AlphaComposite.java revision 0
2362N/A * Copyright 1997-2006 Sun Microsystems, Inc. All Rights Reserved. 0N/A * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 0N/A * This code is free software; you can redistribute it and/or modify it 0N/A * under the terms of the GNU General Public License version 2 only, as 0N/A * published by the Free Software Foundation. Sun designates this 0N/A * particular file as subject to the "Classpath" exception as provided 0N/A * by Sun in the LICENSE file that accompanied this code. 0N/A * This code is distributed in the hope that it will be useful, but WITHOUT 0N/A * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 0N/A * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 0N/A * version 2 for more details (a copy is included in the LICENSE file that 0N/A * accompanied this code). 0N/A * You should have received a copy of the GNU General Public License version 0N/A * 2 along with this work; if not, write to the Free Software Foundation, 2362N/A * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 2362N/A * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, 0N/A * CA 95054 USA or visit www.sun.com if you need additional information or 0N/A * have any questions. 0N/A * The <code>AlphaComposite</code> class implements basic alpha 0N/A * compositing rules for combining source and destination colors 0N/A * to achieve blending and transparency effects with graphics and 0N/A * The specific rules implemented by this class are the basic set 0N/A * of 12 rules described in 0N/A * T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84, 0N/A * The rest of this documentation assumes some familiarity with the 0N/A * definitions and concepts outlined in that paper. 0N/A * This class extends the standard equations defined by Porter and 0N/A * Duff to include one additional factor. 0N/A * An instance of the <code>AlphaComposite</code> class can contain 0N/A * an alpha value that is used to modify the opacity or coverage of 0N/A * every source pixel before it is used in the blending equations. 0N/A * It is important to note that the equations defined by the Porter 0N/A * and Duff paper are all defined to operate on color components 0N/A * that are premultiplied by their corresponding alpha components. 0N/A * Since the <code>ColorModel</code> and <code>Raster</code> classes 0N/A * allow the storage of pixel data in either premultiplied or 0N/A * non-premultiplied form, all input data must be normalized into 0N/A * premultiplied form before applying the equations and all results 0N/A * might need to be adjusted back to the form required by the destination 0N/A * before the pixel values are stored. 0N/A * Also note that this class defines only the equations 0N/A * for combining color and alpha values in a purely mathematical 0N/A * sense. The accurate application of its equations depends 0N/A * on the way the data is retrieved from its sources and stored 0N/A * in its destinations. 0N/A * See <a href="#caveats">Implementation Caveats</a> 0N/A * for further information. 0N/A * The following factors are used in the description of the blending 0N/A * equation in the Porter and Duff paper: 0N/A * <table summary="layout"> 0N/A * <tr><th align=left>Factor <th align=left>Definition 0N/A * <tr><td><em>A<sub>s</sub></em><td>the alpha component of the source pixel 0N/A * <tr><td><em>C<sub>s</sub></em><td>a color component of the source pixel in premultiplied form 0N/A * <tr><td><em>A<sub>d</sub></em><td>the alpha component of the destination pixel 0N/A * <tr><td><em>C<sub>d</sub></em><td>a color component of the destination pixel in premultiplied form 0N/A * <tr><td><em>F<sub>s</sub></em><td>the fraction of the source pixel that contributes to the output 0N/A * <tr><td><em>F<sub>d</sub></em><td>the fraction of the destination pixel that contributes 0N/A * <tr><td><em>A<sub>r</sub></em><td>the alpha component of the result 0N/A * <tr><td><em>C<sub>r</sub></em><td>a color component of the result in premultiplied form 0N/A * Using these factors, Porter and Duff define 12 ways of choosing 0N/A * the blending factors <em>F<sub>s</sub></em> and <em>F<sub>d</sub></em> to 0N/A * produce each of 12 desirable visual effects. 0N/A * The equations for determining <em>F<sub>s</sub></em> and <em>F<sub>d</sub></em> 0N/A * are given in the descriptions of the 12 static fields 0N/A * that specify visual effects. 0N/A * the description for 0N/A * <a href="#SRC_OVER"><code>SRC_OVER</code></a> 0N/A * specifies that <em>F<sub>s</sub></em> = 1 and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>). 