Compositing and Blending Level 1

Abstract

Compositing describes how shapes of different elements are combined into a single image. There are various possible approaches for compositing. Previous versions of SVG and CSS used Simple Alpha Compositing. In this model, each element is rendered into its own buffer and is then merged with its backdrop using the Porter Duff source-over operator. This specification will define a new compositing model that expands upon the Simple Alpha Compositing model by offering:

• additional Porter Duff compositing operators
• advanced blending modes which allow control of how colors mix in the areas where shapes overlap
• compositing groups

In addition, this specification will define CSS properties for blending and group isolation and the properties of the ‘globalcompositeoperation’ attribute as defined in [2DCONTEXT2].

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

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 “compositing-1” in the subject, preferably like this: “[compositing-1] …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 include 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.

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The following features are at risk:

1. Force ‘isolation’ if an SVG images is linked using the ‘img’ element.

The Candidate Recommendation period will end around 17 August 2014. There is currently no preliminary implementation report. During this period, the WG will complete its test suite and create an implementation report.

Table of Contents

• 1. Introduction
• 2. Reading This Document
  • 2.1. Module interactions
  • 2.2. Values
• 3. Specifying Blending in CSS
  • 3.1. Order of graphical operations
  • 3.2. Behavior specific to HTML
  • 3.3. Behavior specific to SVG
  • 3.4. CSS Properties
    • 3.4.1. The ‘mix-blend-mode’ property
    • 3.4.2. The ‘isolation’ property
    • 3.4.3. The ‘background-blend-mode’ property
• 4. Specifying Compositing and Blending in Canvas 2D
• 5. Introduction to compositing
  • 5.1. Simple alpha compositing
    • 5.1.1. Examples of simple alpha compositing
• 6. General Formula for Compositing and Blending
• 7. Backdrop calculation
  • 7.1. Examples of backdrop calculation
• 8. Compositing Groups
  • 8.1. Group invariance
  • 8.2. Isolated Groups
  • 8.3. The Page Group
• 9. Advanced compositing features
  • 9.1. The Porter Duff Compositing Operators
    • 9.1.1. Clear
    • 9.1.2. Copy
    • 9.1.3. Destination
    • 9.1.4. Source Over
    • 9.1.5. Destination Over
    • 9.1.6. Source In
    • 9.1.7. Destination In
    • 9.1.8. Source Out
    • 9.1.9. Destination Out
    • 9.1.10. Source Atop
    • 9.1.11. Destination Atop
    • 9.1.12. XOR
    • 9.1.13. Lighter
  • 9.2. Group compositing behavior with Porter Duff modes
• 10. Blending
  • 10.1. Separable blend modes
    • 10.1.1. ‘normal’ blend mode
    • 10.1.2. ‘multiply’ blend mode
    • 10.1.3. ‘screen’ blend mode
    • 10.1.4. ‘overlay’ blend mode
    • 10.1.5. ‘darken’ blend mode
    • 10.1.6. ‘lighten’ blend mode
    • 10.1.7. ‘color-dodge’ blend mode
    • 10.1.8. ‘color-burn’ blend mode
    • 10.1.9. ‘hard-light’ blend mode
    • 10.1.10. ‘soft-light’ blend mode
    • 10.1.11. ‘difference’ blend mode
    • 10.1.12. ‘exclusion’ blend mode
  • 10.2. Non-separable blend modes
    • 10.2.1. ‘hue’ blend mode
    • 10.2.2. ‘saturation’ blend mode
    • 10.2.3. ‘color’ blend mode
    • 10.2.4. ‘luminosity’ blend mode
  • 10.3. Effect of group isolation on blending
• 11. Security issues with compositing and blending
• 12. Conformance
• 13. References
  • 13.1. Normative References
  • 13.2. Informative References
• 14. Changes

1. Introduction

This subsection is non-normative.
The first part of this document describes the properties used to control the compositing in CSS. The second part will describe the algorithms of Porter Duff compositing and blending.

2. Reading This Document

Each section of this document is normative unless otherwise specified.

2.1. Module interactions

This specification defines a set of CSS properties that affect the visual rendering of elements to which those properties are applied; these effects are applied after elements have been sized and positioned according to the Visual formatting model from [CSS21]. Some values of these properties result in the creation of a containing block, and/or the creation of a stacking context.

The ‘background-blend-mode’ property also builds upon the properties defined in the CSS Backgrounds and Borders module.[CSS3BG]

This specification also enhances the rules as specified in Section 14.2 Simple alpha compositing of [SVG11] and simple alpha compositing of [CSS3COLOR].

This module also extends the ‘globalcompositeoperation’ as defined in [2DCONTEXT2].

