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Compositing describes how shapes of different elements are combined into a single image. There are various possible approaches for compositing. Previous versions of SVG 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:
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This is the first public working draft of this module.
normal
’ blend mode
multiply
’ blend mode
screen
’ blend mode
overlay
’ blend mode
darken
’ blend mode
lighten
’ blend mode
color-dodge
’ blend mode
color-burn
’ blend mode
hard-light
’ blend mode
soft-light
’ blend mode
difference
’ blend mode
exclusion
’ blend mode
The first part of this document will describe the algorithms of Porter
Duff compositing and blending. The second part describes the properties
used to control the compositing in CSS.
Compositing is the combining of a graphic element with its backdrop.
The figure below, gives a basic example of compositing. The graphic element, a ... in this case, is overlaid on top of the backdrop to produce a composite image. Where the graphic element is opaque, it obscures the backdrop and where the graphic element is partially transparent, some of the backdrop can be seen through.
Add a pretty picture. I'm thinking a house with a window that is transparent overlaid on a mountain scene.
In the model described in this specification there are two steps to the overall compositing operation - Porter Duff compositing and blending. Porter Duff compositing takes into account the overall shape of the graphic element and its opacity, as well as the opacity and shape of the backdrop, and determines where the backdrop is visible, where the graphic element is visible and where one is visible through the other. 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 PD 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. Shape either exists at a particular point or it does 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.
The simple alpha compositing model used in previous versions of SVG allowed for the illusion of partial or full transparency. While this specification provides the author the choice of many Porter Duff compositing operators and many blending modes, the simple alpha compositing model forced a single Porter Duff compositing operator and a single blend mode.
The formula for simple alpha compositing is
co = Cs x αs + Cb x αb x (1 - αs)
Where
co: the 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 pixel 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
Often, it can be more efficient to store a pre-multiplied
value
for the color and opacity. The pre-multiplied value is given
by
cs = Cs x αs
with
cs: the pre-multiplied value
Cs: the color value
αs: the alpha value
Thus the formula for simple alpha compositing using pre-multplied values becomes
co = cs + cb x (1 - αs)
To extract the color component of a pre-multiplied value, the formula is reversed:
Co = co / αo
Figure 1
Figure 1 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)
Figure 2
Figure 2 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 = 1Calculating the color component of the resultant composite
Co = co / αo Co = RGB(0, 0, 1) / 1 Co = RGB(0, 0, 1)
Figure 3
Figure 3 shows 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 = 1Calculating 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
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 = 0.75Calculating 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)
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
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.
Figure 5
Figure 5 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.
Figure 6
In figure 6, the shape in the backdrop has an alpha value. The alpha value of the backdrop shape is preserved when the backdrop is calculated.
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 no groups. See Group Invariance. The result of this is that single elements behave as if they were in a group by themselves.
A compositing group is rendered by first compositing the elements of the group onto the inital 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.
An important behavior 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 knockout, isolate, blending modes other than normal or Porter Duff compositing operators other than source-over, groups may no longer be invariant.
Isolated groups are controlled with the isolation property.
In an isolated group, the initial backdrop shall be black and fully
transparent - RGBA(0, 0, 0, 0).
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.
In a knockout group, each individual element shall be composited with the initial backdrop rather than with the stack of preceeding elements in the group. When calculating the backdrop for an element inside a knockout group, the elements of the group are ignored. Instead, only the elements that are behind the knockout group are included in the backdrop.
The above example demonstrates two versions of a group containing three
squares (red, green, blue) that are 50% opaque. The group is composited
over a grey striped background.
On the left, the group has the ‘knock-out
’
property set to ‘knock-out
’. On the right,
the group has the ‘knock-out
’ property set
to ‘preserve
’.
If the ‘knock-out
’ property has the value
‘knock-out
’, each element within the group
is only composited with the elements underneath the 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 defined by the User
Agent. Typically this will be white with 100% opacity.
The page group may be used as an element in another graphical composition.
For example,
an SVG file contains a red object at 50% opacity,
The user agent composites the page group onto a white background with 100%
opacity.
The results are as follows:
co = RGB(255, 0, 0) * .5 + RGB(255, 255, 255) * 1 * (1 - .5) co = RGB(127, 0, 0) + RGB(127, 127, 127) co = RGB(255, 127, 127)
which is the color value ultimately displayed by the user agent.
Simple alpha compositing uses the
source-over Porter Duff compositing operator. Additional compositing
operators exist and may be specified with the alpha-compositing property. The
additional compositing operators allow for more complex interactions
between the shapes of elements being composited. The compositing operators
are described in ‘The Porter Duff compositing operators
’. The
operators that applies to an element or group is selected using the alpha-compositing property.
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.
The four regions are:
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 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:
Both | αs x αb |
---|---|
Source only | αs(1 – αb) |
Destination only | αb(1 – αs) |
None | (1 – αs)(1 – αb) |
The figure above represents coverage of 0.5 for both source and destination.