0N/A * Once a set of equations for determining the blending factors is 0N/A * known they can then be applied to each pixel to produce a result 0N/A * using the following set of equations: 0N/A * <em>F<sub>s</sub></em> = <em>f</em>(<em>A<sub>d</sub></em>) 0N/A * <em>F<sub>d</sub></em> = <em>f</em>(<em>A<sub>s</sub></em>) 0N/A * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*<em>F<sub>s</sub></em> + <em>A<sub>d</sub></em>*<em>F<sub>d</sub></em> 0N/A * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*<em>F<sub>s</sub></em> + <em>C<sub>d</sub></em>*<em>F<sub>d</sub></em></pre> 0N/A * The following factors will be used to discuss our extensions to 0N/A * the blending equation in the Porter and Duff paper: 0N/A * <table summary="layout"> 0N/A * <tr><th align=left>Factor <th align=left>Definition 0N/A * <tr><td><em>C<sub>sr</sub></em> <td>one of the raw color components of the source pixel 0N/A * <tr><td><em>C<sub>dr</sub></em> <td>one of the raw color components of the destination pixel 0N/A * <tr><td><em>A<sub>ac</sub></em> <td>the "extra" alpha component from the AlphaComposite instance 0N/A * <tr><td><em>A<sub>sr</sub></em> <td>the raw alpha component of the source pixel 0N/A * <tr><td><em>A<sub>dr</sub></em><td>the raw alpha component of the destination pixel 0N/A * <tr><td><em>A<sub>df</sub></em> <td>the final alpha component stored in the destination 0N/A * <tr><td><em>C<sub>df</sub></em> <td>the final raw color component stored in the destination 0N/A * <h3>Preparing Inputs</h3> 0N/A * The <code>AlphaComposite</code> class defines an additional alpha 0N/A * value that is applied to the source alpha. 0N/A * This value is applied as if an implicit SRC_IN rule were first 0N/A * applied to the source pixel against a pixel with the indicated 0N/A * alpha by multiplying both the raw source alpha and the raw 0N/A * source colors by the alpha in the <code>AlphaComposite</code>. 0N/A * This leads to the following equation for producing the alpha 0N/A * used in the Porter and Duff blending equation: 0N/A * <em>A<sub>s</sub></em> = <em>A<sub>sr</sub></em> * <em>A<sub>ac</sub></em> </pre> 0N/A * All of the raw source color components need to be multiplied 0N/A * by the alpha in the <code>AlphaComposite</code> instance. 0N/A * Additionally, if the source was not in premultiplied form 0N/A * then the color components also need to be multiplied by the 0N/A * Thus, the equation for producing the source color components 0N/A * for the Porter and Duff equation depends on whether the source 0N/A * pixels are premultiplied or not: 0N/A * <em>C<sub>s</sub></em> = <em>C<sub>sr</sub></em> * <em>A<sub>sr</sub></em> * <em>A<sub>ac</sub></em> (if source is not premultiplied) 0N/A * <em>C<sub>s</sub></em> = <em>C<sub>sr</sub></em> * <em>A<sub>ac</sub></em> (if source is premultiplied) </pre> 0N/A * No adjustment needs to be made to the destination alpha: 0N/A * <em>A<sub>d</sub></em> = <em>A<sub>dr</sub></em> </pre> 0N/A * The destination color components need to be adjusted only if 0N/A * they are not in premultiplied form: 0N/A * <em>C<sub>d</sub></em> = <em>C<sub>dr</sub></em> * <em>A<sub>d</sub></em> (if destination is not premultiplied) 0N/A * <em>C<sub>d</sub></em> = <em>C<sub>dr</sub></em> (if destination is premultiplied) </pre> 0N/A * <h3>Applying the Blending Equation</h3> 0N/A * The adjusted <em>A<sub>s</sub></em>, <em>A<sub>d</sub></em>, 0N/A * <em>C<sub>s</sub></em>, and <em>C<sub>d</sub></em> are used in the standard 0N/A * Porter and Duff equations to calculate the blending factors 0N/A * <em>F<sub>s</sub></em> and <em>F<sub>d</sub></em> and then the resulting 0N/A * premultiplied components <em>A<sub>r</sub></em> and <em>C<sub>r</sub></em>. 0N/A * <h3>Preparing Results</h3> 0N/A * The results only need to be adjusted if they are to be stored 0N/A * back into a destination buffer that holds data that is not 2557N/A * premultiplied, using the following equations: 0N/A * <em>A<sub>df</sub></em> = <em>A<sub>r</sub></em> 0N/A * <em>C<sub>df</sub></em> = <em>C<sub>r</sub></em> (if dest is premultiplied) 0N/A * <em>C<sub>df</sub></em> = <em>C<sub>r</sub></em> / <em>A<sub>r</sub></em> (if dest is not premultiplied) </pre> 0N/A * Note that since the division is undefined if the resulting alpha 0N/A * is zero, the division in that case is omitted to avoid the "divide 0N/A * by zero" and the color components are left as 0N/A * <h3>Performance Considerations</h3> 0N/A * For performance reasons, it is preferrable that 0N/A * <code>Raster</code> objects passed to the <code>compose</code> 0N/A * method of a {@link CompositeContext} object created by the 0N/A * <code>AlphaComposite</code> class have premultiplied data. 0N/A * If either the source <code>Raster</code> 0N/A * or the destination <code>Raster</code> 0N/A * is not premultiplied, however, 0N/A * appropriate conversions are performed before and after the compositing 0N/A * <h3><a name="caveats">Implementation Caveats</a></h3> 0N/A * Many sources, such as some of the opaque image types listed 0N/A * in the <code>BufferedImage</code> class, do not store alpha values 0N/A * for their pixels. Such sources supply an alpha of 1.0 for 0N/A * all of their pixels. 0N/A * Many destinations also have no place to store the alpha values 0N/A * that result from the blending calculations performed by this class. 0N/A * Such destinations thus implicitly discard the resulting 0N/A * alpha values that this class produces. 