- All Implemented Interfaces:
- Composite
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 BufferedImageclass, 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_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.
- 
 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 SRCmode 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, round-off 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:
- 
Field SummaryFieldsModifier and TypeFieldDescriptionstatic final AlphaCompositeAlphaCompositeobject that implements the opaque CLEAR rule with an alpha of 1.0f.static final intBoth the color and the alpha of the destination are cleared (Porter-Duff Clear rule).static final AlphaCompositeAlphaCompositeobject that implements the opaque DST rule with an alpha of 1.0f.static final intThe destination is left untouched (Porter-Duff Destination rule).static final intThe 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 final intThe part of the destination lying inside of the source replaces the destination (Porter-Duff Destination In Source rule).static final intThe part of the destination lying outside of the source replaces the destination (Porter-Duff Destination Held Out By Source rule).static final intThe destination is composited over the source and the result replaces the destination (Porter-Duff Destination Over Source rule).static final AlphaCompositeAlphaCompositeobject that implements the opaque DST_ATOP rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque DST_IN rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque DST_OUT rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque DST_OVER rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque SRC rule with an alpha of 1.0f.static final intThe source is copied to the destination (Porter-Duff Source rule).static final intThe part of the source lying inside of the destination is composited onto the destination (Porter-Duff Source Atop Destination rule).static final intThe part of the source lying inside of the destination replaces the destination (Porter-Duff Source In Destination rule).static final intThe part of the source lying outside of the destination replaces the destination (Porter-Duff Source Held Out By Destination rule).static final intThe source is composited over the destination (Porter-Duff Source Over Destination rule).static final AlphaCompositeAlphaCompositeobject that implements the opaque SRC_ATOP rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque SRC_IN rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque SRC_OUT rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque SRC_OVER rule with an alpha of 1.0f.static final AlphaCompositeAlphaCompositeobject that implements the opaque XOR rule with an alpha of 1.0f.static final intThe 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).
- 
Method SummaryModifier and TypeMethodDescriptioncreateContext(ColorModel srcColorModel, ColorModel dstColorModel, RenderingHints hints) Creates a context for the compositing operation.derive(float alpha) Returns a similarAlphaCompositeobject that uses the specified alpha value.derive(int rule) Returns a similarAlphaCompositeobject that uses the specified compositing rule.booleanDetermines whether the specified object is equal to thisAlphaComposite.floatgetAlpha()Returns the alpha value of thisAlphaComposite.static AlphaCompositegetInstance(int rule) Creates anAlphaCompositeobject with the specified rule.static AlphaCompositegetInstance(int rule, float alpha) Creates anAlphaCompositeobject with the specified rule and the constant alpha to multiply with the alpha of the source.intgetRule()Returns the compositing rule of thisAlphaComposite.inthashCode()Returns the hashcode for this composite.
- 
Field Details- 
CLEARBoth 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:
 
- 
SRCThe 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:
 
- 
DSTThe destination is left untouched (Porter-Duff Destination rule).Fs = 0 and Fd = 1, thus: Ar = Ad Cr = Cd - Since:
- 1.4
- See Also:
 
- 
SRC_OVERThe 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:
 
- 
DST_OVERThe 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:
 
- 
SRC_INThe 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:
 
- 
DST_INThe 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:
 
- 
SRC_OUTThe 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:
 
- 
DST_OUTThe 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:
 
- 
SRC_ATOPThe 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:
 
- 
DST_ATOPThe 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:
 
- 
XORThe 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:
 
- 
ClearAlphaCompositeobject that implements the opaque CLEAR rule with an alpha of 1.0f.- See Also:
 
- 
SrcAlphaCompositeobject that implements the opaque SRC rule with an alpha of 1.0f.- See Also:
 
- 
DstAlphaCompositeobject that implements the opaque DST rule with an alpha of 1.0f.- Since:
- 1.4
- See Also:
 
- 
SrcOverAlphaCompositeobject that implements the opaque SRC_OVER rule with an alpha of 1.0f.- See Also:
 
- 
DstOverAlphaCompositeobject that implements the opaque DST_OVER rule with an alpha of 1.0f.- See Also:
 
- 
SrcInAlphaCompositeobject that implements the opaque SRC_IN rule with an alpha of 1.0f.- See Also:
 
- 
DstInAlphaCompositeobject that implements the opaque DST_IN rule with an alpha of 1.0f.- See Also:
 
- 
SrcOutAlphaCompositeobject that implements the opaque SRC_OUT rule with an alpha of 1.0f.- See Also:
 
- 
DstOutAlphaCompositeobject that implements the opaque DST_OUT rule with an alpha of 1.0f.- See Also:
 
- 
SrcAtopAlphaCompositeobject that implements the opaque SRC_ATOP rule with an alpha of 1.0f.- Since:
- 1.4
- See Also:
 
- 
DstAtopAlphaCompositeobject that implements the opaque DST_ATOP rule with an alpha of 1.0f.- Since:
- 1.4
- See Also:
 
- 
XorAlphaCompositeobject that implements the opaque XOR rule with an alpha of 1.0f.- Since:
- 1.4
- See Also:
 
 
- 
- 
Method Details- 
getInstanceCreates anAlphaCompositeobject with the specified rule.
- 
getInstanceCreates anAlphaCompositeobject 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.- alphamust be a floating point number in the inclusive range [0.0, 1.0].
- Returns:
- the AlphaCompositeobject created
- Throws:
- IllegalArgumentException- if- alphais less than 0.0 or greater than 1.0, or if- ruleis 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
 
- 
createContextpublic 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:
- createContextin interface- Composite
- Parameters:
- srcColorModel- the- ColorModelof the source
- dstColorModel- the- ColorModelof the destination
- hints- the hint that the context object uses to choose between rendering alternatives
- Returns:
- the CompositeContextobject to be used to perform compositing operations.
 
- 
getAlphapublic float getAlpha()Returns the alpha value of thisAlphaComposite. If thisAlphaCompositedoes not have an alpha value, 1.0 is returned.- Returns:
- the alpha value of this AlphaComposite.
 
- 
getRulepublic int getRule()Returns the compositing rule of thisAlphaComposite.- Returns:
- the compositing rule of this AlphaComposite.
 
- 
deriveReturns a similarAlphaCompositeobject 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 AlphaCompositeobject derived from this object that uses the specified compositing rule.
- Throws:
- IllegalArgumentException- if- ruleis 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
 
- 
deriveReturns a similarAlphaCompositeobject 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.- alphamust be a floating point number in the inclusive range [0.0, 1.0].
- Returns:
- an AlphaCompositeobject derived from this object that uses the specified alpha value.
- Throws:
- IllegalArgumentException- if- alphais less than 0.0 or greater than 1.0
- Since:
- 1.6
 
- 
hashCodepublic int hashCode()Returns the hashcode for this composite.
- 
equalsDetermines whether the specified object is equal to thisAlphaComposite.The result is trueif and only if the argument is notnulland is anAlphaCompositeobject that has the same compositing rule and alpha value as this object.
 
-