jdk.awt.AlphaComposite
The AlphaComposite class implements basic alpha compositing rules for combining source and destination colors to achieve blending and transparency effects with graphics and images. The specific rules implemented by this class are the basic set of 12 rules described in T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84, 253-259. The rest of this documentation assumes some familiarity with the definitions and concepts outlined in that paper.
This class extends the standard equations defined by Porter and Duff to include one additional factor. An instance of the AlphaComposite class can contain an alpha value that is used to modify the opacity or coverage of every source pixel before it is used in the blending equations.
It is important to note that the equations defined by the Porter and Duff paper are all defined to operate on color components that are premultiplied by their corresponding alpha components. Since the ColorModel and Raster classes allow the storage of pixel data in either premultiplied or non-premultiplied form, all input data must be normalized into premultiplied form before applying the equations and all results might need to be adjusted back to the form required by the destination before the pixel values are stored.
Also note that this class defines only the equations for combining color and alpha values in a purely mathematical sense. The accurate application of its equations depends on the way the data is retrieved from its sources and stored in its destinations. See Implementation Caveats for further information.
The following factors are used in the description of the blending equation in the Porter and Duff paper:
Factor Definition Asthe alpha component of the source pixel Csa color component of the source pixel in premultiplied form Adthe alpha component of the destination pixel Cda color component of the destination pixel in premultiplied form Fsthe fraction of the source pixel that contributes to the output Fdthe fraction of the destination pixel that contributes to the output Arthe alpha component of the result Cra color component of the result in premultiplied form
Using these factors, Porter and Duff define 12 ways of choosing the blending factors Fs and Fd to produce each of 12 desirable visual effects. The equations for determining Fs and Fd are given in the descriptions of the 12 static fields that specify visual effects. For example, the description for SRC_OVER specifies that Fs = 1 and Fd = (1-As). Once a set of equations for determining the blending factors is known they can then be applied to each pixel to produce a result using the following set of equations:
Fs = f(Ad)
Fd = f(As)
Ar = As*Fs Ad*Fd
Cr = Cs*Fs Cd*Fd
The following factors will be used to discuss our extensions to the blending equation in the Porter and Duff paper:
Factor Definition Csr one of the raw color components of the source pixel Cdr one of the raw color components of the destination pixel Aac the "extra" alpha component from the AlphaComposite instance Asr the raw alpha component of the source pixel Adrthe raw alpha component of the destination pixel Adf the final alpha component stored in the destination Cdf the final raw color component stored in the destination
Preparing Inputs
The AlphaComposite class defines an additional alpha value that is applied to the source alpha. This value is applied as if an implicit SRC_IN rule were first applied to the source pixel against a pixel with the indicated alpha by multiplying both the raw source alpha and the raw source colors by the alpha in the AlphaComposite. This leads to the following equation for producing the alpha used in the Porter and Duff blending equation:
As = Asr * Aac
All of the raw source color components need to be multiplied by the alpha in the AlphaComposite instance. Additionally, if the source was not in premultiplied form then the color components also need to be multiplied by the source alpha. Thus, the equation for producing the source color components for the Porter and Duff equation depends on whether the source pixels are premultiplied or not:
Cs = Csr * Asr * Aac (if source is not premultiplied)
Cs = Csr * Aac (if source is premultiplied)
No adjustment needs to be made to the destination alpha:
Ad = Adr
The destination color components need to be adjusted only if they are not in premultiplied form:
Cd = Cdr * Ad (if destination is not premultiplied)
Cd = Cdr (if destination is premultiplied)
Applying the Blending Equation
The adjusted As, Ad, Cs, and Cd are used in the standard Porter and Duff equations to calculate the blending factors Fs and Fd and then the resulting premultiplied components Ar and Cr.
Preparing Results
The results only need to be adjusted if they are to be stored back into a destination buffer that holds data that is not premultiplied, using the following equations:
Adf = Ar
Cdf = Cr (if dest is premultiplied)
Cdf = Cr / Ar (if dest is not premultiplied)
Note that since the division is undefined if the resulting alpha is zero, the division in that case is omitted to avoid the "divide by zero" and the color components are left as all zeros.
Performance Considerations
For performance reasons, it is preferable that Raster objects passed to the compose method of a CompositeContext object created by the AlphaComposite class have premultiplied data. If either the source Raster or the destination Raster is not premultiplied, however, appropriate conversions are performed before and after the compositing operation.
Implementation Caveats
Many sources, such as some of the opaque image types listed in the BufferedImage class, do not store alpha values for their pixels. Such sources supply an alpha of 1.0 for all of their pixels.
