
For example, the browser might not understand how to render SVG images,
and the PNG may be temporarily 404
(returning an HTML 404 page, which the browser can't decode as an image)
due to a server move,
so the GIF is used until one of the previous problems corrects itself.
<pre>background-image: image("wavy.svg", "wavy.png" , "wavy.gif");</pre>
</div>
<div class='example'>
The fallback color can be used to ensure that text is still readable
even when the image fails to load.
For example, the following code works fine if the image is rectangular and has no transparency:
<pre>
body { color: black; background: white; }
p.special { color: white; background: url("dark.png") black; }
</pre>
When the image doesn't load,
the background color is still there to ensure that the white text is readable.
However, if the image has some transparency,
the black will be visible behind it,
which is probably not desired.
The ''image()'' function addresses this:
<pre>
body { color: black; background: white; }
p.special { color: white; background: image("dark.png", black); }
</pre>
Now, the black won't show at all if the image loads,
but if for whatever reason the image fails,
it'll pop in and prevent the white text from being set against a white background.
</div>
If a URL uses a fragment identifier syntax that the implementation does not understand,
or does not consider valid for that type of image,
the URL must be treated as representing an <a>invalid image</a>.
<span class='note'>This error-handling is limited to image(),
and not in the definition of URL,
for legacy compat reasons.</span>
<div class='example'>
For example, if a future specification defined a way to refer to a specific frame of an animated GIF with a fragment identifier,
an author could write the following to get newer browsers to use the GIF's frame,
and older browsers to instead download the fallback image:
<pre>background-image: image("cat_meme.gif#frame=5", "lolcat.png");</pre>
</div>
<!-- XXX SVG fragment example with spriting / fallbacks (once SVG can provide this) -->
<h4 id="color-images">
Solid-color Images</h4>
<div class='example'>
At times, one may need a solid-color image for a property or function that does not accept the <<color>> type directly.
The ''image()'' function can be used for this:
by specifying <em>only</em> a color without any URLs,
the function immediately falls back to representing a solid-color image of the chosen color.
<pre>background-image: image(rgba(0,0,255,.5)), url("bg-image.png");</pre>
In the above, the background is the image "bg-image.png",
overlaid with partially-transparent blue.
</div>
<h4 id='bidi-images'>
Bidi-sensitive Images</h4>
Before listing any <code><image-src>s</code>,
the author may specify a directionality for the image,
similar to adding a <code>dir</code> attribute to an element in HTML.
If a directional image is used on or in an element with opposite <a href="http://www.w3.org/TR/CSS21/visuren.html#propdef-direction">direction</a>,
the image must be flipped in the inline direction
(as if it was transformed by, e.g., <code>scaleX(-1)</code>, if the inline direction is the X axis).
Note: Absent this declaration,
images default to no directionality at all,
and thus don't care about the directionality of the surrounding element.
<div class='example'>
A list may use an arrow for a bullet that points into the content.
If the list can contain both LTR and RTL text,
though, the bullet may be on the left or the right,
and an image designed to point into the text on one side will point out of the text on the other side.
This can be fixed with code like:
<pre>
<ul style="list-style-image: image(ltr 'arrow.png');">
<li dir='ltr'>My bullet is on the left!</li>
<li dir='rtl'>MY BULLET IS ON THE RIGHT!</li>
</ul>
</pre>
This should render something like:
<pre>
⇒ My bullet is on the left!
!THGIR EHT NO SI TELLUB YM ⇐
</pre>
In LTR list items, the image will be used as-is.
In the RTL list items, however,
it will be flipped in the inline direction,
so it still points into the content.
