AlphaComposite.java

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package java.awt;

import java.awt.image.ColorModel;
import javax.tools.annotation.GenerateNativeHeader;
import sun.java2d.SunCompositeContext;

/**
 * The <code>AlphaComposite</code> 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.
 *
 * <p>
 * This class extends the standard equations defined by Porter and
 * Duff to include one additional factor.
 * An instance of the <code>AlphaComposite</code> 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.
 *
 * <p>
 * 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 <code>ColorModel</code> and <code>Raster</code> 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.
 *
 * <p>
 * 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 <a href="#caveats">Implementation Caveats</a>
 * for further information.
 *
 * <p>
 * The following factors are used in the description of the blending
 * equation in the Porter and Duff paper:
 *
 * <blockquote>
 * <table summary="layout">
 * <tr><th align=left>Factor&nbsp;&nbsp;<th align=left>Definition
 * <tr><td><em>A<sub>s</sub></em><td>the alpha component of the source pixel
 * <tr><td><em>C<sub>s</sub></em><td>a color component of the source pixel in premultiplied form
 * <tr><td><em>A<sub>d</sub></em><td>the alpha component of the destination pixel
 * <tr><td><em>C<sub>d</sub></em><td>a color component of the destination pixel in premultiplied form
 * <tr><td><em>F<sub>s</sub></em><td>the fraction of the source pixel that contributes to the output
 * <tr><td><em>F<sub>d</sub></em><td>the fraction of the destination pixel that contributes
 * to the output
 * <tr><td><em>A<sub>r</sub></em><td>the alpha component of the result
 * <tr><td><em>C<sub>r</sub></em><td>a color component of the result in premultiplied form
 * </table>
 * </blockquote>
 *
 * <p>
 * Using these factors, Porter and Duff define 12 ways of choosing
 * the blending factors <em>F<sub>s</sub></em> and <em>F<sub>d</sub></em> to
 * produce each of 12 desirable visual effects.
 * The equations for determining <em>F<sub>s</sub></em> and <em>F<sub>d</sub></em>
 * are given in the descriptions of the 12 static fields
 * that specify visual effects.
 * For example,
 * the description for
 * <a href="#SRC_OVER"><code>SRC_OVER</code></a>
 * specifies that <em>F<sub>s</sub></em> = 1 and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>).
 * 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:
 *
 * <pre>
 *      <em>F<sub>s</sub></em> = <em>f</em>(<em>A<sub>d</sub></em>)
 *      <em>F<sub>d</sub></em> = <em>f</em>(<em>A<sub>s</sub></em>)
 *      <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>
 *      <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>
 *
 * <p>
 * The following factors will be used to discuss our extensions to
 * the blending equation in the Porter and Duff paper:
 *
 * <blockquote>
 * <table summary="layout">
 * <tr><th align=left>Factor&nbsp;&nbsp;<th align=left>Definition
 * <tr><td><em>C<sub>sr</sub></em> <td>one of the raw color components of the source pixel
 * <tr><td><em>C<sub>dr</sub></em> <td>one of the raw color components of the destination pixel
 * <tr><td><em>A<sub>ac</sub></em>  <td>the "extra" alpha component from the AlphaComposite instance
 * <tr><td><em>A<sub>sr</sub></em> <td>the raw alpha component of the source pixel
 * <tr><td><em>A<sub>dr</sub></em><td>the raw alpha component of the destination pixel
 * <tr><td><em>A<sub>df</sub></em> <td>the final alpha component stored in the destination
 * <tr><td><em>C<sub>df</sub></em> <td>the final raw color component stored in the destination
 * </table>
 *</blockquote>
 *
 * <h3>Preparing Inputs</h3>
 *
 * <p>
 * The <code>AlphaComposite</code> 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 <code>AlphaComposite</code>.
 * This leads to the following equation for producing the alpha
 * used in the Porter and Duff blending equation:
 *
 * <pre>
 *      <em>A<sub>s</sub></em> = <em>A<sub>sr</sub></em> * <em>A<sub>ac</sub></em> </pre>