2.2. Values

This specification follows the CSS property definition conventions 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 inherit keyword as their property value. For readability it has not been repeated explicitly.

3. Specifying Blending in CSS

3.1. Order of graphical operations

The compositing model must follow the SVG compositing model [SVG11]: first any filter effect is applied, then any clipping, masking, blending and compositing.

3.2. Behavior specific to HTML

Everything in CSS that creates a stacking context must be considered an ‘isolated’ group. HTML elements themselves should not create groups.

An element that has blending applied, must blend with all the underlying content of the stacking context [CSS21] that that element belongs to.

3.3. Behavior specific to SVG

By default, every element must create a non-isolated group.

However, certain operations in SVG will create isolated groups. If one of the following features is used, the group must become isolated:

• opacity
• filters
• 3D transforms (2D transforms must NOT cause isolation)
• blending
• masking

3.4. CSS Properties

3.4.1. The ‘mix-blend-mode’ property

The blend mode defines the formula that must be used to mix the colors with the backdrop. This behavior is described in more detail in Blending.

The syntax of the property of <blend-mode> is given with:

<blend-mode> = normal | multiply | screen | overlay | darken | lighten | color-dodge |
  color-burn | hard-light | soft-light | difference | exclusion | hue | 
  saturation | color | luminosity

Applying a blendmode other than ‘normal’ to the element must establish a new stacking context [CSS21]. This group must then be blended and composited with the stacking context that contains the element.

Given the following sample markup:

<body>
   <img src="ducky.png"/>
</body>"

And the following style rule:

body { background-color: green; }

... will produce the following result:

Partially transparent image on a green backdrop

If we change the style rule to include blending:

body { background-color: green; }
img { mix-blend-mode: multiply; }

... the output will be the image blending with the green background of the <body> element.

Blending of a transparent image on a green backdrop.

Given the following svg code:

<svg>
  <circle cx="40" cy="40" r="40" fill="red"/>
  <circle cx="80" cy="40" r="40" fill="green"/>
  <circle cx="60" cy="80" r="40" fill="blue"/>
</svg>

And the following style rule:

circle { mix-blend-mode: screen; }

... the output will be blending of the 3 circles. Each circle is rendered from bottom to top. Where the elements overlap, the blend mode produces a change in color.

Example of 3 circles blending with a screen blend mode

In the following style sheet and document fragment:

body { background-color: green; }
div { background-color: red; width: 200px; opacity: .95}
img { mix-blend-mode: difference; }

<body>
  <div>
     <img src="ducky.png"/>
  </div>
</body>

... the ‘opacity’ on the <div> element is causing the creation of a stacking context. This causes the creation of a new group so the image doesn't blend with the color of the <body>.

Example of blending within a stacking context.

Note how the image is not blending with the green color.

Given the following sample markup:

<body>
   <div>
     <p>'overlay' blending on text</p>
   </div>
</body>

And the following style rule:

div { background-image: url('texture.png'); }
@font-face {
  font-family: "Mythos Std";
  src: url("http://myfontvendor.com/mythos.otf");
}
p {
  mix-blend-mode: overlay;
  font-family: "Mythos Std" 
}

Text with a blend overlay on top of an image.

3.4.2. The ‘isolation’ property

In SVG, this defines whether an element is isolated or not.
For CSS, setting ‘isolation’ to ‘isolate’ will turn the element into a stacking context.

By default, elements use the ‘auto’ keyword which implies that they are not isolated. However operations that cause the creation of stacking context [CSS21] must cause a group to be isolated. These operations are described in ‘behavior specific to HTML’ and ‘behavior specific to SVG’.

The syntax of the property of <isolation-mode> is given with:

<isolation-mode> = auto | isolate

In CSS, a background image or the content of an <img> must always be rendered into an isolated group.
For instance, if you link to an SVG file through the ‘img’ tag, the artwork of that SVG will not blend with the backdrop of the content.

In SVG, ‘mask’ always creates an isolated group.

3.4.3. The ‘background-blend-mode’ property

Defines the blending mode of each background layer.

Each background layer must blend with the element's background layer that is below it and the element's background color. Background layers must not blend with the content that is behind the element, instead they must act as if they are rendered into an isolated group.

The description of the ‘background-blend-mode’ property is as follows:

The ‘background-blend-mode’ list must be applied in the same order as ‘background-image’[CSS3BG]. This means that the first element in the list will apply to the layer that is on top. If a property doesn't have enough comma-separated values to match the number of layers, the UA must calculate its used value by repeating the list of values until there are enough.

If the ‘background’ [CSS3BG] shorthand is used, the ‘background-blend-mode’ property for that element must be reset to its initial value.

