/*
 * @(#)AlphaComposite.java	1.37 00/02/02
 *
 * Copyright 1997-2000 Sun Microsystems, Inc. All Rights Reserved.
 * 
 * This software is the proprietary information of Sun Microsystems, Inc.  
 * Use is subject to license terms.
 * 
 */

package java.awt;

import java.awt.image.ColorModel;

/**
 * This <code>AlphaComposite</code> 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.
 *<p>
 * 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.
 * <p>
 * The following abbreviations are used in the description of the rules:
 * <ul>
 * <li>Cs = one of the color components of the source pixel.
 * <li>Cd = one of the color components of the destination pixel.
 * <li>As = alpha component of the source pixel.
 * <li>Ad = alpha component of the destination pixel.
 * <li>Fs = fraction of the source pixel that contributes to the output.
 * <li>Fd = fraction of the input destination pixel that contributes to the
 * output.
 * </ul>
 *<p>
 * The color and alpha components produced by the compositing operation are
 * calculated as follows:
 *<pre>
 * 	Cd = Cs*Fs + Cd*Fd
 * 	Ad = As*Fs + Ad*Fd
 *</pre>
 * 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.
 *<p>
 * For performance reasons, it is preferrable that Rasters passed to the compose
 * method of a {@link CompositeContext} object created by the 
 * <code>AlphaComposite</code> class have premultiplied data.
 * If either source or destination Rasters are not premultiplied, however,
 * appropriate conversions are performed before and after the compositing
 * operation.
 *<p>
 * 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 Composite
 * @see CompositeContext
 * @version 10 Feb 1997
 */


public final class AlphaComposite implements Composite {
    /**
     * 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.
     *<p>
     * Fs = 0 and Fd = 0, thus:
     *<pre>
     * 	Cd = 0
     * 	Ad = 0
     *</pre>
     * 
     */
    public static final int	CLEAR		= 1;

    /**
     * Porter-Duff Source rule.
     * The source is copied to the destination.
     * The destination is not used as input.
     *<p>
     * Fs = 1 and Fd = 0, thus:
     *<pre>
     * 	Cd = Cs
     * 	Ad = As
     *</pre>
     */
    public static final int	SRC		= 2;

    /**
     * Porter-Duff Source Over Destination rule.
     * The source is composited over the destination.
     *<p>
     * Fs = 1 and Fd = (1-As), thus:
     *<pre>
     * 	Cd = Cs + Cd*(1-As)
     * 	Ad = As + Ad*(1-As)
     *</pre>
     */
    public static final int	SRC_OVER	= 3;

    /**
     * Porter-Duff Destination Over Source rule.
     * The destination is composited over the source and
     * the result replaces the destination.
     *<p>
     * Fs = (1-Ad) and Fd = 1, thus:
     *<pre>
     * 	Cd = Cs*(1-Ad) + Cd
     * 	Ad = As*(1-Ad) + Ad
     *</pre>
     */
    public static final int	DST_OVER	= 4;

    /**
     * Porter-Duff Source In Destination rule.
     * The part of the source lying inside of the destination replaces
     * the destination.
     *<p>
     * Fs = Ad and Fd = 0, thus:
     *<pre>
     * 	Cd = Cs*Ad
     * 	Ad = As*Ad
     *</pre>
     */
    public static final int	SRC_IN		= 5;

    /**
     * Porter-Duff Destination In Source rule.
     * The part of the destination lying inside of the source
     * replaces the destination.
     *<p>
     * Fs = 0 and Fd = As, thus:
     *<pre>
     * 	Cd = Cd*As
     * 	Ad = Ad*As
     *</pre>
     */
    public static final int	DST_IN		= 6;

    /**
     * Porter-Duff Source Held Out By Destination rule.
     * The part of the source lying outside of the destination
     * replaces the destination.
     *<p>
     * Fs = (1-Ad) and Fd = 0, thus:
     *<pre>
     * 	Cd = Cs*(1-Ad)
     * 	Ad = As*(1-Ad)
     *</pre>
     */
    public static final int	SRC_OUT		= 7;

    /**
     * Porter-Duff Destination Held Out By Source rule.
     * The part of the destination lying outside of the source
     * replaces the destination.
     *<p>
     * Fs = 0 and Fd = (1-As), thus:
     *<pre>
     * 	Cd = Cd*(1-As)
     * 	Ad = Ad*(1-As)
     *</pre>
     */
    public static final int	DST_OUT		= 8;

    /**
     * <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 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);

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

    float extraAlpha;
    int rule;

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

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

    /**
     * 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 #SRC_OVER}, {@link #DST_OVER}, {@link #SRC_IN}, 
     *         {@link #DST_IN}, {@link #SRC_OUT}, or {@link #DST_OUT}
     */
    public static AlphaComposite getInstance(int rule) {
	switch (rule) {
	case CLEAR:
	    return Clear;
	case SRC:
	    return Src;
	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;
	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, 1.0]. 
     */
    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 AlphaCompositeContext(srcColorModel, dstColorModel,
                                         rule, extraAlpha);
    }

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

    /**
     * Tests if the specified {@link Object} is equal to this 
     * <code>AlphaComposite</code> 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;
    }
            
}