0N/A * It is recommended that such destinations should treat their stored 0N/A * color values as non-premultiplied and divide the resulting color 0N/A * values by the resulting alpha value before storing the color 0N/A * 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. * that does not separately store * color components is not a * good candidate for any type of translucent blending. * For example, <code>BufferedImage.TYPE_BYTE_INDEXED</code> * 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^<em>n</em>-1 (where <em>n</em> 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 * 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)</pre> * If integer math were being used and this value were being * <a href="#SRC"><code>SRC</code></a> * 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)</pre> * 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 * (A, R, G, B) = (0x01, 0xff, 0x00, 0x00)</pre> * (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 * (A, R, G, B) = (0x01, 0x01, 0x00, 0x00)</pre> * 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. * 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. * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = 0, thus: * <em>A<sub>r</sub></em> = 0 * <em>C<sub>r</sub></em> = 0 public static final int CLEAR =
1;
* The source is copied to the destination * (Porter-Duff Source rule). * The destination is not used as input. * <em>F<sub>s</sub></em> = 1 and <em>F<sub>d</sub></em> = 0, thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em> public static final int SRC =
2;
* The destination is left untouched * (Porter-Duff Destination rule). * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = 1, thus: * <em>A<sub>r</sub></em> = <em>A<sub>d</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>d</sub></em> public static final int DST =
9;
// Note that DST was added in 1.4 so it is numbered out of order... * The source is composited over the destination * (Porter-Duff Source Over Destination rule). * <em>F<sub>s</sub></em> = 1 and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>), thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em> + <em>A<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em> + <em>C<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) * The destination is composited over the source and * the result replaces the destination * (Porter-Duff Destination Over Source rule). * <em>F<sub>s</sub></em> = (1-<em>A<sub>d</sub></em>) and <em>F<sub>d</sub></em> = 1, thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) + <em>A<sub>d</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) + <em>C<sub>d</sub></em> * The part of the source lying inside of the destination replaces * (Porter-Duff Source In Destination rule). * <em>F<sub>s</sub></em> = <em>A<sub>d</sub></em> and <em>F<sub>d</sub></em> = 0, thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*<em>A<sub>d</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*<em>A<sub>d</sub></em> public static final int SRC_IN =
5;
* The part of the destination lying inside of the source * replaces the destination * (Porter-Duff Destination In Source rule). * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = <em>A<sub>s</sub></em>, thus: * <em>A<sub>r</sub></em> = <em>A<sub>d</sub></em>*<em>A<sub>s</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>d</sub></em>*<em>A<sub>s</sub></em> public static final int DST_IN =
6;
* The part of the source lying outside of the destination * replaces the destination * (Porter-Duff Source Held Out By Destination rule). * <em>F<sub>s</sub></em> = (1-<em>A<sub>d</sub></em>) and <em>F<sub>d</sub></em> = 0, thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) public static final int SRC_OUT =
7;
* The part of the destination lying outside of the source * replaces the destination * (Porter-Duff Destination Held Out By Source rule). * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>), thus: * <em>A<sub>r</sub></em> = <em>A<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) * <em>C<sub>r</sub></em> = <em>C<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) public static final int DST_OUT =
8;
// Rule 9 is DST which is defined above where it fits into the // list logically, rather than numerically // public static final int DST = 9; * The part of the source lying inside of the destination * is composited onto the destination * (Porter-Duff Source Atop Destination rule). * <em>F<sub>s</sub></em> = <em>A<sub>d</sub></em> and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>), thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*<em>A<sub>d</sub></em> + <em>A<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) = <em>A<sub>d</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*<em>A<sub>d</sub></em> + <em>C<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) * 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). * <em>F<sub>s</sub></em> = (1-<em>A<sub>d</sub></em>) and <em>F<sub>d</sub></em> = <em>A<sub>s</sub></em>, thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) + <em>A<sub>d</sub></em>*<em>A<sub>s</sub></em> = <em>A<sub>s</sub></em> * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) + <em>C<sub>d</sub></em>*<em>A<sub>s</sub></em> * The part of the source that lies outside of the destination * is combined with the part of the destination that lies outside * (Porter-Duff Source Xor Destination rule). * <em>F<sub>s</sub></em> = (1-<em>A<sub>d</sub></em>) and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>), thus: * <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) + <em>A<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) * <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>*(1-<em>A<sub>d</sub></em>) + <em>C<sub>d</sub></em>*(1-<em>A<sub>s</sub></em>) public static final int XOR =
