Class AlphaComposite

All Implemented Interfaces:
Composite

public final class AlphaComposite
extends Object
implements 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
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. 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
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

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.
See Also:
Composite, CompositeContext

Fields

Modifier and Type Field and Description
static AlphaComposite Clear

AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f.
static int CLEAR

Both the color and the alpha of the destination are cleared (Porter-Duff Clear rule).
static AlphaComposite Dst

AlphaComposite object that implements the opaque DST rule with an alpha of 1.0f.
static int DST

The destination is left untouched (Porter-Duff Destination rule).
static 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).
static int DST_IN

The part of the destination lying inside of the source replaces the destination (Porter-Duff Destination In Source rule).
static int DST_OUT

The part of the destination lying outside of the source replaces the destination (Porter-Duff Destination Held Out By Source rule).
static int DST_OVER

The destination is composited over the source and the result replaces the destination (Porter-Duff Destination Over Source rule).
static AlphaComposite DstAtop

AlphaComposite object that implements the opaque DST_ATOP rule with an alpha of 1.0f.
static AlphaComposite DstIn

AlphaComposite object that implements the opaque DST_IN rule with an alpha of 1.0f.
static AlphaComposite DstOut

AlphaComposite object that implements the opaque DST_OUT rule with an alpha of 1.0f.
static AlphaComposite DstOver

AlphaComposite object that implements the opaque DST_OVER rule with an alpha of 1.0f.
static AlphaComposite Src

AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f.
static int SRC

The source is copied to the destination (Porter-Duff Source rule).
static int SRC_ATOP

The part of the source lying inside of the destination is composited onto the destination (Porter-Duff Source Atop Destination rule).
static int SRC_IN

The part of the source lying inside of the destination replaces the destination (Porter-Duff Source In Destination rule).
static int SRC_OUT

The part of the source lying outside of the destination replaces the destination (Porter-Duff Source Held Out By Destination rule).
static int SRC_OVER

The source is composited over the destination (Porter-Duff Source Over Destination rule).
static AlphaComposite SrcAtop

AlphaComposite object that implements the opaque SRC_ATOP rule with an alpha of 1.0f.
static AlphaComposite SrcIn

AlphaComposite object that implements the opaque SRC_IN rule with an alpha of 1.0f.
static AlphaComposite SrcOut

AlphaComposite object that implements the opaque SRC_OUT rule with an alpha of 1.0f.
static AlphaComposite SrcOver

AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f.
static AlphaComposite Xor

AlphaComposite object that implements the opaque XOR rule with an alpha of 1.0f.
static 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).

Methods

Modifier and Type Method and Description
CompositeContext createContext(ColorModel srcColorModel, ColorModel dstColorModel, RenderingHints hints)

Creates a context for the compositing operation.
AlphaComposite derive(float alpha)

Returns a similar AlphaComposite object that uses the specified alpha value.
AlphaComposite derive(int rule)

Returns a similar AlphaComposite object that uses the specified compositing rule.
boolean equals(Object obj)

Determines whether the specified object is equal to this AlphaComposite.
float getAlpha()

Returns the alpha value of this AlphaComposite.
static AlphaComposite getInstance(int rule)

Creates an AlphaComposite object with the specified rule.
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.
int getRule()

Returns the compositing rule of this AlphaComposite.
int hashCode()

Returns the hashcode for this composite.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

Fields

CLEAR

@Native
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
See Also:
Constant Field Values

SRC

@Native
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
See Also:
Constant Field Values

DST

@Native
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
See Also:
Constant Field Values

SRC_OVER

@Native
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)
See Also:
Constant Field Values

DST_OVER

@Native
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
See Also:
Constant Field Values

SRC_IN

@Native
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
See Also:
Constant Field Values

DST_IN

@Native
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
See Also:
Constant Field Values

SRC_OUT

@Native
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)
See Also:
Constant Field Values

DST_OUT

@Native
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)
See Also:
Constant Field Values

SRC_ATOP

@Native
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
See Also:
Constant Field Values

DST_ATOP

@Native
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
See Also:
Constant Field Values

XOR

@Native
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
See Also:
Constant Field Values

Clear

public static final AlphaComposite Clear

AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f.

See Also:
CLEAR

Src

public static final AlphaComposite Src

AlphaComposite object that implements the opaque SRC rule with an alpha of 1.0f.

See Also:
SRC

Dst

public static final AlphaComposite Dst

AlphaComposite object that implements the opaque DST rule with an alpha of 1.0f.

Since:
1.4
See Also:
DST

SrcOver

public static final AlphaComposite SrcOver

AlphaComposite object that implements the opaque SRC_OVER rule with an alpha of 1.0f.

See Also:
SRC_OVER

DstOver

public static final AlphaComposite DstOver

AlphaComposite object that implements the opaque DST_OVER rule with an alpha of 1.0f.

See Also:
DST_OVER

SrcIn

public static final AlphaComposite SrcIn

AlphaComposite object that implements the opaque SRC_IN rule with an alpha of 1.0f.

See Also:
SRC_IN

DstIn

public static final AlphaComposite DstIn

AlphaComposite object that implements the opaque DST_IN rule with an alpha of 1.0f.

See Also:
DST_IN

SrcOut

public static final AlphaComposite SrcOut

AlphaComposite object that implements the opaque SRC_OUT rule with an alpha of 1.0f.

See Also:
SRC_OUT

DstOut

public static final AlphaComposite DstOut

AlphaComposite object that implements the opaque DST_OUT rule with an alpha of 1.0f.

See Also:
DST_OUT

SrcAtop

public static final AlphaComposite SrcAtop

AlphaComposite object that implements the opaque SRC_ATOP rule with an alpha of 1.0f.

Since:
1.4
See Also:
SRC_ATOP

DstAtop

public static final AlphaComposite DstAtop

AlphaComposite object that implements the opaque DST_ATOP rule with an alpha of 1.0f.

Since:
1.4
See Also:
DST_ATOP

Xor

public static final AlphaComposite Xor

AlphaComposite object that implements the opaque XOR rule with an alpha of 1.0f.

Since:
1.4
See Also:
XOR

Methods

getInstance

public static AlphaComposite getInstance(int rule)

Creates an AlphaComposite object with the specified rule.

Parameters:
rule - the compositing rule
Throws:
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

getInstance

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.

Parameters:
rule - the compositing rule
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].
Throws:
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

createContext

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.

Specified by:
createContext in interface Composite
Parameters:
srcColorModel - the ColorModel of the source
dstColorModel - the ColorModel of the destination
hints - the hint that the context object uses to choose between rendering alternatives
Returns:
the CompositeContext object to be used to perform compositing operations.

getAlpha

public float getAlpha()

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.

getRule

public int getRule()

Returns the compositing rule of this AlphaComposite.

Returns:
the compositing rule of this AlphaComposite.

derive

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.

Parameters:
rule - the compositing rule
Returns:
an AlphaComposite object derived from this object that uses the specified compositing rule.
Throws:
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
Since:
1.6

derive

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.

Parameters:
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].
Returns:
an AlphaComposite object derived from this object that uses the specified alpha value.
Throws:
IllegalArgumentException - if alpha is less than 0.0 or greater than 1.0
Since:
1.6

hashCode

public int hashCode()

Returns the hashcode for this composite.

Overrides:
hashCode in class Object
Returns:
a hash code for this composite.
See Also:
Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)

equals

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.

Overrides:
equals in class Object
Parameters:
obj - the Object to test for equality
Returns:
true if obj equals this AlphaComposite; false otherwise.
See Also:
Object.hashCode(), HashMap

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