Many destinations also have no place to store the alpha values that result from the blending calculations performed by this class. Such destinations thus implicitly discard the resulting alpha values that this class produces. It is recommended that such destinations should treat their stored color values as non-premultiplied and divide the resulting color values by the resulting alpha value before storing the color values and discarding the alpha value.
The accuracy of the results depends on the manner in which pixels are stored in the destination. An image format that provides at least 8 bits of storage per color and alpha component is at least adequate for use as a destination for a sequence of a few to a dozen compositing operations. An image format with fewer than 8 bits of storage per component is of limited use for just one or two compositing operations before the rounding errors dominate the results. An image format that does not separately store color components is not a good candidate for any type of translucent blending. For example, BufferedImage.TYPE_BYTE_INDEXED should not be used as a destination for a blending operation because every operation can introduce large errors, due to the need to choose a pixel from a limited palette to match the results of the blending equations.
Nearly all formats store pixels as discrete integers rather than the floating point values used in the reference equations above. The implementation can either scale the integer pixel values into floating point values in the range 0.0 to 1.0 or use slightly modified versions of the equations that operate entirely in the integer domain and yet produce analogous results to the reference equations.
Typically the integer values are related to the floating point values in such a way that the integer 0 is equated to the floating point value 0.0 and the integer 2^n-1 (where n is the number of bits in the representation) is equated to 1.0. For 8-bit representations, this means that 0x00 represents 0.0 and 0xff represents 1.0.
The internal implementation can approximate some of the equations and it can also eliminate some steps to avoid unnecessary operations. For example, consider a discrete integer image with non-premultiplied alpha values that uses 8 bits per component for storage. The stored values for a nearly transparent darkened red might be:
(A, R, G, B) = (0x01, 0xb0, 0x00, 0x00)
If integer math were being used and this value were being composited in SRC mode with no extra alpha, then the math would indicate that the results were (in integer format):
(A, R, G, B) = (0x01, 0x01, 0x00, 0x00)
Note that the intermediate values, which are always in premultiplied form, would only allow the integer red component to be either 0x00 or 0x01. When we try to store this result back into a destination that is not premultiplied, dividing out the alpha will give us very few choices for the non-premultiplied red value. In this case an implementation that performs the math in integer space without shortcuts is likely to end up with the final pixel values of:
(A, R, G, B) = (0x01, 0xff, 0x00, 0x00)
(Note that 0x01 divided by 0x01 gives you 1.0, which is equivalent to the value 0xff in an 8-bit storage format.)
Alternately, an implementation that uses floating point math might produce more accurate results and end up returning to the original pixel value with little, if any, roundoff error. Or, an implementation using integer math might decide that since the equations boil down to a virtual NOP on the color values if performed in a floating point space, it can transfer the pixel untouched to the destination and avoid all the math entirely.
These implementations all attempt to honor the same equations, but use different tradeoffs of integer and floating point math and reduced or full equations. To account for such differences, it is probably best to expect only that the premultiplied form of the results to match between implementations and image formats. In this case both answers, expressed in premultiplied form would equate to:
(A, R, G, B) = (0x01, 0x01, 0x00, 0x00)
and thus they would all match.
Because of the technique of simplifying the equations for calculation efficiency, some implementations might perform differently when encountering result alpha values of 0.0 on a non-premultiplied destination. Note that the simplification of removing the divide by alpha in the case of the SRC rule is technically not valid if the denominator (alpha) is 0. But, since the results should only be expected to be accurate when viewed in premultiplied form, a resulting alpha of 0 essentially renders the resulting color components irrelevant and so exact behavior in this case should not be expected.
The AlphaComposite class implements basic alpha
compositing rules for combining source and destination colors
to achieve blending and transparency effects with graphics and
images.
The specific rules implemented by this class are the basic set
of 12 rules described in
T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84,
253-259.
The rest of this documentation assumes some familiarity with the
definitions and concepts outlined in that paper.
This class extends the standard equations defined by Porter and
Duff to include one additional factor.
An instance of the AlphaComposite class can contain
an alpha value that is used to modify the opacity or coverage of
every source pixel before it is used in the blending equations.
It is important to note that the equations defined by the Porter
and Duff paper are all defined to operate on color components
that are premultiplied by their corresponding alpha components.
Since the ColorModel and Raster classes
allow the storage of pixel data in either premultiplied or
non-premultiplied form, all input data must be normalized into
premultiplied form before applying the equations and all results
might need to be adjusted back to the form required by the destination
before the pixel values are stored.
Also note that this class defines only the equations
for combining color and alpha values in a purely mathematical
sense. The accurate application of its equations depends
on the way the data is retrieved from its sources and stored
in its destinations.