</div>
<!--
iiii tttt
i::::i ttt:::t
iiii t:::::t
t:::::t
iiiiiii mmmmmmm mmmmmmm aaaaaaaaaaaaa ggggggggg ggggg eeeeeeeeeeee ssssssssss eeeeeeeeeeee ttttttt:::::ttttttt
i:::::i mm:::::::m m:::::::mm a::::::::::::a g:::::::::ggg::::g ee::::::::::::ee ss::::::::::s ee::::::::::::ee t:::::::::::::::::t
i::::i m::::::::::mm::::::::::m aaaaaaaaa:::::a g:::::::::::::::::g e::::::eeeee:::::eess:::::::::::::s e::::::eeeee:::::eet:::::::::::::::::t
i::::i m::::::::::::::::::::::m a::::ag::::::ggggg::::::gge::::::e e:::::es::::::ssss:::::se::::::e e:::::etttttt:::::::tttttt
i::::i m:::::mmm::::::mmm:::::m aaaaaaa:::::ag:::::g g:::::g e:::::::eeeee::::::e s:::::s ssssss e:::::::eeeee::::::e t:::::t
i::::i m::::m m::::m m::::m aa::::::::::::ag:::::g g:::::g e:::::::::::::::::e s::::::s e:::::::::::::::::e t:::::t
i::::i m::::m m::::m m::::m a::::aaaa::::::ag:::::g g:::::g e::::::eeeeeeeeeee s::::::s e::::::eeeeeeeeeee t:::::t
i::::i m::::m m::::m m::::ma::::a a:::::ag::::::g g:::::g e:::::::e ssssss s:::::s e:::::::e t:::::t tttttt
i::::::im::::m m::::m m::::ma::::a a:::::ag:::::::ggggg:::::g e::::::::e s:::::ssss::::::se::::::::e t::::::tttt:::::t
i::::::im::::m m::::m m::::ma:::::aaaa::::::a g::::::::::::::::g e::::::::eeeeeeee s::::::::::::::s e::::::::eeeeeeee tt::::::::::::::t
i::::::im::::m m::::m m::::m a::::::::::aa:::a gg::::::::::::::g ee:::::::::::::e s:::::::::::ss ee:::::::::::::e tt:::::::::::tt
iiiiiiiimmmmmm mmmmmm mmmmmm aaaaaaaaaa aaaa gggggggg::::::g eeeeeeeeeeeeee sssssssssss eeeeeeeeeeeeee ttttttttttt
g:::::g
gggggg g:::::g
g:::::gg gg:::::g
g::::::ggg:::::::g
gg:::::::::::::g
ggg::::::ggg
gggggg
-->
<h3 id='image-set-notation'>
Resolution Negotiation: the ''image-set()'' notation</h3>
Delivering the most appropriate image resolution for a user's device can be a difficult task.
Ideally, images should be in the same resolution as the device they're being viewed in,
which can vary between users.
However, other factors can factor into the decision of which image to send;
for example, if the user is on a slow mobile connection,
they may prefer to receive lower-res images
rather than waiting for a large proper-res image to load.
The ''image-set()'' function allows an author to ignore most of these issues,
simply providing multiple resolutions of an image
and letting the UA decide which is most appropriate in a given situation.
<p class='issue'>
This solution assumes that resolution is a proxy for filesize,
and therefore doesn't appropriately handle multi-resolution sets of vector images,
or mixing vector images with raster ones (e.g. for icons).
For example, use a vector for high-res,
pixel-optimized bitmap for low-res,
and same vector again for low-bandwidth (because it's much smaller, even though it's higher resolution).
The syntax for ''image-set()'' is:
<pre class='prod'>
<dfn>image-set()</dfn> = image-set( [ <<image-set-decl>>, ]* [ <<image-set-decl>> | <<color>>] )
<dfn><image-set-decl></dfn> = [ <<image>> | <<string>> ] <<resolution>></pre>
The ''image-set()'' function can not be nested inside of itself,
either directly
or indirectly
(as an argument to another <<image>> type).
<p class='issue'>
WebKit's experimental implementation has an ''x'' unit
which is identical to ''dppx''.
Should we add it?
Each <<string>> or <<url>> inside ''image-set()'' represents an image,
just like in ''image()''.
Every <<image-set-decl>> in a given ''image-set()'' must have a different <<resolution>>,
or else the function is invalid.
The value that a given ''image-set()'' represents is determined by
the UA first ordering the set of <<image-set-decl>>s
in a UA-determined order
(which should be based on the <<resolution>> of each <<image-set-decl>>,
but may include other factors that the UA deems relevant,
such as connection speed).