Both = 0.5 x 0.5 = 0.25 Source (1 – 0.5) = 0.25 Destination – 0.5) = 0.25 None = (1 – 0.5)(1 – 0.5) = 0.25
Therefore, the area of each region is 25% in this example.
The landmark 1984 paper [3] 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)
No regions are enabled.
Fa = 0; Fb = 0 co = 0 αo = 0
Only the source will be present.
Fa = 1; Fb = 0 co = αs x Cs αo = αs
Only the destination will be present.
Fa = 0; Fb = 1 co = αb x Cb αo = αb
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)
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
The source that overlaps the destination, replaces the destination.
Fa = αb; Fb = 0 co = αs x Cs x αb αo = αs x αb
Destination which overlaps the source, replaces the source.
Fa = 0; Fb = αs co = αb x Cb x αs αo = αb x αs
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)
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)
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)
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
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)
Display the sum of the source image and destination image. It is defined
in the Porter Duff paper [3] as the ‘plus
’
operator.
Fa = 1; Fb = 1 co = αs x Cs + αb x Cb; αo = αs + αb
When compositing the elements within an isolated group, the elements are composited over an empty (RGBA(0, 0, 0, 0)) 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.
Every element within a knock-out group is composited with the initial backdrop. This means, that for every element within the group, the backdrop for the compositing of that element, is the initial backdrop.
In the example below, the elements within the group (the circle and the
square) are composited using the source-atop operator,
with only the hexagon. This has the effect of "knocking out" the circle,
where it is overlapped by the square.
Additionally, because the source-atop Porter Duff operator is used, the
source shape (either the square or the circle) is only placed where the
backdrop exists (the backdrop being the hexagon for both compositing
operations within the group).
When compositing, the areas of the composite that may be modified by the compositing operation, must fall within the shape of the element being composited (i.e. where α > 0). This is known as "clip to self" in some graphics libraries. The alternative is to not clip the compositing operation at all. The results can be seen below. Some of the Porter Duff operators are unchanged, because they normally have no effect outside the source region. The changes can be seen in the clear, source, source-in, destination-in, source-out and destination-atop.
Blending is the aspect of compositing that calculates the mixing of colors where the source element and backdrop overlap. Blending takes the colors of the source element and mixes them with the backdrop in areas 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 calcualtions 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 realised. 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 lowers.
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 subsitute 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
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.
normal
’ blend modeThis is the default attribute which specifies no blending. The blending formula simply selects the source color.
B(Cb, Cs) = Cs
multiply
’ blend modeThe 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
screen
’ blend modeMultiplies 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)
overlay
’ blend modeMultiplies 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.
darken
’ blend modeSelects 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)
lighten
’ blend modeSelects 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)
color-dodge
’ blend modeBrightens the backdrop color to reflect the source color. Painting with black produces no changes.
if(Cs < 1) B(Cb, Cs) = min(1, Cb / (1 - Cs)) else B(Cb, Cs) = 1
color-burn
’ blend modeDarkens the backdrop color to reflect the source color. Painting with white produces no change.
if(Cs > 0) B(Cb, Cs) = 1 - min(1, (1 - Cb) / Cs) else B(Cb, Cs) = 0
hard-light
’ blend modeMultiplies 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.
soft-light
’ blend modeDarkens 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)
difference
’ blend modeSubtracts 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 |
exclusion
’ blend modeProduces 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
Nonseparable blend modes consider all color components in combination
as opposed to the seperable ones that look at each component
individually.
All of these blend modes conceptually entail the following steps:
a) Convert the backdrop and source colors from the
blending color space to an intermediate hue-saturation-luminosity
representation.
b) Create a new color from some combination of hue, saturation, and
luminosity components selected from the backdrop
and source colors.
c) 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 + (((Cred - 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;
hue
’ blend modeCreates 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))
saturation
’ blend modeCreates 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))
color
’ blend modeCreates 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))
luminosity
’ blend modeCreates 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))
In the following example, the elements used to construct the paper
aeroplane are within a group. Each of these elements has the blend-mode property set to multiply.
The aeroplane on the left is a normal group, the aeroplane 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 aeroplane 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 blend-mode (the default blend-mode
applied to the group).
If an element specifies non-default blending, compositing or ‘opacity
’, its transform-style
and that of all of its children will revert to ‘flat
’.
The application of non-default blending or compositing to an element
formatted with the CSS box model also establishes a stacking context the
same way that CSS ‘opacity
’ does. One of the consequences is that
elements with z-index will not honor the depth of elements outside of the
group.
Everything in CSS that creates a stacking context is
considered a group. HTML elements themselves do not create groups.
If content uses CSS filters, they will be applied before blending and compositing.
In SVG, every element will create a group.
Most implementations can optimize this by not treating elements with
default arguments as groups.
The compositing model follows the SVG
compositing model: first any filter effect is applied, then any
clipping, masking, blending and compositing
alpha-compositing
’
property Defines the compositing mode used when compositing an element onto a
page or group.
This behavior is described in more detail in Advanced compositing features.