See Implementation Caveats
for further information.
The following factors are used in the description of the blending
equation in the Porter and Duff paper:
Factor Definition
Asthe alpha component of the source pixel
Csa color component of the source pixel in premultiplied form
Adthe alpha component of the destination pixel
Cda color component of the destination pixel in premultiplied form
Fsthe fraction of the source pixel that contributes to the output
Fdthe fraction of the destination pixel that contributes
to the output
Arthe alpha component of the result
Cra color component of the result in premultiplied form
Using these factors, Porter and Duff define 12 ways of choosing
the blending factors Fs and Fd to
produce each of 12 desirable visual effects.
The equations for determining Fs and Fd
are given in the descriptions of the 12 static fields
that specify visual effects.
For example,
the description for
SRC_OVER
specifies that Fs = 1 and Fd = (1-As).
Once a set of equations for determining the blending factors is
known they can then be applied to each pixel to produce a result
using the following set of equations:
Fs = f(Ad)
Fd = f(As)
Ar = As*Fs Ad*Fd
Cr = Cs*Fs Cd*Fd
The following factors will be used to discuss our extensions to
the blending equation in the Porter and Duff paper:
Factor Definition
Csr one of the raw color components of the source pixel
Cdr one of the raw color components of the destination pixel
Aac the "extra" alpha component from the AlphaComposite instance
Asr the raw alpha component of the source pixel
Adrthe raw alpha component of the destination pixel
Adf the final alpha component stored in the destination
Cdf the final raw color component stored in the destination
Preparing Inputs
The AlphaComposite class defines an additional alpha
value that is applied to the source alpha.
This value is applied as if an implicit SRC_IN rule were first
applied to the source pixel against a pixel with the indicated
alpha by multiplying both the raw source alpha and the raw
source colors by the alpha in the AlphaComposite.
This leads to the following equation for producing the alpha
used in the Porter and Duff blending equation:
As = Asr * Aac
All of the raw source color components need to be multiplied
by the alpha in the AlphaComposite instance.
Additionally, if the source was not in premultiplied form
then the color components also need to be multiplied by the
source alpha.
Thus, the equation for producing the source color components
for the Porter and Duff equation depends on whether the source
pixels are premultiplied or not:
Cs = Csr * Asr * Aac (if source is not premultiplied)
Cs = Csr * Aac (if source is premultiplied)
No adjustment needs to be made to the destination alpha:
Ad = Adr
The destination color components need to be adjusted only if
they are not in premultiplied form:
Cd = Cdr * Ad (if destination is not premultiplied)
Cd = Cdr (if destination is premultiplied)
Applying the Blending Equation
The adjusted As, Ad,
Cs, and Cd are used in the standard
Porter and Duff equations to calculate the blending factors
Fs and Fd and then the resulting
premultiplied components Ar and Cr.
Preparing Results
The results only need to be adjusted if they are to be stored
back into a destination buffer that holds data that is not
premultiplied, using the following equations:
Adf = Ar
Cdf = Cr (if dest is premultiplied)
Cdf = Cr / Ar (if dest is not premultiplied)
Note that since the division is undefined if the resulting alpha
is zero, the division in that case is omitted to avoid the "divide
by zero" and the color components are left as
all zeros.
Performance Considerations
For performance reasons, it is preferable that
Raster objects passed to the compose
method of a CompositeContext object created by the
AlphaComposite class have premultiplied data.
If either the source Raster
or the destination Raster
is not premultiplied, however,
appropriate conversions are performed before and after the compositing
operation.
Implementation Caveats
Many sources, such as some of the opaque image types listed
in the BufferedImage class, do not store alpha values
for their pixels. Such sources supply an alpha of 1.0 for
all of their pixels.
Many destinations also have no place to store the alpha values
that result from the blending calculations performed by this class.
Such destinations thus implicitly discard the resulting
alpha values that this class produces.
It is recommended that such destinations should treat their stored
color values as non-premultiplied and divide the resulting color
values by the resulting alpha value before storing the color
values and discarding the alpha value.
The accuracy of the results depends on the manner in which pixels
are stored in the destination.
An image format that provides at least 8 bits of storage per color
and alpha component is at least adequate for use as a destination
for a sequence of a few to a dozen compositing operations.
An image format with fewer than 8 bits of storage per component
is of limited use for just one or two compositing operations
before the rounding errors dominate the results.
An image format
that does not separately store
color components is not a
good candidate for any type of translucent blending.
For example, BufferedImage.TYPE_BYTE_INDEXED
should not be used as a destination for a blending operation
because every operation
can introduce large errors, due to
the need to choose a pixel from a limited palette to match the
results of the blending equations.