 *
 * All of the raw source color components need to be multiplied
 * by the alpha in the <code>AlphaComposite</code> 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:
 *
 * <pre>
 *      <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)
 *      <em>C<sub>s</sub></em> = <em>C<sub>sr</sub></em> * <em>A<sub>ac</sub></em>           (if source is premultiplied) </pre>
 *
 * No adjustment needs to be made to the destination alpha:
 *
 * <pre>
 *      <em>A<sub>d</sub></em> = <em>A<sub>dr</sub></em> </pre>
 *
 * <p>
 * The destination color components need to be adjusted only if
 * they are not in premultiplied form:
 *
 * <pre>
 *      <em>C<sub>d</sub></em> = <em>C<sub>dr</sub></em> * <em>A<sub>d</sub></em>    (if destination is not premultiplied)
 *      <em>C<sub>d</sub></em> = <em>C<sub>dr</sub></em>         (if destination is premultiplied) </pre>
 *
 * <h3>Applying the Blending Equation</h3>
 *
 * <p>
 * The adjusted <em>A<sub>s</sub></em>, <em>A<sub>d</sub></em>,
 * <em>C<sub>s</sub></em>, and <em>C<sub>d</sub></em> are used in the standard
 * Porter and Duff equations to calculate the blending factors
 * <em>F<sub>s</sub></em> and <em>F<sub>d</sub></em> and then the resulting
 * premultiplied components <em>A<sub>r</sub></em> and <em>C<sub>r</sub></em>.
 *
 * <p>
 * <h3>Preparing Results</h3>
 *
 * <p>
 * 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:
 *
 * <pre>
 *      <em>A<sub>df</sub></em> = <em>A<sub>r</sub></em>
 *      <em>C<sub>df</sub></em> = <em>C<sub>r</sub></em>                 (if dest is premultiplied)
 *      <em>C<sub>df</sub></em> = <em>C<sub>r</sub></em> / <em>A<sub>r</sub></em>            (if dest is not premultiplied) </pre>
 *
 * 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.
 *
 * <p>
 * <h3>Performance Considerations</h3>
 *
 * <p>
 * For performance reasons, it is preferrable that
 * <code>Raster</code> objects passed to the <code>compose</code>
 * method of a {@link CompositeContext} object created by the
 * <code>AlphaComposite</code> class have premultiplied data.
 * If either the source <code>Raster</code>
 * or the destination <code>Raster</code>
 * is not premultiplied, however,
 * appropriate conversions are performed before and after the compositing
 * operation.
 *
 * <h3><a name="caveats">Implementation Caveats</a></h3>
 *
 * <ul>
 * <li>
 * Many sources, such as some of the opaque image types listed
 * in the <code>BufferedImage</code> class, do not store alpha values
 * for their pixels.  Such sources supply an alpha of 1.0 for
 * all of their pixels.
 *
 * <p>
 * <li>
 * 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.
 *
 * <p>
 * <li>
 * 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, <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.
 *
 * <p>
 * <li>
 * 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.
 *
 * <p>
 * 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
 * 1.0.
 *
 * <p>
 * <li>
 * 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:
 *
 * <pre>
 *    (A, R, G, B) = (0x01, 0xb0, 0x00, 0x00)</pre>
 *
 * <p>
 * If integer math were being used and this value were being
 * composited in
 * <a href="#SRC"><code>SRC</code></a>
 * mode with no extra alpha, then the math would
 * indicate that the results were (in integer format):
 *
 * <pre>
 *    (A, R, G, B) = (0x01, 0x01, 0x00, 0x00)</pre>
 *
 * <p>
 * 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:
 *
 * <pre>
 *    (A, R, G, B) = (0x01, 0xff, 0x00, 0x00)</pre>
 *
 * <p>
 * (Note that 0x01 divided by 0x01 gives you 1.0, which is equivalent
 * to the value 0xff in an 8-bit storage format.)
 *
 * <p>
 * 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.
 *
 * <p>
 * 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:
 *
 * <pre>
 *    (A, R, G, B) = (0x01, 0x01, 0x00, 0x00)</pre>
 *
 * <p>
 * and thus they would all match.
 *
 * <p>
 * <li>
 * 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.
 * </ul>
 * @see Composite
 * @see CompositeContext
 */