Given the following sample markup:

<body>
   <div></div>
</body>

And the following style rule:

body { background-color: green; }
div { 
  width: 200px;
  height: 200px;
  background-size: 200px 200px;
  background-repeat:no-repeat;
  background-image: linear-gradient(to right, #000000 0%,#ffffff 100%), url('ducky.png');
  background-blend-mode: difference, normal;
}

Blending of 2 background images.

Note that the gradient is not blending with the color of <body>. Instead it retains its original color.

4. Specifying Compositing and Blending in Canvas 2D

The canvas 2d context defines the globalCompositeOperation attribute that is used to set the current compositing and blending operator.

This property takes the following value:

The syntax of the property of <composite-mode> is given with:

<composite-mode> = clear | copy | destination | source-over | destination-over | source-in | 
destination-in | source-out | destination-out | source-atop | destination-atop | xor | lighter

5. Introduction to compositing

Compositing is the combining of a graphic element with its backdrop.

In the model described in this specification there are two steps to the overall compositing operation - Porter-Duff compositing and blending. The blending step determines how the colors from the graphic element and the backdrop interact.

Typically, the blending step is performed first, followed by the Porter-Duff compositing step. In the blending step, the resultant color from the mix of the element and the the backdrop is calculated. The graphic element's color is replaced with this resultant color. The graphic element is then composited with the backdrop using the specified compositing operator.

Shape is defined by the mathematical description of the shape. A particular point is either inside the shape or it is not. There are no gradations.

Opacity is described using an alpha value, stored alongside the color value for each particular point. The alpha value is between 0 and 1, inclusive. A value of 0 means that the pixel has no coverage at that point, and is therefore transparent; i.e. there is no color contribution from any geometry because the geometry does not overlap this pixel. A value of 1 means that the pixel is fully opaque; the geometry completely overlaps the pixel.

5.1. Simple alpha compositing

The formula for simple alpha compositing is

co = Cs x αs + Cb x αb x (1 - αs)

Where

• co: the premultiplied pixel value after compositing
• Cs: the color value of the source graphic element being composited
• αs: the alpha value of the source graphic element being composited
• Cb: the color value of the backdrop
• αb: the alpha value of the backdrop

All values are between 0 and 1 inclusive.

The pixel value after compositing (co) is given by adding the contributions from the source graphic element [Cs x αs] and the backdrop [Cb x αb x (1 - αs)]. For both the graphic element and the backdrop, the color values are multiplied by the alpha to determine the amount of color that contributes. With zero alpha meaning that the color does not contribute and partial alpha means that some percentage of the color contributes. The contribution of the backdrop is further reduced based on the opacity of the graphic element. Conceptually, (1 - αs) of the backdrop shows through the graphic element, meaning that if the graphic element is fully opaque (αs=1) then no backdrop shows through.

The simple alpha compositing formula listed above gives a resultant color which is the result of the weighted average of the backdrop color and graphic element color, with the weighting determined by the backdrop and graphic element alphas. The resultant alpha value of the composite is simply the sum of the contributed alpha of the composited elements. The formula for the resultant alpha of the composite is

αo = αs + αb x (1 - αs)

Where

• αo: the alpha value of the composite
• αs: the alpha value of the graphic element being composited
• αb: the alpha value of the backdrop

cs = Cs x αs

co = cs + cb x (1 - αs)

Co = co / αo

5.1.1. Examples of simple alpha compositing

This describes the most basic case. It consists of 1 shape that is filled with a solid color (α = 1). The shape is composited with an empty background. The empty background has no effect on the resultant composite.

Cs = RGB(1,0,0)
αs = 1
Cb = RGB(0,0,0)
αb = 0

co = Cs x αs + Cb x αb x (1 - αs)
co = RGB(1,0,0) x 1 + RGB(0,0,0) x 0 x (1 - 1)
co = RGB(1,0,0) x 1
co = RGB(1,0,0)

This is a more complex example. There is no transparency, but the 2 shapes intersect.

Applying the compositing formula in the area of intersection, gives:

Cs = RGB(0,0,1)
αs = 1
Cb = RGB(1,0,0)
αb = 1

co = Cs x αs + Cb x αb x (1 - αs)
co = RGB(0,0,1) x 1 + RGB(1,0,0) x 1 x (1 - 1)
co = RGB(0,0,1) x 1 + RGB(1,0,0) x 1 x 0
co = RGB(0,0,1) x 1
co = RGB(0,0,1)

Calculating the alpha of the resultant composite

αo = αs + αb x (1 - αs)
αo = 1 + 1 x (1 - 1)
αo = 1

Calculating the color component of the resultant composite

Co = co / αo
Co = RGB(0, 0, 1) / 1
Co = RGB(0, 0, 1)

This is an example where the shape has some transparency, but the backdrop is fully opaque.