The function then represents the first such image that is not an <a>invalid image</a>.
If a fallback color was provided,
the UA may decide at any point that the remaining <<image-set-decl>>s are unsuitable,
and choose to use the fallback color instead,
in which case the function represents a solid-color image of the given color with no <a>intrinsic dimensions</a>.
If <em>all</em> of the <<image-set-decl>>s represent <a>invalid images</a>,
then if a fallback color was provided,
the function represents a solid-color image of the given color with no <a>intrinsic dimensions</a>;
otherwise, the function represents an <a>invalid image</a>.
<p class='issue'>
HTML currently doesnt' do fallback if the first-chosen image doesn't load.
We should harmonize with HTML,
either by convincing them to do fallback,
or by removing our own fallback.
If an <<image-set-decl>> is chosen and successfully loaded,
the associated <<resolution>> is the image's <a>intrinsic resolution</a>.
<p class='issue'>
It's been suggested that we don't need a full <<resolution>> here,
and should instead just use the ''x'' unit by itself.
This seems silly to me,
since the ''x'' unit is just an easier synonym for ''dppx'',
but thoughts?
Beyond theoretical purity considerations,
it seems that ''dpi'' can be useful for use-cases involving printing,
where dpi is actually used to talk about images
and having an integer correlation between dots and pixels
is either unnecessary or strictly non-sensical
(as the screening resolution can depend on the colors being used).
<div class='example'>
This example shows how to use ''image-set()'' to provide an image in three versions:
a "normal" version,
a "high-res" version,
and an extra-high resolution version for use in high-quality printing
(as printers can have <em>extremely</em> high resolution):
<pre>
background-image: image-set( "foo.png" 1x,
"foo-2x.png" 2x,
"foo-print.png" 600dpi );
</pre>
</div>
<!--
EEEEEEEEEEEEEEEEEEEEEElllllll tttt
E::::::::::::::::::::El:::::l ttt:::t
E::::::::::::::::::::El:::::l t:::::t
EE::::::EEEEEEEEE::::El:::::l t:::::t
E:::::E EEEEEE l::::l eeeeeeeeeeee mmmmmmm mmmmmmm eeeeeeeeeeee nnnn nnnnnnnn ttttttt:::::ttttttt
E:::::E l::::l ee::::::::::::ee mm:::::::m m:::::::mm ee::::::::::::ee n:::nn::::::::nn t:::::::::::::::::t
E::::::EEEEEEEEEE l::::l e::::::eeeee:::::eem::::::::::mm::::::::::m e::::::eeeee:::::een::::::::::::::nn t:::::::::::::::::t
E:::::::::::::::E l::::l e::::::e e:::::em::::::::::::::::::::::me::::::e e:::::enn:::::::::::::::ntttttt:::::::tttttt
E:::::::::::::::E l::::l e:::::::eeeee::::::em:::::mmm::::::mmm:::::me:::::::eeeee::::::e n:::::nnnn:::::n t:::::t
E::::::EEEEEEEEEE l::::l e:::::::::::::::::e m::::m m::::m m::::me:::::::::::::::::e n::::n n::::n t:::::t
E:::::E l::::l e::::::eeeeeeeeeee m::::m m::::m m::::me::::::eeeeeeeeeee n::::n n::::n t:::::t
E:::::E EEEEEE l::::l e:::::::e m::::m m::::m m::::me:::::::e n::::n n::::n t:::::t tttttt
EE::::::EEEEEEEE:::::El::::::le::::::::e m::::m m::::m m::::me::::::::e n::::n n::::n t::::::tttt:::::t
E::::::::::::::::::::El::::::l e::::::::eeeeeeee m::::m m::::m m::::m e::::::::eeeeeeee n::::n n::::n tt::::::::::::::t
E::::::::::::::::::::El::::::l ee:::::::::::::e m::::m m::::m m::::m ee:::::::::::::e n::::n n::::n tt:::::::::::tt
EEEEEEEEEEEEEEEEEEEEEEllllllll eeeeeeeeeeeeee mmmmmm mmmmmm mmmmmm eeeeeeeeeeeeee nnnnnn nnnnnn ttttttttttt
-->
<h3 id='element-notation'>
Using Elements as Images: the ''element()'' notation</h3>
The ''element()'' function allows an author to use an element in the document as an image.