Nearly all formats store pixels as discrete integers rather than
the floating point values used in the reference equations above.
The implementation can either scale the integer pixel
values into floating point values in the range 0.0 to 1.0 or
use slightly modified versions of the equations
that operate entirely in the integer domain and yet produce
analogous results to the reference equations.
Typically the integer values are related to the floating point
values in such a way that the integer 0 is equated
to the floating point value 0.0 and the integer
2^n-1 (where n is the number of bits
in the representation) is equated to 1.0.
For 8-bit representations, this means that 0x00
represents 0.0 and 0xff represents
1.0.
The internal implementation can approximate some of the equations
and it can also eliminate some steps to avoid unnecessary operations.
For example, consider a discrete integer image with non-premultiplied
alpha values that uses 8 bits per component for storage.
The stored values for a
nearly transparent darkened red might be:
(A, R, G, B) = (0x01, 0xb0, 0x00, 0x00)
If integer math were being used and this value were being
composited in
SRC
mode with no extra alpha, then the math would
indicate that the results were (in integer format):
(A, R, G, B) = (0x01, 0x01, 0x00, 0x00)
Note that the intermediate values, which are always in premultiplied
form, would only allow the integer red component to be either 0x00
or 0x01. When we try to store this result back into a destination
that is not premultiplied, dividing out the alpha will give us
very few choices for the non-premultiplied red value.
In this case an implementation that performs the math in integer
space without shortcuts is likely to end up with the final pixel
values of:
(A, R, G, B) = (0x01, 0xff, 0x00, 0x00)
(Note that 0x01 divided by 0x01 gives you 1.0, which is equivalent
to the value 0xff in an 8-bit storage format.)
Alternately, an implementation that uses floating point math
might produce more accurate results and end up returning to the
original pixel value with little, if any, roundoff error.
Or, an implementation using integer math might decide that since
the equations boil down to a virtual NOP on the color values
if performed in a floating point space, it can transfer the
pixel untouched to the destination and avoid all the math entirely.
These implementations all attempt to honor the
same equations, but use different tradeoffs of integer and
floating point math and reduced or full equations.
To account for such differences, it is probably best to
expect only that the premultiplied form of the results to
match between implementations and image formats. In this
case both answers, expressed in premultiplied form would
equate to:
(A, R, G, B) = (0x01, 0x01, 0x00, 0x00)
and thus they would all match.
Because of the technique of simplifying the equations for
calculation efficiency, some implementations might perform
differently when encountering result alpha values of 0.0
on a non-premultiplied destination.
Note that the simplification of removing the divide by alpha
in the case of the SRC rule is technically not valid if the
denominator (alpha) is 0.
But, since the results should only be expected to be accurate
when viewed in premultiplied form, a resulting alpha of 0
essentially renders the resulting color components irrelevant
and so exact behavior in this case should not be expected.*-clearclj
Static Constant.
AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-dstclj
Static Constant.
AlphaComposite object that implements the opaque DST rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque DST rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-dst-atopclj
Static Constant.
AlphaComposite object that implements the opaque DST_ATOP rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque DST_ATOP rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-dst-inclj
Static Constant.
AlphaComposite object that implements the opaque DST_IN rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque DST_IN rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-dst-outclj
Static Constant.
AlphaComposite object that implements the opaque DST_OUT rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque DST_OUT rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-dst-overclj
Static Constant.
AlphaComposite object that implements the opaque DST_OVER rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque DST_OVER rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-srcclj
Static Constant.
AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-src-atopclj
Static Constant.
AlphaComposite object that implements the opaque SRC_ATOP rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque SRC_ATOP rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-src-inclj
Static Constant.
AlphaComposite object that implements the opaque SRC_IN rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque SRC_IN rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-src-outclj
Static Constant.
AlphaComposite object that implements the opaque SRC_OUT rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque SRC_OUT rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-src-overclj
Static Constant.
AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*-xorclj
Static Constant.