Applying the compositing formula in the area of intersection, gives:

Cs = RGB(0,0,1)
αs = 0.5
Cb = RGB(1,0,0)
αb = 1

co = Cs x αs + Cb x αb x (1 - αs)
co = RGB(0,0,1) x 0.5 + RGB(1,0,0) x 1 x (1 - 0.5)
co = RGB(0,0,1) x 0.5 + RGB(1,0,0) x 0.5
co = RGB(0.5,0,0.5)

Calculating the alpha of the resultant composite

αo = αs + αb x (1 - αs)
αo = 0.5 + 1 x (1 - 0.5)
αo = 1

Calculating the color component of the resultant composite

Co = co / αo
Co = RGB(0.5, 0, 0.5) / 1
Co = RGB(0.5, 0, 0.5)

Figure 4 shows an example where both the shape and the backdrop are transparent.

Applying the compositing formula in the area of intersection, gives:

Cs = RGB(0,0,1)
αs = 0.5
Cb = RGB(1,0,0)
αb = 0.5

co = Cs x αs + Cb x αb x (1 - αs)
co = RGB(0,0,1) x 0.5 + RGB(1,0,0) x 0.5 x (1 - 0.5)
co = RGB(0,0,1) x 0.5 + RGB(1,0,0) x 0.25
co = RGB(0.25, 0, 0.5)

Calculating the alpha of the resultant composite

αo = αs + αb x (1 - αs)
αo = 0.5 + 0.5 x (1 - 0.5)
αo = 0.75

Calculating the color component of the resultant composite

Co = co / αo
Co = RGB(0.25, 0, 0.5) / 0.75
Co = RGB(0.33, 0, 0.66)

6. General Formula for Compositing and Blending

The general formula for compositing and blending which allows for selection of the compositing operator and blending function comprises two steps. The terms used in these functions will be described in detail in the following sections.

Apply the blend in place

Cs = (1 - αb) x Cs + αb x B(Cb, Cs)

Composite

Co = αs x Fa x Cs + αb x Fb x Cb

Where:

• Cs: is the source color
• Cb: is the backdrop color
• αs: is the source alpha
• αb: is the backdrop alpha
• B(Cb, Cs): is the mixing function
• Fa: is defined by the Porter Duff operator in use
• Fb: is defined by the Porter Duff operator in use

7. Backdrop calculation

The backdrop is the content behind the element and is what the element is composited with. This means that the backdrop is the result of compositing all previous elements.

7.1. Examples of backdrop calculation

This example has 2 simple shapes. The backdrop for the blue shape includes the bottom right corner of the red shape . The dotted line shows the area that is examined during compositing of the blue shape.

The shape in the backdrop has an alpha value. The alpha value of the backdrop shape is preserved when the backdrop is calculated.

8. Compositing Groups

Compositing groups allow more control over the interaction of compositing with the backdrop. Groups can be used to specify how a compositing effect within a group will interact with the content that is already in the scene (the backdrop).

Compositing groups may be made up of any number of elements, and may contain other compositing groups.

The default properties of a compositing group shall cause no visual difference compared to having no group. See Group Invariance.

A compositing group is rendered by first compositing the elements of the group onto the initial backdrop. The result of this is a single element containing color and alpha information. This element is then composited onto the group backdrop. Steps shall be taken to ensure the group backdrop makes only a single contribution to the final composite.

initial backdrop

The initial backdrop is the backdrop used for compositing the group's first element. This will be the same as the group backdrop in a non-isolated group, or a fully transparent backdrop for an isolated group.

group backdrop

The group backdrop is the result of compositing all elements up to but not including the first element in the group.

8.1. Group invariance

An important property of simple alpha compositing is its group invariance. This behavior is preserved in the more complex model described in this specification. Adding or removing grouping with default attributes shall not show visual differences.

so: A + B + C = A + (B + C) = (A + B) + C

When adding attributes to the group such as ‘isolate’, blending modes other than ‘normal’ or Porter Duff compositing operators other than ‘source-over’, groups may no longer be invariant.

8.2. Isolated Groups

In an isolated group, the initial backdrop shall be black and fully transparent.

In this instance, the initial backdrop is different than the group backdrop. The only interaction with the group backdrop shall occur when the group's computed color, shape and alpha are composited with it.

See ‘Isolated groups and Porter Duff modes’ for a description of the effect of isolated groups on compositing. See ‘Effect of group isolation on blending’ for a description of the effect of isolated groups on blending.

8.3. The Page Group

The top level group is the page group. All other elements and groups are composited into this group. The page group is an isolated group.

The page group is composited with a backdrop color of white with 100% opacity.