As the referenced element changes appearance,
the image changes as well.
This can be used, for example,
to create live previews of the next/previous slide in a slideshow,
or to reference a canvas element for a fancy generated gradient or even an animated background.
Note: The ''element()'' function only reproduces the <em>appearance</em> of the referenced element,
not the actual content and its structure.
Authors should only use this for decorative purposes,
and must not use ''element()'' to reproduce an element with significant content across the page.
Instead, just insert multiple copies of the element into the document.
The syntax for ''element()'' is:
<pre class=prod><dfn>element()</dfn> = element( <<id-selector>> )</pre>
where <<id-selector>> is an ID selector [[!SELECT]].
<p class='issue'>
Do we need to be able to refer to elements in external documents
(such as SVG paint servers)?
Or is it enough to just use url() for this?
<p class='issue'>
This name conflicts with a somewhat similar function in GCPM.
This needs to be resolved somehow.
<p class='issue'>
Want the ability to do "reflections" of an element,
either as a background-image on the element or in a pseudo-element.
This needs to be specially-handled to avoid triggering the cycle-detection.
<p class='issue'>
When we have overflow:paged,
how can we address a single page in the view?
The ''element()'' function references the element matched by its argument.
The ID is first looked up in the <a idl>elementSources</a> map,
as described in that section.
If it's not found,
it's then matched against the document.
If multiple elements are matched,
the function references the first such element.
The image represented by the ''element()'' function can vary based on whether the element is visible in the document:
<dl>
<dt>an <a title="element-not-rendered">element that is rendered</a> and is not a descendant of a replaced element
<dd>
The function represents an image with its intrinsic size equal to the <dfn export>decorated bounding box</dfn> of the referenced element:
<ul>
<li>
for an element rendered using a CSS rendering model,
the <a>decorated bounding box</a> is the smallest axis-aligned rectangle
that contains the <a href="http://www.w3.org/TR/2011/CR-css3-background-20110215/#border-image-area">border image areas</a> of all the fragments of the principal box
<li>
for an element rendered using the SVG rendering model,
<a href="http://www.w3.org/TR/SVGTiny12/intro.html#TermDecoratedBoundingBox">the decorated bounding box is defined by SVG</a>
</ul>
Note: Because images clip anything outside their bounds by default,
this means that decorations that extend outside the <a>decorated bounding box</a>,
like box shadows,
may be clipped.
The image is constructed by rendering the referenced element and its descendants
(at the same size that they would be in the document)
over an infinite ''transparent'' canvas,
positioned so that the edges of the <a>decorated bounding box</a> are flush with the edges of the image.
<p class='issue'>
Requiring some degree of stacking context on the element appears to be required for an efficient implementation.
Do we need a full stacking context, or just a pseudo-stacking context?
Should it need to be a stacking context normally,
or can we just render it as a stacking context when rendering it to element()?
If the referenced element has a transform applied to it or an ancestor,
the transform must be ignored when rendering the element as an image. [[!CSS3-TRANSFORMS]]
If the referenced element is broken across pages,
the element is displayed as if the page content areas were joined flush in the pagination direction,
with pages' edges corresponding to the initial containing block's start edge aligned.
<span class='note'>Elements broken across lines or columns are just rendered with their <a>decorated bounding box</a>.</span>
Implementations may either re-use existing bitmap data generated for the referenced element
or regenerate the display of the element to maximize quality at the image's size
(for example, if the implementation detects that the referenced element is an SVG fragment);
in the latter case, the layout of the referenced element in the image must not be changed by the regeneration process.
That is, the image must look identical to the referenced element,
modulo rasterization quality.