AlphaComposite object that implements the opaque XOR rule with an alpha of 1.0f.
type: java.awt.AlphaComposite
Static Constant. AlphaComposite object that implements the opaque XOR rule with an alpha of 1.0f. type: java.awt.AlphaComposite
*get-instanceclj
(*get-instance rule)(*get-instance rule alpha)Creates an AlphaComposite object with the specified rule and the constant alpha to multiply with the alpha of the source. The source is multiplied with the specified alpha before being composited with the destination.
rule - the compositing rule - int
alpha - the constant alpha to be multiplied with the alpha of the source. alpha must be a floating point number in the inclusive range [0.0, 1.0]. - float
returns: java.awt.AlphaComposite
throws: java.lang.IllegalArgumentException - if alpha is less than 0.0 or greater than 1.0, or if rule is not one of the following: CLEAR, SRC, DST, SRC_OVER, DST_OVER, SRC_IN, DST_IN, SRC_OUT, DST_OUT, SRC_ATOP, DST_ATOP, or XOR
Creates an AlphaComposite object with the specified rule and the constant alpha to multiply with the alpha of the source. The source is multiplied with the specified alpha before being composited with the destination. rule - the compositing rule - `int` alpha - the constant alpha to be multiplied with the alpha of the source. alpha must be a floating point number in the inclusive range [0.0, 1.0]. - `float` returns: `java.awt.AlphaComposite` throws: java.lang.IllegalArgumentException - if alpha is less than 0.0 or greater than 1.0, or if rule is not one of the following: CLEAR, SRC, DST, SRC_OVER, DST_OVER, SRC_IN, DST_IN, SRC_OUT, DST_OUT, SRC_ATOP, DST_ATOP, or XOR
create-contextclj
(create-context this src-color-model dst-color-model hints)Creates a context for the compositing operation. The context contains state that is used in performing the compositing operation.
src-color-model - the ColorModel of the source - java.awt.image.ColorModel
dst-color-model - the ColorModel of the destination - java.awt.image.ColorModel
hints - the hint that the context object uses to choose between rendering alternatives - java.awt.RenderingHints
returns: the CompositeContext object to be used to perform
compositing operations. - java.awt.CompositeContext
Creates a context for the compositing operation. The context contains state that is used in performing the compositing operation. src-color-model - the ColorModel of the source - `java.awt.image.ColorModel` dst-color-model - the ColorModel of the destination - `java.awt.image.ColorModel` hints - the hint that the context object uses to choose between rendering alternatives - `java.awt.RenderingHints` returns: the CompositeContext object to be used to perform compositing operations. - `java.awt.CompositeContext`
deriveclj
(derive this rule)Returns a similar AlphaComposite object that uses the specified compositing rule. If this object already uses the specified compositing rule, this object is returned.
rule - the compositing rule - int
returns: an AlphaComposite object derived from
this object that uses the specified compositing rule. - java.awt.AlphaComposite
throws: java.lang.IllegalArgumentException - if rule is not one of the following: CLEAR, SRC, DST, SRC_OVER, DST_OVER, SRC_IN, DST_IN, SRC_OUT, DST_OUT, SRC_ATOP, DST_ATOP, or XOR
Returns a similar AlphaComposite object that uses the specified compositing rule. If this object already uses the specified compositing rule, this object is returned. rule - the compositing rule - `int` returns: an AlphaComposite object derived from this object that uses the specified compositing rule. - `java.awt.AlphaComposite` throws: java.lang.IllegalArgumentException - if rule is not one of the following: CLEAR, SRC, DST, SRC_OVER, DST_OVER, SRC_IN, DST_IN, SRC_OUT, DST_OUT, SRC_ATOP, DST_ATOP, or XOR
equalsclj
(equals this obj)Determines whether the specified object is equal to this AlphaComposite.
The result is true if and only if the argument is not null and is an AlphaComposite object that has the same compositing rule and alpha value as this object.
obj - the Object to test for equality - java.lang.Object
returns: true if obj equals this
AlphaComposite; false otherwise. - boolean
Determines whether the specified object is equal to this AlphaComposite. The result is true if and only if the argument is not null and is an AlphaComposite object that has the same compositing rule and alpha value as this object. obj - the Object to test for equality - `java.lang.Object` returns: true if obj equals this AlphaComposite; false otherwise. - `boolean`
get-alphaclj
(get-alpha this)Returns the alpha value of this AlphaComposite. If this AlphaComposite does not have an alpha value, 1.0 is returned.
returns: the alpha value of this AlphaComposite. - float
Returns the alpha value of this AlphaComposite. If this AlphaComposite does not have an alpha value, 1.0 is returned. returns: the alpha value of this AlphaComposite. - `float`
get-ruleclj
(get-rule this)Returns the compositing rule of this AlphaComposite.
returns: the compositing rule of this AlphaComposite. - int
Returns the compositing rule of this AlphaComposite. returns: the compositing rule of this AlphaComposite. - `int`
hash-codeclj
(hash-code this)Returns the hashcode for this composite.
returns: a hash code for this composite. - int
Returns the hashcode for this composite. returns: a hash code for this composite. - `int`
cljdoc is a website building & hosting documentation for Clojure/Script libraries
× close