9. Advanced compositing features

Simple alpha compositing uses the source-over Porter Duff compositing operator.

Porter Duff compositing is based on a model of a pixel in which two shapes (source and destination) may contribute to the final color of the pixel. The pixel is divided into 4 sub-pixel regions and each region represents a possible combination of source and destination. [PORTERDUFF]

The four regions are:

Source Only

Where only the source contributes to the pixel color

Destination only

where only the destination contributes to the pixel color

Both

Source and Destination – where both the source and destination may combine to define the pixel color

None

No source or Destination – where neither make a contribution to the final pixel color

Destination is synonymous with backdrop. The term destination is used in this section as this is considered the standard when working with Porter Duff compositing. Additionally, the compositing operators use ‘destination’ in their names.

The contribution from each region to the final pixel color is defined by the coverage of the shape at that pixel, and the operator in use. Coverage is specified in terms of alpha. Full alpha (1) implies full coverage, while zero alpha (0) implies no coverage. This means that the area of each region within the sub-pixel is dependent on the coverage of each shape contributing to the pixel. The area of each region can be calculated with the following equations:

The figure above represents coverage of 0.5 for both source and destination.

Both = 0.5 x 0.5 = 0.25 
Source Only = 0.5 (1 – 0.5) = 0.25
Destination Only = 0.5(1 – 0.5) = 0.25
None = (1 – 0.5)(1 – 0.5) = 0.25

Therefore, the coverage of each region is 0.25 in this example.

9.1. The Porter Duff Compositing Operators

The landmark paper by Thomas Porter and Tom Duff, who worked for Lucasfilm, defined the algebra of compositing and developed the twelve "Porter Duff" operators. These operators control the results of mixing the four sub-pixel regions formed by the overlapping of graphical objects that have an alpha or pixel coverage channel/value. The operators use all practical combinations of the four regions.

There are 12 basic Porter Duff operators, satisfying all possible combinations of source and destination.

From the geometric representation of each operator, the contribution of each shape can be seen to be expressed as a fraction of the total coverage of the output. For example, in source over, the possible contribution of source is full (1) and the possible contribution of destination is whatever is remaining (1 – αs). This is modified by the coverage of source and destination to give the equation for the final coverage of the pixel:

    αo = αs x 1 + αb x (1 – αs)

The fractional terms Fa (1 in this example) and Fb (1 – αs in this example) are defined for each operator and specify the fraction of the shapes that may contribute to the final pixel value. The general form of the equation for coverage is:

    αs x Fa + αb x Fb

and incorporating color gives the general Porter Duff equation

    co = αs x Fa x Cs + αb x Fb x Cb

Where:

• co is the output color pre-multiplied with the output alpha [0 <= co <= 1]
• αs is the coverage of the source Fa is defined by the operator and controls inclusion of the source Cs is the color of the source (not multiplied by alpha)
• αb is the coverage of the destination Fb is defined by the operator and controls inclusion of the destination Cb is the color of the destination (not multiplied by alpha)

9.1.1. Clear

No regions are enabled.

    Fa = 0; Fb = 0
    co = 0
    αo = 0

9.1.2. Copy

Only the source will be present.

    Fa = 1; Fb = 0
    co = αs x Cs
    αo = αs

9.1.3. Destination

Only the destination will be present.

    Fa = 0; Fb = 1
    co = αb x Cb
    αo = αb

9.1.4. Source Over

Source is placed over the destination

    Fa = 1; Fb = 1 – αs
    co = αs x Cs + αb x Cb x (1 – αs)
    αo = αs + αb x (1 – αs)

9.1.5. Destination Over

Destination is placed over the source.

    Fa = 1 – αb; Fb = 1
    co = αs x Cs x (1 – αb) + αb x Cb
    αo = αs x (1 – αb) + αb

9.1.6. Source In

The source that overlaps the destination, replaces the destination.

    Fa = αb; Fb = 0
    co = αs x Cs x αb
    αo = αs x αb

9.1.7. Destination In

Destination which overlaps the source, replaces the source.

    Fa = 0; Fb = αs
    co = αb x Cb x αs 
    αo = αb x αs

9.1.8. Source Out

Source is placed, where it falls outside of the destination.

    Fa = 1 – αb; Fb = 0
    co = αs x Cs x (1 – αb)
    αo = αs x (1 – αb)

9.1.9. Destination Out

Destination is placed, where it falls outside of the source.

    Fa = 0; Fb = 1 – αs
    co = αb x Cb x (1 – αs)
    αo = αb x (1 – αs)

9.1.10. Source Atop

Source which overlaps the destination, replaces the destination. Destination is placed elsewhere.