<div class='example'>
As a somewhat silly example, a <code><p></code> element can be reused as a background elsewhere in the document:
<pre>
<style>
#src { color: white; background: lime; width: 300px; height: 40px; }
#dst { color: black; background: element(#src); padding: 20px; margin: 20px 0; }
</style>
<p id='src'>I'm an ordinary element!</p>
<p id='dst'>I'm using the previous element as my background!</p>
</pre>
<img src="images/element-function.png" alt="">
</div>
<dt>an <a title='element-not-rendered'>element that is not rendered</a>, but which provides a <a>paint source</a>
<dd>
The function represents an image with the intrinsic size and appearance of the <a>paint source</a>.
The host language defines the size and appearance of paint sources.
<div class='example'>
For example, the ''element()'' function can reference an SVG <code><pattern></code> element in an HTML document:
<pre>
<!DOCTYPE html>
<svg>
<defs>
<pattern id='pattern1'>
<path d='...'>
</pattern>
</defs>
</svg>
<p style="background: element(#pattern1)">
I'm using the pattern as a background!
If the pattern is changed or animated,
my background will be updated too!
</p>
</pre>
HTML also defines that a handful of elements,
such as <code><canvas></code>, <code><img></code>, and <code><video></code>,
provide a paint source.
This means that CSS can, for example,
reference a canvas that's being drawn into,
but not displayed in the page:
<pre>
<!DOCTYPE html>
<script>
var canvas = document.querySelector('#animated-bullet');
canvas.width = 20; canvas.height = 20;
drawAnimation(canvas);
</script>
<canvas id='animated-bullet' style='display:none'></canvas>
<ul style="list-style-image: element(#animated-bullet);">
<li>I'm using the canvas as a bullet!</li>
<li>So am I!</li>
<li>As the canvas is changed over time with Javascript,
we'll all update our bullet image with it!</li>
</ul>
</pre>
</div>
<dt>anything else
<dd>
The function represents an <a>invalid image</a>.
<div class='example'>
For example, all of the following ''element()'' uses will result in a transparent background:
<pre>
<!DOCTYPE html>
<p id='one' style="display:none;">one</p>
<iframe src="http://example.com">
<p id='two'>I'm fallback content!</p>
</iframe>
<ul>
<li style="background: element(#one);">
A display:none element isn't rendered, and a P element
doesn't provide a paint source.
</li>
<li style="background: element(#two);">
The descendants of a replaced element like an IFRAME
can't be used in element() either.
</li>
<li style="background: element(#three);">
There's no element with an id of "three", so this also
gets rendered as a transparent image.
</li>
</ul>
</pre>
</div>
</dl>
An element is <dfn export id='element-not-rendered' title='element-not-rendered'>not rendered</dfn> if it does not have an associated box.
This can happen, for example,
if the element or an ancestor is ''display:none''.
Host languages may define additional ways in which an element can be considered not rendered;
for example, in SVG,
any descendant of a <code><defs></code> element is considered to be not rendered.
<div class='example'>
The ''element()'' function can be put to many uses.
For example, it can be used to show a preview of the previous or next slide in a slideshow:
<pre>
<!DOCTYPE html>
<script>
function navigateSlides() {
var currentSlide = ...;
document.querySelector('#prev-slide').id = '';
document.querySelector('#next-slide').id = '';
currentSlide.previousElementSibling.id = 'prev-slide';
currentSlide.nextElementSibling.id = 'next-slide';
}
</script>
<style>
#prev-preview, #next-preview {
position: fixed;
...
}
#prev-preview { background: element(#prev-slide); }
#next-preview { background: element(#next-slide); }
</style>
<a id='prev-preview'>Previous Slide</a>
<a id='next-preview'>Next Slide</a>
<section class='slide'>...</section>
<section class='slide current-slide'>...</section>
...
</pre>
In this example, the <code>navigateSlides</code> function updates the ids of the next and previous slides,
which are then displayed in small floating boxes alongside the slides.
Since you can't interact with the slides through the ''element()'' function (it's just an image),
you could even use <code>click</code> handlers on the preview boxes to help navigate through the page.
</div>
<h4 id='paint-sources'>
Paint Sources</h4>
Host languages may define that some elements provide a <dfn export>paint source</dfn>.
Paint sources have an intrinsic width, height, and appearance,
separate from the process of rendering,
and so may be used as images even when they're <a title='element-not-rendered'>not rendered</a>.