/* No native methods here, but the constants are needed in the supporting JNI code */
@GenerateNativeHeader
public final class AlphaComposite implements Composite {
    /**
     * 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.
     *<p>
     * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = 0, thus:
     *<pre>
     *  <em>A<sub>r</sub></em> = 0
     *  <em>C<sub>r</sub></em> = 0
     *</pre>
     */
    public static final int     CLEAR           = 1;

    /**
     * The source is copied to the destination
     * (Porter-Duff Source rule).
     * The destination is not used as input.
     *<p>
     * <em>F<sub>s</sub></em> = 1 and <em>F<sub>d</sub></em> = 0, thus:
     *<pre>
     *  <em>A<sub>r</sub></em> = <em>A<sub>s</sub></em>
     *  <em>C<sub>r</sub></em> = <em>C<sub>s</sub></em>
     *</pre>
     */
    public static final int     SRC             = 2;

    /**
     * The destination is left untouched
     * (Porter-Duff Destination rule).
     *<p>
     * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = 1, thus:
     *<pre>
     *  <em>A<sub>r</sub></em> = <em>A<sub>d</sub></em>
     *  <em>C<sub>r</sub></em> = <em>C<sub>d</sub></em>
     *</pre>
     * @since 1.4
     */
    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).
     *<p>
     * <em>F<sub>s</sub></em> = 1 and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>), thus:
     *<pre>
     *  <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>)
     *</pre>
     */
    public static final int     SRC_OVER        = 3;

    /**
     * The destination is composited over the source and
     * the result replaces the destination
     * (Porter-Duff Destination Over Source rule).
     *<p>
     * <em>F<sub>s</sub></em> = (1-<em>A<sub>d</sub></em>) and <em>F<sub>d</sub></em> = 1, thus:
     *<pre>
     *  <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>
     *</pre>
     */
    public static final int     DST_OVER        = 4;

    /**
     * The part of the source lying inside of the destination replaces
     * the destination
     * (Porter-Duff Source In Destination rule).
     *<p>
     * <em>F<sub>s</sub></em> = <em>A<sub>d</sub></em> and <em>F<sub>d</sub></em> = 0, thus:
     *<pre>
     *  <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>
     *</pre>
     */
    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).
     *<p>
     * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = <em>A<sub>s</sub></em>, thus:
     *<pre>
     *  <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>
     *</pre>
     */
    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).
     *<p>
     * <em>F<sub>s</sub></em> = (1-<em>A<sub>d</sub></em>) and <em>F<sub>d</sub></em> = 0, thus:
     *<pre>
     *  <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>)
     *</pre>
     */
    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).
     *<p>
     * <em>F<sub>s</sub></em> = 0 and <em>F<sub>d</sub></em> = (1-<em>A<sub>s</sub></em>), thus:
     *<pre>
     *  <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>)
     *</pre>
     */
    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).
     *<p>
     * <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:
     *<pre>
     *  <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>)
     *</pre>
     * @since 1.4
     */
    public static final int     SRC_ATOP        = 10;

    /**
     * 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).
     *<p>
     * <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:
     *<pre>
     *  <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>
     *</pre>
     * @since 1.4
     */
    public static final int     DST_ATOP        = 11;

    /**
     * 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).
     *<p>
     * <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:
     *<pre>
     *  <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>)
     *</pre>
     * @since 1.4
     */
    public static final int     XOR             = 12;

    /**
     * <code>AlphaComposite</code> object that implements the opaque CLEAR rule
     * with an alpha of 1.0f.
     * @see #CLEAR
     */
    public static final AlphaComposite Clear    = new AlphaComposite(CLEAR);

    /**
     * <code>AlphaComposite</code> object that implements the opaque SRC rule
     * with an alpha of 1.0f.
     * @see #SRC
     */
    public static final AlphaComposite Src      = new AlphaComposite(SRC);

    /**
     * <code>AlphaComposite</code> object that implements the opaque DST rule
     * with an alpha of 1.0f.
     * @see #DST
     * @since 1.4
     */
    public static final AlphaComposite Dst      = new AlphaComposite(DST);

    /**
     * <code>AlphaComposite</code> object that implements the opaque SRC_OVER rule
     * with an alpha of 1.0f.
     * @see #SRC_OVER
     */
    public static final AlphaComposite SrcOver  = new AlphaComposite(SRC_OVER);

    /**
     * <code>AlphaComposite</code> object that implements the opaque DST_OVER rule
     * with an alpha of 1.0f.
     * @see #DST_OVER
     */
    public static final AlphaComposite DstOver  = new AlphaComposite(DST_OVER);

    /**
     * <code>AlphaComposite</code> object that implements the opaque SRC_IN rule
     * with an alpha of 1.0f.
     * @see #SRC_IN
     */
    public static final AlphaComposite SrcIn    = new AlphaComposite(SRC_IN);

    /**
     * <code>AlphaComposite</code> object that implements the opaque DST_IN rule
     * with an alpha of 1.0f.
     * @see #DST_IN
     */
    public static final AlphaComposite DstIn    = new AlphaComposite(DST_IN);

    /**
     * <code>AlphaComposite</code> object that implements the opaque SRC_OUT rule
     * with an alpha of 1.0f.
     * @see #SRC_OUT
     */
    public static final AlphaComposite SrcOut   = new AlphaComposite(SRC_OUT);

