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java.awt
Class AlphaComposite

java.lang.Object
  |
  +--java.awt.AlphaComposite

public final class AlphaComposite

extends Object

implements Composite

This AlphaComposite class implements the basic alpha compositing rules for combining source and destination pixels to achieve blending and transparency effects with graphics and images. The rules implemented by this class are a subset of the Porter-Duff rules described in T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84, 253-259.

If any input does not have an alpha channel, an alpha value of 1.0, which is completely opaque, is assumed for all pixels. A constant alpha value can also be specified to be multiplied with the alpha value of the source pixels.

The following abbreviations are used in the description of the rules:

• Cs = one of the color components of the source pixel.
• Cd = one of the color components of the destination pixel.
• As = alpha component of the source pixel.
• Ad = alpha component of the destination pixel.
• Fs = fraction of the source pixel that contributes to the output.
• Fd = fraction of the input destination pixel that contributes to the output.

The color and alpha components produced by the compositing operation are calculated as follows:

 	Cd = Cs*Fs + Cd*Fd
 	Ad = As*Fs + Ad*Fd

where Fs and Fd are specified by each rule. The above equations assume that both source and destination pixels have the color components premultiplied by the alpha component. Similarly, the equations expressed in the definitions of compositing rules below assume premultiplied alpha.

For performance reasons, it is preferrable that Rasters passed to the compose method of a CompositeContext object created by the AlphaComposite class have premultiplied data. If either source or destination Rasters are not premultiplied, however, appropriate conversions are performed before and after the compositing operation.

The alpha resulting from the compositing operation is stored in the destination if the destination has an alpha channel. Otherwise, the resulting color is divided by the resulting alpha before being stored in the destination and the alpha is discarded. If the alpha value is 0.0, the color values are set to 0.0.

See Also:

Composite, CompositeContext

Field Summary

static AlphaComposite	Clear AlphaComposite object that implements the opaque CLEAR rule with an alpha of 1.0f.
static int	CLEAR Porter-Duff Clear rule.
static int	DST_IN Porter-Duff Destination In Source rule.
static int	DST_OUT Porter-Duff Destination Held Out By Source rule.
static int	DST_OVER Porter-Duff Destination Over Source rule.
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 Porter-Duff Source rule.
static int	SRC_IN Porter-Duff Source In Destination rule.
static int	SRC_OUT Porter-Duff Source Held Out By Destination rule.
static int	SRC_OVER Porter-Duff Source Over Destination rule.
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.

Method Summary

CompositeContext	createContext(ColorModel srcColorModel, ColorModel dstColorModel, RenderingHints hints) Creates a context for the compositing operation.
boolean	equals(Object obj) Tests if the specified Object is equal to this AlphaComposite object.
float	getAlpha() Returns the alpha value of thisAlphaComposite.
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

Field Detail

CLEAR

public static final int CLEAR

Porter-Duff Clear rule. Both the color and the alpha of the destination are cleared. Neither the source nor the destination is used as input.

Fs = 0 and Fd = 0, thus:

 	Cd = 0
 	Ad = 0

SRC

public static final int SRC

Porter-Duff Source rule. The source is copied to the destination. The destination is not used as input.

Fs = 1 and Fd = 0, thus:

 	Cd = Cs
 	Ad = As

SRC_OVER

public static final int SRC_OVER

Porter-Duff Source Over Destination rule. The source is composited over the destination.

Fs = 1 and Fd = (1-As), thus:

 	Cd = Cs + Cd*(1-As)
 	Ad = As + Ad*(1-As)

DST_OVER

public static final int DST_OVER

Porter-Duff Destination Over Source rule. The destination is composited over the source and the result replaces the destination.

Fs = (1-Ad) and Fd = 1, thus:

 	Cd = Cs*(1-Ad) + Cd
 	Ad = As*(1-Ad) + Ad

SRC_IN

public static final int SRC_IN

Porter-Duff Source In Destination rule. The part of the source lying inside of the destination replaces the destination.

Fs = Ad and Fd = 0, thus:

 	Cd = Cs*Ad
 	Ad = As*Ad

DST_IN

public static final int DST_IN

Porter-Duff Destination In Source rule. The part of the destination lying inside of the source replaces the destination.

Fs = 0 and Fd = As, thus:

 	Cd = Cd*As
 	Ad = Ad*As

SRC_OUT

public static final int SRC_OUT

Porter-Duff Source Held Out By Destination rule. The part of the source lying outside of the destination replaces the destination.

Fs = (1-Ad) and Fd = 0, thus:

 	Cd = Cs*(1-Ad)
 	Ad = As*(1-Ad)

DST_OUT

public static final int DST_OUT

Porter-Duff Destination Held Out By Source rule. The part of the destination lying outside of the source replaces the destination.

Fs = 0 and Fd = (1-As), thus:

 	Cd = Cd*(1-As)
 	Ad = Ad*(1-As)

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

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

Method Detail

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, SRC_OVER, DST_OVER, SRC_IN, DST_IN, SRC_OUT, or DST_OUT

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].

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

Returns:

the CompositeContext object to be used to perform compositing operations.

getAlpha

public float getAlpha()

Returns the alpha value of thisAlphaComposite. 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.

hashCode

public int hashCode()

Returns the hashcode for this composite.

Overrides:

hashCode in class Object

Returns:

a hash code for this composite.

equals

public boolean equals(Object obj)

Tests if the specified Object is equal to this AlphaComposite object.

Overrides:

equals in class Object

Parameters:

obj - the Object to test for equality

Returns:

true if obj equals this AlphaComposite; false otherwise.