Filter Effects Module Level 2

Editor’s Draft,

This version:
https://drafts.fxtf.org/filters-2/
Latest version:
https://www.w3.org/TR/filter-effects-2/
Previous Versions:
https://www.w3.org/TR/2014/WD-filter-effects-1-20141125/
https://www.w3.org/TR/2013/WD-filter-effects-1-20131126/
https://www.w3.org/TR/2013/WD-filter-effects-20130523/
https://www.w3.org/TR/2012/WD-filter-effects-20121025/
Feedback:
public-fx@w3.org with subject line “[filter-effects-2] … message topic …” (archives)
Test Suite:
http://test.csswg.org/suites/filter-effects/nightly-unstable/
Editors:
(Adobe Systems Inc)
(Apple Inc)
Issues List:
in Bugzilla

Abstract

Filter effects are a way of processing an element’s rendering before it is displayed in the document. Typically, rendering an element via CSS or SVG can conceptually described as if the element, including its children, are drawn into a buffer (such as a raster image) and then that buffer is composited into the elements parent. Filters apply an effect before the compositing stage. Examples of such effects are blurring, changing color intensity and warping the image.

This is Level 2 of the Filter Effects Module.

Status of this document

This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

The (archived) public mailing list public-fx@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “filter-effects-2” in the subject, preferably like this: “[filter-effects-2] …summary of comment…

This document was produced by the CSS Working Group (part of the Style Activity) and the SVG Working Group (part of the Graphics Activity).

This document was produced by groups operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures (CSS) and a public list of any patent disclosures (SVG) made in connection with the deliverables of each group; these pages also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

1. Introduction

This section is not normative

A filter effect is a graphical operation that is applied to an element as it is drawn into the document. It is an image-based effect, in that it takes zero or more images as input, a number of parameters specific to the effect, and then produces an image as output. The output image is either rendered into the document instead of the original element, used as an input image to another filter effect, or provided as a CSS image value.

This is Level 2 of the Filter Effects Module. It is currently written as a "delta", describing any differences from Level 1.

2. Backdrop filters: the backdrop-filter property

The description of the backdrop-filter property is as follows:

Name: backdrop-filter
Value: none | <filter-function-list>
Initial: none
Applies to: All elements. In SVG, it applies to container elements without the defs element and all graphics elements
Inherited: no
Percentages: n/a
Media: visual
Computed value: as specified
Animatable: yes

If the value of the backdrop-filter property is none then there is no filter effect applied. Otherwise, the list of functions are applied in the order provided.

The first filter function or filter reference in the list takes the element’s BackgroundImage as the input image. Subsequent operations take the output from the previous filter function or filter reference as the input image. filter element reference functions can specify an alternate input, but still uses the previous output as its SourceGraphic.

Filter functions must operate in the sRGB color space.

A computed value of other than none results in the creation of a stacking context [CSS21] the same way that CSS opacity does. All the elements descendants are rendered together as a group with the filter effect applied to the group as a whole.

The result of the backdrop-filter is rendered before the other painting operations for the element. That is, behind any background of the element.

If the filter functions would have produced a result that extended beyond the bounds of the input, it is clipped to the original bounds.

The use of this property may have an adverse effect on on performance, especially when applied to a number of elements, or a large area of the page. Authors should be careful to use it on a minimal amount of content.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS21]
Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. REC. URL: http://www.w3.org/TR/CSS2
[SVG2]
Nikos Andronikos; et al. Scalable Vector Graphics (SVG) 2. 15 September 2015. WD. URL: http://www.w3.org/TR/SVG2/
[CSS-COLOR-4]
CSS Color Module Level 4 URL: https://drafts.csswg.org/css-color-4/
[CSS-VALUES]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 3. 11 June 2015. CR. URL: http://www.w3.org/TR/css-values/
[FILTERS-1]
Filter Effects Level 1 URL: https://www.w3.org/TR/filter-effects-1/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Property Index

Name Value Initial Applies to Inh. %ages Media Ani­mat­able Com­puted value
backdrop-filter none | <filter-function-list> none All elements. In SVG, it applies to container elements without the defs element and all graphics elements no n/a visual yes as specified