    /**
     * <code>AlphaComposite</code> object that implements the opaque DST_OUT rule
     * with an alpha of 1.0f.
     * @see #DST_OUT
     */
    public static final AlphaComposite DstOut   = new AlphaComposite(DST_OUT);

    /**
     * <code>AlphaComposite</code> object that implements the opaque SRC_ATOP rule
     * with an alpha of 1.0f.
     * @see #SRC_ATOP
     * @since 1.4
     */
    public static final AlphaComposite SrcAtop  = new AlphaComposite(SRC_ATOP);

    /**
     * <code>AlphaComposite</code> object that implements the opaque DST_ATOP rule
     * with an alpha of 1.0f.
     * @see #DST_ATOP
     * @since 1.4
     */
    public static final AlphaComposite DstAtop  = new AlphaComposite(DST_ATOP);

    /**
     * <code>AlphaComposite</code> object that implements the opaque XOR rule
     * with an alpha of 1.0f.
     * @see #XOR
     * @since 1.4
     */
    public static final AlphaComposite Xor      = new AlphaComposite(XOR);

    private static final int MIN_RULE = CLEAR;
    private static final int MAX_RULE = XOR;

    float extraAlpha;
    int rule;

    private AlphaComposite(int rule) {
        this(rule, 1.0f);
    }

    private AlphaComposite(int rule, float alpha) {
        if (rule < MIN_RULE || rule > MAX_RULE) {
            throw new IllegalArgumentException("unknown composite rule");
        }
        if (alpha >= 0.0f && alpha <= 1.0f) {
            this.rule = rule;
            this.extraAlpha = alpha;
        } else {
            throw new IllegalArgumentException("alpha value out of range");
        }
    }

    /**
     * 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}
     */
    public static AlphaComposite getInstance(int rule) {
        switch (rule) {
        case CLEAR:
            return Clear;
        case SRC:
            return Src;
        case DST:
            return Dst;
        case SRC_OVER:
            return SrcOver;
        case DST_OVER:
            return DstOver;
        case SRC_IN:
            return SrcIn;
        case DST_IN:
            return DstIn;
        case SRC_OUT:
            return SrcOut;
        case DST_OUT:
            return DstOut;
        case SRC_ATOP:
            return SrcAtop;
        case DST_ATOP:
            return DstAtop;
        case XOR:
            return Xor;
        default:
            throw new IllegalArgumentException("unknown composite rule");
        }
    }

    /**
     * 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
     * with the destination.
     * @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,&nbsp;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}
     */
    public static AlphaComposite getInstance(int rule, float alpha) {
        if (alpha == 1.0f) {
            return getInstance(rule);
        }
        return new AlphaComposite(rule, alpha);
    }

    /**
     * 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.
     */
    public CompositeContext createContext(ColorModel srcColorModel,
                                          ColorModel dstColorModel,
                                          RenderingHints hints) {
        return new SunCompositeContext(this, srcColorModel, dstColorModel);
    }

    /**
     * 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>.
     */
    public float getAlpha() {
        return extraAlpha;
    }

    /**
     * Returns the compositing rule of this <code>AlphaComposite</code>.
     * @return the compositing rule of this <code>AlphaComposite</code>.
     */
    public int getRule() {
        return rule;
    }

    /**
     * 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}
     * @since 1.6
     */
    public AlphaComposite derive(int rule) {
        return (this.rule == rule)
            ? this
            : getInstance(rule, this.extraAlpha);
    }

    /**
     * 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,&nbsp;1.0].
     * @throws IllegalArgumentException if
     *         <code>alpha</code> is less than 0.0 or greater than 1.0
     * @since 1.6
     */
    public AlphaComposite derive(float alpha) {
        return (this.extraAlpha == alpha)
            ? this
            : getInstance(this.rule, alpha);
    }

    /**
     * Returns the hashcode for this composite.
     * @return      a hash code for this composite.
     */
    public int hashCode() {
        return (Float.floatToIntBits(extraAlpha) * 31 + rule);
    }

    /**
     * Determines whether the specified object is equal to this
     * <code>AlphaComposite</code>.
     * <p>
     * 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.
     */
    public boolean equals(Object obj) {
        if (!(obj instanceof AlphaComposite)) {
            return false;
        }

        AlphaComposite ac = (AlphaComposite) obj;

        if (rule != ac.rule) {
            return false;
        }

        if (extraAlpha != ac.extraAlpha) {
            return false;
        }

        return true;
    }

}