In HTML, the <code><img></code>, <code><video></code>, and <code><canvas></code> elements provide paint sources
(defined in each element's section in <a href='http://www.whatwg.org/specs/web-apps/current-work/multipage/'>HTML5</a>).
In SVG, any element that provides a <a href='http://www.w3.org/TR/SVG/pservers.html'>paint server</a> provides a paint source.
<span class='note'>Note: In SVG1.1,
the <code><linearGradient></code>,
<code><radialGradient></code>,
and <code><pattern></code> elements
provide paint sources.</span>
They are drawn as described in the spec,
with the coordinate systems defined as follows:
<dl>
<dt>objectBoundingBox
<dd>
The coordinate system has its origin at the top left corner of the rectangle defined by the <a>concrete object
2334
size</a> that it's being drawn into,
and the same width and height as the <a>concrete object size</a>.
A single <a href="http://www.w3.org/TR/SVG/coords.html#Units">user coordinate</a> is the width and height of the <a>concrete object size</a>.
<dt>userSpaceOnUse
<dd>
The coordinate system has its origin at the top left corner of the rectangle defined by the <a>concrete object size</a> that it's being drawn into,
and the same width and height as the <a>concrete object size</a>.
<a href="http://www.w3.org/TR/SVG/coords.html#Units">User coordinates</a> are sized equivalently to the CSS ''px'' unit.
</dl>
Note: It is expected that a future version of this module will define ways to refer to paint sources in external documents,
or ones that are created solely by script and never inserted into a document at all.
<h4 id='elementmap'>
The <code>ElementMap</code> interface</h4>
The ''element()'' function normally selects elements within a document,
but elements that provide a <a>paint source</a> don't necessarily need to be in-document.
For example, an HTML <code><canvas></code> element can be created, maintained, and drawn into entirely in script,
with no need for it to be inserted into the document directly.
All that's needed is a way to refer to the element,
as an ID selector cannot select elements outside of the document.
The <a idl>elementSources</a> Map object provides this.
<pre class='idl'>
partial interface CSS {
[SameObject] readonly attribute Map elementSources;
};
</pre>
Any entries in the <a idl>elementSources</a> map with a string k
E88C
ey
and a value that is an object providing a <a>paint source</a>
are made available to the ''element()'' function.
Whenever ''element()'' uses an <<id-selector>>,
the ID's value (without the leading <code>#</code> character)
is first looked up in the <a idl>elementSources</a> map:
<ul>
<li>
If it's found,
and the object associated with it provides a <a>paint source</a>,
the ''element()'' function represents that paint source.
<li>
If it's found,
but the object associated with it <em>doesn't</em> provide a <a>paint source</a>,
the ''element()'' function represent an <a>invalid image</a>.
<li>
If the ID isn't found in the map at all,
it's then looked for in the document as normal.
</ul>
<div class='example'>
For example, fancy animating backgrounds can be done with an external canvas:
<pre>
<script>
var bg = document.createElement('canvas');
bg.height = 200;
bg.width = 1000;
drawFancyBackground(bg);
CSS.elementSources.set('fancy', bg);
</script>
<style>
h1 {
background-image: element(#fancy);
}
</style>
</pre>
As the "fancy" canvas is drawn into and animated,
the backgrounds of all the H1 elements will automatically update in tandem.
</div>
<h4 id='element-cycles'>
Cycle Detection</h4>
The ''element()'' function can produce nonsensical circular relationships,
such as an element using itself as its own background.
These relationships can be easily and reliably detected and resolved, however,
by keeping track of a dependency graph and using common cycle-detection algorithms.
The dependency graph consists of edges such that:
<ul>
<li>
every element depends on its children
<li>
for any element A with a property using the ''element()'' function pointing to an element B,
A depends on B
<li>
if a host language defines a way for elements to refer to the rendering of other elements,
the referencing element depends on the referenced element.
For example, in SVG,
a <code><use></code> element depends on the element it referenced.
</ul>
If the graph contains a cycle,
any ''element()'' functions participating in the cycle are <a>invalid images</a>.