    Fa = αb; Fb = 1 – αs
    co = αs x Cs x αb + αb x Cb x (1 – αs)
    αo = αs x αb + αb x (1 – αs)

9.1.11. Destination Atop

Destination which overlaps the source replaces the source. Source is placed elsewhere.

    Fa = 1 - αb; Fb = αs
    co = αs x Cs x (1 - αb) + αb x Cb x αs
    αo = αs x (1 - αb) + αb x αs

9.1.12. XOR

The non-overlapping regions of source and destination are combined.

    Fa = 1 - αb; Fb = 1 – αs
    co = αs x Cs x (1 - αb) + αb x Cb x (1 – αs)
    αo = αs x (1 - αb) + αb x (1 – αs)

9.1.13. Lighter

Display the sum of the source image and destination image. It is defined in the Porter Duff paper as the ‘plus’ operator. [PORTERDUFF]

    Fa = 1; Fb = 1
    co = αs x Cs + αb x Cb;
    αo = αs + αb

9.2. Group compositing behavior with Porter Duff modes

When compositing the elements within an isolated group, the elements are composited over a transparent black initial backdrop. If the bottom element in the group uses a Porter Duff compositing operator which is dependent on the backdrop, such as destination, source-in, destination-in, destination-out or source-atop, then the result of the composite will be empty. Subsequent elements within the group are composited with the result of the first composite.

10. Blending

Blending is the aspect of compositing that calculates the mixing of colors where the source element and backdrop overlap.
Conceptually, the colors in the source element are blended in place with the backdrop. After blending, the modified source element is composited with the backdrop. In practice, this is usually all performed in one step.
The blending calculations must not use pre-multiplied color values.

The "mixing" formula is defined as:

	Cm = B(Cb, Cs)

with:

• Cm: the result color after blending
• B: the formula that does the blending
• Cb: the backdrop color
• Cs: the source color

The result of the mixing formula must be clamped to the minimum and maximum values of the color range.

The result of the mixing function is modulated by the backdrop alpha. A fully opaque backdrop allows the mixing function to be fully realized. A transparent backdrop will cause the final result to be a weighted average between the source color and mixed color with the weight controlled by the backdrop alpha. The value of the new color becomes:

	Cr = (1 - αb) x Cs + αb x B(Cb, Cs)

with:

• Cr: the result color
• B: the formula that does the blending
• Cs: the source color
• Cb: the backdrop color
• αb: the backdrop alpha

This example has a red rectangle with a blending mode that is placed on top of a set of green rectangles that have different levels of opacity.

Note how the top rectangle shifts more toward red as the opacity of the backdrop gets smaller.

The following formula gives the color value in the area where the source and backdrop intersects and then composites with the specified Porter Duff compositing formula. For simple alpha blending, the formula thus becomes:

simple alpha compositing:
	co = cs + cb x (1 - αs)
written as non-premultiplied:
	αo x Co = αs x Cs + (1 - αs) x αb x Cb
now substitute the result of blending for Cs:
    αo x Co = αs x ((1 - αb) x Cs + αb x B(Cb, Cs)) + (1 - αs) x αb x Cb
            = αs x (1 - αb) x Cs + αs x αb x B(Cb, Cs) + (1 - αs) x αb x Cb

10.1. Separable blend modes

A blend mode is termed separable if each component of the result color is completely determined by the corresponding components of the constituent backdrop and source colors — that is, if the mixing formula is applied separately to each set of corresponding components.

Each of the following blend modes will apply the blending function B(Cb, Cs) on each of the color components. For simplicity, all the examples in this chapter use source-over compositing.

10.1.1. ‘normal’ blend mode

This is the default attribute which specifies no blending. The blending formula simply selects the source color.

    B(Cb, Cs) = Cs

10.1.2. ‘multiply’ blend mode

The source color is multiplied by the destination color and replaces the destination.

The resultant color is always at least as dark as either the source or destination color. Multiplying any color with black results in black. Multiplying any color with white preserves the original color.

    B(Cb, Cs) = Cb x Cs

10.1.3. ‘screen’ blend mode

Multiplies the complements of the backdrop and source color values, then complements the result.

The result color is always at least as light as either of the two constituent colors. Screening any color with white produces white; screening with black leaves the original color unchanged. The effect is similar to projecting multiple photographic slides simultaneously onto a single screen.

    B(Cb, Cs) = 1 - [(1 - Cb) x (1 - Cs)]
              = Cb + Cs -(Cb x Cs)

10.1.4. ‘overlay’ blend mode

Multiplies or screens the colors, depending on the backdrop color value.

Source colors overlay the backdrop while preserving its highlights and shadows. The backdrop color is not replaced but is mixed with the source color to reflect the lightness or darkness of the backdrop.

