All Implemented Interfaces:
AlphaCompositeclass 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.
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 As the alpha component of the source pixel Cs a color component of the source pixel in premultiplied form Ad the alpha component of the destination pixel Cd a color component of the destination pixel in premultiplied form Fs the fraction of the source pixel that contributes to the output Fd the fraction of the destination pixel that contributes to the output Ar the alpha component of the result Cr a 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.
the description for
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 Adr the 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
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
This leads to the following equation for producing the alpha
used in the Porter and Duff blending equation:
As = Asr * AacAll of the raw source color components need to be multiplied by the alpha in the
AlphaCompositeinstance. 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)
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.
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.
For performance reasons, it is preferrable that
Raster objects passed to the
method of a CompositeContext object created by the
AlphaComposite class have premultiplied data.
If either the source
or the destination
is not premultiplied, however,
appropriate conversions are performed before and after the compositing
BufferedImageclass, do not store alpha values for their pixels. Such sources supply an alpha of 1.0 for all of their pixels.
BufferedImage.TYPE_BYTE_INDEXEDshould 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.
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.
(A, R, G, B) = (0x01, 0xb0, 0x00, 0x00)
If integer math were being used and this value were being
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.
|public static final int||CLEAR||Both the color and the alpha of the destination are cleared
(Porter-Duff Clear rule).
Neither the source nor the destination is used as input.
Fs = 0 and Fd = 0, thus:
Ar = 0 Cr = 0
|public static final int||SRC||The source is copied to the destination
(Porter-Duff Source rule).
The destination is not used as input.
Fs = 1 and Fd = 0, thus:
Ar = As Cr = Cs
|public static final int||DST||The destination is left untouched
(Porter-Duff Destination rule).
Fs = 0 and Fd = 1, thus:
Ar = Ad Cr = Cd
|public static final int||SRC_OVER||The source is composited over the destination
(Porter-Duff Source Over Destination rule).
Fs = 1 and Fd = (1-As), thus:
Ar = As + Ad*(1-As) Cr = Cs + Cd*(1-As)
|public static final int||DST_OVER||The destination is composited over the source and
the result replaces the destination
(Porter-Duff Destination Over Source rule).
Fs = (1-Ad) and Fd = 1, thus:
Ar = As*(1-Ad) + Ad Cr = Cs*(1-Ad) + Cd
|public static final int||SRC_IN||The part of the source lying inside of the destination replaces
(Porter-Duff Source In Destination rule).
Fs = Ad and Fd = 0, thus:
Ar = As*Ad Cr = Cs*Ad
|public static final int||DST_IN||The part of the destination lying inside of the source
replaces the destination
(Porter-Duff Destination In Source rule).
Fs = 0 and Fd = As, thus:
Ar = Ad*As Cr = Cd*As
|public static final int||SRC_OUT||The part of the source lying outside of the destination
replaces the destination
(Porter-Duff Source Held Out By Destination rule).
Fs = (1-Ad) and Fd = 0, thus:
Ar = As*(1-Ad) Cr = Cs*(1-Ad)
|public static final int||DST_OUT||The part of the destination lying outside of the source
replaces the destination
(Porter-Duff Destination Held Out By Source rule).
Fs = 0 and Fd = (1-As), thus:
Ar = Ad*(1-As) Cr = Cd*(1-As)
|public static final int||SRC_ATOP||The part of the source lying inside of the destination
is composited onto the destination
(Porter-Duff Source Atop Destination rule).
Fs = Ad and Fd = (1-As), thus:
Ar = As*Ad + Ad*(1-As) = Ad Cr = Cs*Ad + Cd*(1-As)
|public static final int||DST_ATOP||The part of the destination lying inside of the source
is composited over the source and replaces the destination
(Porter-Duff Destination Atop Source rule).
Fs = (1-Ad) and Fd = As, thus:
Ar = As*(1-Ad) + Ad*As = As Cr = Cs*(1-Ad) + Cd*As
|public static final int||XOR||The part of the source that lies outside of the destination
is combined with the part of the destination that lies outside
of the source
(Porter-Duff Source Xor Destination rule).
Fs = (1-Ad) and Fd = (1-As), thus:
Ar = As*(1-Ad) + Ad*(1-As) Cr = Cs*(1-Ad) + Cd*(1-As)
|public static final AlphaComposite||Clear|
|public static final AlphaComposite||Src|
|public static final AlphaComposite||Dst|
|public static final AlphaComposite||SrcOver|
|public static final AlphaComposite||DstOver|
|public static final AlphaComposite||SrcIn|
|public static final AlphaComposite||DstIn|
|public static final AlphaComposite||SrcOut|
|public static final AlphaComposite||DstOut|
|public static final AlphaComposite||SrcAtop|
|public static final AlphaComposite||DstAtop|
|public static final AlphaComposite||Xor|
|Method from java.awt.AlphaComposite Summary:|
|createContext, derive, derive, equals, getAlpha, getInstance, getInstance, getRule, hashCode|
|Methods from java.lang.Object:|
|clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait|
|Method from java.awt.AlphaComposite Detail:|
public CompositeContext createContext(ColorModel srcColorModel, ColorModel dstColorModel, RenderingHints hints)
public AlphaComposite derive(int rule)
The result is
public float getAlpha()
public static AlphaComposite getInstance(int rule)
public static AlphaComposite getInstance(int rule, float alpha)
public int getRule()
public int hashCode()