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java.awt
public final class: AlphaComposite [javadoc | source]
java.lang.Object
   java.awt.AlphaComposite

All Implemented Interfaces:
    Composite

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 preferrable 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

Field Summary
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
    since: 1.4 -
 
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 the destination (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)
    since: 1.4 -
 
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
    since: 1.4 -
 
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)
    since: 1.4 -
 
public static final  AlphaComposite Clear    AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f. 
public static final  AlphaComposite Src    AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f. 
public static final  AlphaComposite Dst    AlphaComposite object that implements the opaque DST rule with an alpha of 1.0f.
    Also see:
    DST
    since: 1.4 -
 
public static final  AlphaComposite SrcOver    AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f. 
public static final  AlphaComposite DstOver    AlphaComposite object that implements the opaque DST_OVER rule with an alpha of 1.0f. 
public static final  AlphaComposite SrcIn    AlphaComposite object that implements the opaque SRC_IN rule with an alpha of 1.0f. 
public static final  AlphaComposite DstIn    AlphaComposite object that implements the opaque DST_IN rule with an alpha of 1.0f. 
public static final  AlphaComposite SrcOut    AlphaComposite object that implements the opaque SRC_OUT rule with an alpha of 1.0f. 
public static final  AlphaComposite DstOut    AlphaComposite object that implements the opaque DST_OUT rule with an alpha of 1.0f. 
public static final  AlphaComposite SrcAtop    AlphaComposite object that implements the opaque SRC_ATOP rule with an alpha of 1.0f. 
public static final  AlphaComposite DstAtop    AlphaComposite object that implements the opaque DST_ATOP rule with an alpha of 1.0f. 
public static final  AlphaComposite Xor    AlphaComposite object that implements the opaque XOR rule with an alpha of 1.0f.
    Also see:
    XOR
    since: 1.4 -
 
 float extraAlpha     
 int rule     
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) 
    Creates a context for the compositing operation. The context contains state that is used in performing the compositing operation.
 public AlphaComposite derive(int 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.
 public AlphaComposite derive(float alpha) 
    Returns a similar AlphaComposite object that uses the specified alpha value. If this object already has the specified alpha value, this object is returned.
 public boolean equals(Object 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.

 public float getAlpha() 
    Returns the alpha value of this AlphaComposite. If this AlphaComposite does not have an alpha value, 1.0 is returned.
 public static AlphaComposite getInstance(int rule) 
    Creates an AlphaComposite object with the specified rule.
 public static AlphaComposite getInstance(int rule,
    float 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.
 public int getRule() 
    Returns the compositing rule of this AlphaComposite.
 public int hashCode() 
    Returns the hashcode for this composite.