    B(Cb, Cs) = HardLight(Cs, Cb)

Overlay is the inverse of the ‘hardlight’ blend mode. See the definition of ‘hardlight’ for the formula.

10.1.5. ‘darken’ blend mode

Selects the darker of the backdrop and source colors.

The backdrop is replaced with the source where the source is darker; otherwise, it is left unchanged.

    B(Cb, Cs) = min(Cb, Cs)

10.1.6. ‘lighten’ blend mode

Selects the lighter of the backdrop and source colors.

The backdrop is replaced with the source where the source is lighter; otherwise, it is left unchanged.

    B(Cb, Cs) = max(Cb, Cs)

The result must be rounded down if it exceeds the range.

10.1.7. ‘color-dodge’ blend mode

Brightens the backdrop color to reflect the source color. Painting with black produces no changes.

    if(Cb == 0)
        B(Cb, Cs) = 0
    else if(Cs == 1)
        B(Cb, Cs) = 1
    else
        B(Cb, Cs) = min(1, Cb / (1 - Cs))

10.1.8. ‘color-burn’ blend mode

Darkens the backdrop color to reflect the source color. Painting with white produces no change.

    if(Cb == 1)
        B(Cb, Cs) = 1
    else if(Cs == 0)
        B(Cb, Cs) = 0
    else
        B(Cb, Cs) = 1 - min(1, (1 - Cb) / Cs)

10.1.9. ‘hard-light’ blend mode

Multiplies or screens the colors, depending on the source color value. The effect is similar to shining a harsh spotlight on the backdrop.

    if(Cs <= 0.5)
        B(Cb, Cs) = Multiply(Cb, 2 x Cs)
    else
        B(Cb, Cs) = Screen(Cb, 2 x Cs -1)	

See the definition of ‘multiply’ and ‘screen’ for their formulas.

10.1.10. ‘soft-light’ blend mode

Darkens or lightens the colors, depending on the source color value. The effect is similar to shining a diffused spotlight on the backdrop

    if(Cs <= 0.5)
        B(Cb, Cs) = Cb - (1 - 2 x Cs) x Cb x (1 - Cb)
    else
        B(Cb, Cs) = Cb + (2 x Cs - 1) x (D(Cb) - Cb)
with
    if(Cb <= 0.25)
        D(Cb) = ((16 * Cb - 12) x Cb + 4) x Cb
    else
        D(Cb) = sqrt(Cb)

10.1.11. ‘difference’ blend mode

Subtracts the darker of the two constituent colors from the lighter color.

Painting with white inverts the backdrop color; painting with black produces no change.

    B(Cb, Cs) = | Cb - Cs |

10.1.12. ‘exclusion’ blend mode

Produces an effect similar to that of the Difference mode but lower in contrast. Painting with white inverts the backdrop color; painting with black produces no change

    B(Cb, Cs) = Cb + Cs - 2 x Cb x Cs

10.2. Non-separable blend modes

Nonseparable blend modes consider all color components in combination as opposed to the separable ones that look at each component individually. All of these blend modes conceptually entail the following steps:

1. Convert the backdrop and source colors from the blending color space to an intermediate hue-saturation-luminosity representation.
2. Create a new color from some combination of hue, saturation, and luminosity components selected from the backdrop and source colors.
3. Convert the result back to the original color space.

The nonseparable blend mode formulas make use of several auxiliary functions:

    Lum(C) = 0.3 x Cred + 0.59 x Cgreen + 0.11 x Cblue
    
    ClipColor(C)
        L = Lum(C)
        n = min(Cred, Cgreen, Cblue)
        x = max(Cred, Cgreen, Cblue)
        if(n < 0)
            C = L + (((C - L) * L) / (L - n))
                      
        if(x > 1)
            C = L + (((C - L) * (1 - L)) / (x - L))
        
        return C
    
    SetLum(C, l)
        d = l - Lum(C)
        Cred = Cred + d
        Cgreen = Cgreen + d
        Cblue = Cblue + d
        return ClipColor(C)
        
    Sat(C) = max(Cred, Cgreen, Cblue) - min(Cred, Cgreen, Cblue)
        
The subscripts min, mid, and max in the next function refer to the color
components having the minimum, middle, and maximum values upon entry to the function.

    SetSat(C, s)
        if(Cmax > Cmin)
            Cmid = (((Cmid - Cmin) x s) / (Cmax - Cmin))
            Cmax = s
        else
            Cmid = Cmax = 0
        Cmin = 0
        return C;

10.2.1. ‘hue’ blend mode

Creates a color with the hue of the source color and the saturation and luminosity of the backdrop color.