<!--
dddddddd
CCCCCCCCCCCCC ffffffffffffffff d::::::d
CCC::::::::::::C f::::::::::::::::f d::::::d
CC:::::::::::::::C f::::::::::::::::::f d::::::d
C:::::CCCCCCCC::::C f::::::fffffff:::::f d:::::d
C:::::C CCCCCCrrrrr rrrrrrrrr ooooooooooo ssssssssss ssssssssss f:::::f ffffffaaaaaaaaaaaaa ddddddddd:::::d eeeeeeeeeeee
C:::::C r::::rrr:::::::::r oo:::::::::::oo ss::::::::::s ss::::::::::s f:::::f a::::::::::::a dd::::::::::::::d ee::::::::::::ee
C:::::C r:::::::::::::::::r o:::::::::::::::oss:::::::::::::s ss:::::::::::::s f:::::::ffffff aaaaaaaaa:::::a d::::::::::::::::d e::::::eeeee:::::ee
C:::::C rr::::::rrrrr::::::ro:::::ooooo:::::os::::::ssss:::::ss::::::ssss:::::sf::::::::::::f a::::ad:::::::ddddd:::::d e::::::e e:::::e
C:::::C r:::::r r:::::ro::::o o::::o s:::::s ssssss s:::::s ssssss f::::::::::::f aaaaaaa:::::ad::::::d d:::::d e:::::::eeeee::::::e
C:::::C r:::::r rrrrrrro::::o o::::o s::::::s s::::::s f:::::::ffffff aa::::::::::::ad:::::d d:::::d e:::::::::::::::::e
C:::::C r:::::r o::::o o::::o s::::::s s::::::s f:::::f a::::aaaa::::::ad:::::d d:::::d e::::::eeeeeeeeeee
C:::::C CCCCCC r:::::r o::::o o::::ossssss s:::::s ssssss s:::::s f:::::f a::::a a:::::ad:::::d d:::::d e:::::::e
C:::::CCCCCCCC::::C r:::::r o:::::ooooo:::::os:::::ssss::::::ss:::::ssss::::::sf:::::::f a::::a a:::::ad::::::ddddd::::::dde::::::::e
CC:::::::::::::::C r:::::r o:::::::::::::::os::::::::::::::s s::::::::::::::s f:::::::f a:::::aaaa::::::a d:::::::::::::::::d e::::::::eeeeeeee
CCC::::::::::::C r:::::r oo:::::::::::oo s:::::::::::ss s:::::::::::ss f:::::::f a::::::::::aa:::a d:::::::::ddd::::d ee:::::::::::::e
CCCCCCCCCCCCC rrrrrrr ooooooooooo sssssssssss sssssssssss fffffffff aaaaaaaaaa aaaa ddddddddd ddddd eeeeeeeeeeeeee
-->
<h3 id='cross-fade-function'>
Combining images: the ''cross-fade()'' notation</h3>
When transitioning between images,
CSS requires a way to explicitly refer to the intermediate image
that is a combination of the start and end images.
This is accomplished with the ''cross-fade()'' function,
which indicates the two images to be combined
and how far along in the transition the combination is.
Note: Authors can also use the ''cross-fade()'' function for many simple image manipulations,
such as tinting an image with a solid color
or highlighting a particular area of the page by combining an image with a radial gradient.
The syntax for ''cross-fade()'' is defined as:
<pre class=prod><dfn>cross-fade()</dfn> = cross-fade( <<percentage>>? <<image>> [, <<image>> | <<color>> ]? )</pre>
The function represents an image generated by
combining two images.
The <<percentage>> represents how much of the first image is retained
when it is blended with the second image.
The <<percentage>> must be between ''0%'' and ''100%'' inclusive;
any other value is invalid.
If omitted,
it defaults to the value ''50%''.
If the last argument is a <<color>>,
it represents a solid-color image with the same intrinsic dimensions as the first image.
If omitted,
it defaults to the color ''transparent''.
More precisely,
given ''cross-fade(<var>p</var> <var>A</var>, <var>B</var>)'',
where <var>A</var> and <var>B</var> are images
and <var>p</var> is a percentage between 0% and 100%,
the function represents an image
with width equal to <code>width<sub>A</sub> × <var>p</var> + width<sub>B</sub> × (1-<var>p</var>)</code>
and height equal to <code>height<sub>A</sub> × <var>p</var> + height<sub>B</sub> × (1-<var>p</var>)</code>.