The description of the ‘alpha-compositing
’ property is as follows:
alpha-compositing
’
Value: | clear | copy | destination | source-over | destination-over | source-in | destination-in | source-out | destination-out | source-atop | destination-atop | xor | lighter |
Initial: | source-over |
Applies to: | All elements. In SVG, it applies to svg, g, use, image, path, rect, circle, ellipse, line, polyline, polygon, text, tspan, and marker. |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
Elements will always composite with ‘clip-to-self
’ set to ‘true
’.
blend-mode
’
property Defines the blend mode used when compositing an element onto the page.
The blend mode defines the formula used to mix colors when shapes
overlap.
This behavior is described in more detail in Blending.
blend-mode
’
Value: | normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity |
Initial: | normal |
Applies to: | All elements. In SVG, it applies to svg, g, use, image, path, rect, circle, ellipse, line, polyline, polygon, text, tspan, and marker. |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
isolation
’
property Defines whether a group is isolated or not.
When a group is isolated, the group's elements will not look at content
outside that group when calculating the backdrop.
The default behavior is for groups to be non-isolated.
This behavior is described in more detail in Isolated Gropus.
isolation
’
Value: | accumulate | isolate |
Initial: | accumulate |
Applies to: | All HTML elements. In SVG, it applies to all container elements
except ‘mask ’
|
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
In CSS, the ‘isolation
’ property of an
<img> or a background image is always ‘isolate
’. For instance, if you link to an SVG file
through the ‘img
’ tag, the content of that
SVG will not blend with its backdrop.
In SVG, ‘mask
’ always creates an isolated group.
knock-out
’
property Defines whether a group is a knock-out group.
When a group is set to ‘knock-out
’, the
elements within the group only composite and blend with elements outside
that group. This effectively ‘knocks out
’ any
element from within the group that is already drawn. The end result is as
if every shape composites with a ‘clear
’
operation (with clip-to-self enabled) first before it blends and
composites.
When ‘knock-out
’ is set to ‘preserve
’, the group is not a knock-out group and
its elements composite normally.
The behavior of this keyword is described in more detail in Knockout Groups.
knock-out
’
Value: | preserve | knock-out |
Initial: | preserve |
Applies to: | All HTML elements. In SVG, it applies to all container elements
except ‘mask ’
|
Inherited: | yes |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
An author might want to specify the blending of multiple backgrounds of an element. The background compositing and blending is always treated as an isolated group.
background-alpha-compositing
’ property The description of the ‘background-alpha-compositing
’ property is
as follows:
background-alpha-compositing
’
Value: | compositing-style [, compositing-style]* |
Initial: | source-over |
Applies to: | All HTML elements" |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
compositing-style = clear | copy | destination | source-over | destination-over | source-in | destination-in | source-out | destination-out | source-atop | destination-atop | xor | lighter
Background images will always composite with ‘clip-to-self
’ set to ‘false
’.
background-blend-mode
’ property The description of the ‘background-blend-mode
’ property is as
follows:
background-blend-mode
’
Value: | blend-style [, blend-style]* |
Initial: | normal |
Applies to: | All HTML elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
blend-style = normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity
box-shadow-blend-mode
’ property Sets the blend mode of the box shadow.
Also needs to be added to the shorthand.
The description of the ‘box-shadow-blend-mode
’ property is as
follows:
box-shadow-blend-mode
’
Value: | blend-style [, blend-style]* |
Initial: | normal |
Applies to: | All HTML elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
blend-style = normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity
text-shadow-blend-mode
’ propertyDRAFT.This proposal needs more discussion.
Set the blend mode of the text shadow.
Also needs to be added to the shorthand.
The description of the ‘text-shadow-blend-mode
’ property is as
follows:
text-shadow-blend-mode
’
Value: | blend-style [, blend-style]* |
Initial: | normal |
Applies to: | All HTML elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
blend-style = normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity
foreground-blend-mode
’ property Sets the blend mode of the text or nested elements.
Also needs to be added to the shorthand.
The description of the ‘foreground-blend-mode
’ property is as
follows:
foreground-blend-mode
’
Value: | blend-style |
Initial: | normal |
Applies to: | All HTML elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Animatable: | no |
blend-style = normal | multiply | screen | overlay | darken | lighten | color-dodge | color-burn | hard-light | soft-light | difference | exclusion | hue | saturation | color | luminosity
To cut down on the number of new keywords, we could extend blend-mode to take the following additional parameters:
A possible drawback is that there are no other keywords that target all these layers at once.
Is there a need to target this part of the element?
Please see the W3 wiki.
It is important that the timing to the blending and compositing
operations is independant of the source and destination pixel. In other
words, operations should be implemented in such a way that they always
take the same amount of time regardless of the pixel values.
If this rule is not followed, an attacker could mount a timing attack.
A timing attack is a method of obtaining information about content that is otherwise protected, based on studying the amount of time it takes for an operation to occur.If, for example, red pixels took longer to draw than green pixels, one might be able to reconstruct a rough image of the element being rendered, even without ever having access to the content of the element.
84 Conference Proceedings, Association for
Computing Machinery, Volume 18, Number 3, July 1984.