12;
* <code>AlphaComposite</code> object that implements the opaque CLEAR rule * <code>AlphaComposite</code> object that implements the opaque SRC rule * <code>AlphaComposite</code> object that implements the opaque DST rule * <code>AlphaComposite</code> object that implements the opaque SRC_OVER rule * <code>AlphaComposite</code> object that implements the opaque DST_OVER rule * <code>AlphaComposite</code> object that implements the opaque SRC_IN rule * <code>AlphaComposite</code> object that implements the opaque DST_IN rule * <code>AlphaComposite</code> object that implements the opaque SRC_OUT rule * <code>AlphaComposite</code> object that implements the opaque DST_OUT rule * <code>AlphaComposite</code> object that implements the opaque SRC_ATOP rule * <code>AlphaComposite</code> object that implements the opaque DST_ATOP rule * <code>AlphaComposite</code> object that implements the opaque XOR rule * Creates an <code>AlphaComposite</code> object with the specified rule. * @param rule the compositing rule * @throws IllegalArgumentException if <code>rule</code> is not one of * the following: {@link #CLEAR}, {@link #SRC}, {@link #DST}, * {@link #SRC_OVER}, {@link #DST_OVER}, {@link #SRC_IN}, * {@link #DST_IN}, {@link #SRC_OUT}, {@link #DST_OUT}, * {@link #SRC_ATOP}, {@link #DST_ATOP}, or {@link #XOR} * Creates an <code>AlphaComposite</code> 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 * @param rule the compositing rule * @param alpha the constant alpha to be multiplied with the alpha of * the source. <code>alpha</code> must be a floating point number in the * inclusive range [0.0, 1.0]. * @throws IllegalArgumentException if * <code>alpha</code> is less than 0.0 or greater than 1.0, or if * <code>rule</code> is not one of * the following: {@link #CLEAR}, {@link #SRC}, {@link #DST}, * {@link #SRC_OVER}, {@link #DST_OVER}, {@link #SRC_IN}, * {@link #DST_IN}, {@link #SRC_OUT}, {@link #DST_OUT}, * {@link #SRC_ATOP}, {@link #DST_ATOP}, or {@link #XOR} * Creates a context for the compositing operation. * The context contains state that is used in performing * the compositing operation. * @param srcColorModel the {@link ColorModel} of the source * @param dstColorModel the <code>ColorModel</code> of the destination * @return the <code>CompositeContext</code> object to be used to perform * compositing operations. * Returns the alpha value of this <code>AlphaComposite</code>. If this * <code>AlphaComposite</code> does not have an alpha value, 1.0 is returned. * @return the alpha value of this <code>AlphaComposite</code>. * Returns the compositing rule of this <code>AlphaComposite</code>. * @return the compositing rule of this <code>AlphaComposite</code>. * Returns a similar <code>AlphaComposite</code> object that uses * the specified compositing rule. * If this object already uses the specified compositing rule, * this object is returned. * @return an <code>AlphaComposite</code> object derived from * this object that uses the specified compositing rule. * @param rule the compositing rule * @throws IllegalArgumentException if * <code>rule</code> is not one of * the following: {@link #CLEAR}, {@link #SRC}, {@link #DST}, * {@link #SRC_OVER}, {@link #DST_OVER}, {@link #SRC_IN}, * {@link #DST_IN}, {@link #SRC_OUT}, {@link #DST_OUT}, * {@link #SRC_ATOP}, {@link #DST_ATOP}, or {@link #XOR} * Returns a similar <code>AlphaComposite</code> object that uses * the specified alpha value. * If this object already has the specified alpha value, * this object is returned. * @return an <code>AlphaComposite</code> object derived from * this object that uses the specified alpha value. * @param alpha the constant alpha to be multiplied with the alpha of * the source. <code>alpha</code> must be a floating point number in the * inclusive range [0.0, 1.0]. * @throws IllegalArgumentException if * <code>alpha</code> is less than 0.0 or greater than 1.0 * Returns the hashcode for this composite. * @return a hash code for this composite. * Determines whether the specified object is equal to this * <code>AlphaComposite</code>. * The result is <code>true</code> if and only if * the argument is not <code>null</code> and is an * <code>AlphaComposite</code> object that has the same * compositing rule and alpha value as this object. * @param obj the <code>Object</code> to test for equality * @return <code>true</code> if <code>obj</code> equals this * <code>AlphaComposite</code>; <code>false</code> otherwise.