The contents of the image must be constructed by
first scaling <var>A</var> and <var>B</var> to the size of the generated image,
then applying <code>dissolve(<var>A</var>,<var>p</var>) plus dissolve(<var>B</var>,1-<var>p</var>)</code>.
The "dissolve()" function and "plus" compositing operator are defined in the literature by Porter-Duff. [[PORTERDUFF]]
<!--
dddddddd
GGGGGGGGGGGGG d::::::d iiii tttt
GGG::::::::::::G d::::::d i::::i ttt:::t
GG:::::::::::::::G d::::::d iiii t:::::t
G:::::GGGGGGGG::::G d:::::d t:::::t
G:::::G GGGGGGrrrrr rrrrrrrrr aaaaaaaaaaaaa ddddddddd:::::d iiiiiii eeeeeeeeeeee nnnn nnnnnnnn ttttttt:::::ttttttt ssssssssss
G:::::G r::::rrr:::::::::r a::::::::::::a dd::::::::::::::d i:::::i ee::::::::::::ee n:::nn::::::::nn t:::::::::::::::::t ss::::::::::s
G:::::G r:::::::::::::::::r aaaaaaaaa:::::a d::::::::::::::::d i::::i e::::::eeeee:::::een::::::::::::::nn t:::::::::::::::::t ss:::::::::::::s
G:::::G GGGGGGGGGGrr::::::rrrrr::::::r a::::ad:::::::ddddd:::::d i::::i e::::::e e:::::enn:::::::::::::::ntttttt:::::::tttttt s::::::ssss:::::s
G:::::G G::::::::G r:::::r r:::::r aaaaaaa:::::ad::::::d d:::::d i::::i e:::::::eeeee::::::e n:::::nnnn:::::n t:::::t s:::::s ssssss
G:::::G GGGGG::::G r:::::r rrrrrrraa::::::::::::ad:::::d d:::::d i::::i e:::::::::::::::::e n::::n n::::n t:::::t s::::::s
G:::::G G::::G r:::::r a::::aaaa::::::ad:::::d d:::::d i::::i e::::::eeeeeeeeeee n::::n n::::n t:::::t s::::::s
G:::::G G::::G r:::::r a::::a a:::::ad:::::d d:::::d i::::i e:::::::e n::::n n::::n t:::::t ttttttssssss s:::::s
G:::::GGGGGGGG::::G r:::::r a::::a a:::::ad::::::ddddd::::::ddi::::::ie::::::::e n::::n n::::n t::::::tttt:::::ts:::::ssss::::::s
GG:::::::::::::::G r:::::r a:::::aaaa::::::a d:::::::::::::::::di::::::i e::::::::eeeeeeee n::::n n::::n tt::::::::::::::ts::::::::::::::s
GGG::::::GGG:::G r:::::r a::::::::::aa:::a d:::::::::ddd::::di::::::i ee:::::::::::::e n::::n n::::n tt:::::::::::tt s:::::::::::ss
GGGGGG GGGG rrrrrrr aaaaaaaaaa aaaa ddddddddd dddddiiiiiiii eeeeeeeeeeeeee nnnnnn nnnnnn ttttttttttt sssssssssss
-->
<h2 id="gradients">
Gradients</h2>
A gradient is an image that smoothly fades from one color to another.
These are commonly used for subtle shading in background images, buttons, and many other things.
The gradient notations described in this section allow an author to specify such an image in a terse syntax,
so that the UA can generate the image automatically when rendering the page.
The syntax of a <<gradient>> is:
<pre class=prod>
<dfn><gradient></dfn> = [
<<linear-gradient()>> | <<repeating-linear-gradient()>> |
<<radial-gradient()>> | <<repeating-radial-gradient()>> |
<<conic-gradient()>> | <<repeating-conic-gradient()>> ]
</pre>
<div class=example>
As with the other <<image>> types defined in this specification,