	B(Cb, Cs) = SetLum(SetSat(Cs, Sat(Cb)), Lum(Cb))

10.2.2. ‘saturation’ blend mode

Creates a color with the saturation of the source color and the hue and luminosity of the backdrop color. Painting with this mode in an area of the backdrop that is a pure gray (no saturation) produces no change.

	B(Cb, Cs) = SetLum(SetSat(Cb, Sat(Cs)), Lum(Cb))

10.2.3. ‘color’ blend mode

Creates a color with the hue and saturation of the source color and the luminosity of the backdrop color. This preserves the gray levels of the backdrop and is useful for coloring monochrome images or tinting color images.

	B(Cb, Cs) = SetLum(Cs, Lum(Cb))

10.2.4. ‘luminosity’ blend mode

Creates a color with the luminosity of the source color and the hue and saturation of the backdrop color. This produces an inverse effect to that of the Color mode.

	B(Cb, Cs) = SetLum(Cb, Lum(Cs))

10.3. Effect of group isolation on blending

In the following example, the elements used to construct the paper airplane are within a group. Each of these elements has their blend mode set to multiply.

The airplane on the left is a normal group, the airplane on the right is an isolated group.

In the isolated group, the elements within the group are composited onto an empty initial backdrop, this stops the elements within the group multiplying with the backdrop. In the normal group, the elements within the group are composited onto the initial backdrop containing the land and sky. Therefore the elements of the airplane multiply with the land and sky. In both instances, the result of the group composite is composited onto the land and sky using the normal mix-blend-mode (the default mix-blend-mode applied to the group).

11. Security issues with compositing and blending

It is important that the timing to the blending and compositing operations is independent of the source and destination pixel. Operations must be implemented in such a way that they always take the same amount of time regardless of the pixel values.

12. 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:

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.

Conformance classes

Conformance to this 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 CSS 2.1 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.

CR exit criteria

For this specification to be advanced to Proposed Recommendation, there must be at least two independent, interoperable implementations of each feature. Each feature may be implemented by a different set of products, there is no requirement that all features be implemented by a single product. For the purposes of this criterion, we define the following terms:

independent

each implementation must be developed by a different party and cannot share, reuse, or derive from code used by another qualifying implementation. Sections of code that have no bearing on the implementation of this specification are exempt from this requirement.

interoperable

passing the respective test case(s) in the official CSS test suite, or, if the implementation is not a Web browser, an equivalent test. Every relevant test in the test suite should have an equivalent test created if such a user agent (UA) is to be used to claim interoperability. In addition if such a UA is to be used to claim interoperability, then there must one or more additional UAs which can also pass those equivalent tests in the same way for the purpose of interoperability. The equivalent tests must be made publicly available for the purposes of peer review.

implementation

a user agent which:

1. implements the specification.
2. is available to the general public. The implementation may be a shipping product or other publicly available version (i.e., beta version, preview release, or “nightly build”). Non-shipping product releases must have implemented the feature(s) for a period of at least one month in order to demonstrate stability.
3. is not experimental (i.e., a version specifically designed to pass the test suite and is not intended for normal usage going forward).

The specification will remain Candidate Recommendation for at least six months.

13. References

13.1. Normative References

[2DCONTEXT2]

Rik Cabanier; et al. HTML Canvas 2D Context, Level 2. 29 October 2013. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2013/WD-2dcontext2-20131029/

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607

[CSS3BG]

Bert Bos; Elika J. Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. 4 February 2014. W3C Last Call Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2014/WD-css3-background-20140204/

[CSS3COLOR]

Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

[RFC2119]

S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt

[SVG11]

Erik Dahlström; et al. Scalable Vector Graphics (SVG) 1.1 (Second Edition). 16 August 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-SVG11-20110816/

13.2. Informative References

[PORTERDUFF]

Thomas Porter; Tom Duff. “Compositing digital images” in: ACM SIGGRAPH Computer Graphics. Volume 18. Issue 3. pp. 253–259. ACM. July 1984. New York, NY, USA. ISSN 0097-8930

14. Changes

The following changes were made relative to the Last Call Working draft of 7 January 2014:

• unneeded normative and informative references were removed

The following changes were made relative to the Last Call Working draft of 10 October 2013:

• knockout was removed from the non-normative section
• removed paragraph on SVG from simple alpha compositing
• updated abstract to include CSS and clarify intent
• removed clip-to-self and its references
• Changed section 5-10 to be normative + clarified notes/examples in those sections

The following changes were made relative to the Working Draft of 2013-06-25:

• clipping was removed as one of the operators that creates an isolated group in SVG
• background-blend-mode was changed so it matches repeating behavior of other background syntaxes.
• The mix-composite property was removed
• all open issues were resolved