Gray8Statistics.java

/*
 * Gray8Statistics.java
 *
 * Created on November 11, 2006, 2:17 PM
 *
 * To change this template, choose Tools | Template Manager
 * and open the template in the editor.
 *
 * Copyright 2007 by Jon A. Webb
 *     This program is free software: you can redistribute it and/or modify
 *    it under the terms of the GNU Lesser General Public License as published by
 *    the Free Software Foundation, either version 3 of the License, or
 *    (at your option) any later version.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU Lesser General Public License for more details.
 *
 *    You should have received a copy of the Lesser GNU General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

package jjil.algorithm;
import jjil.core.Error;
import jjil.core.Gray8Image;
import jjil.core.Image;
import jjil.core.MathPlus;

/**
 * Gray8Statistics is used to measure the mean and variance of a gray
 * image.
 * 
 * 
 * @author webb
 */
public class Gray8Statistics {
    private int nMean; // mean image value, times 256
    private int nVariance; // image variance, times 256
    
    /**
     * Creates a new instance of Gray8Statistics
     */
    public Gray8Statistics() {
    }
    
    /** Estimate the mean and variance of an input gray image.
     *
     * @param image the input image.
     * @throws jjil.core.Error if the input image is not gray.
     */
    public void push(Image image) throws jjil.core.Error
    {
        if (!(image instanceof Gray8Image)) {
            throw new Error(
                			Error.PACKAGE.ALGORITHM,
                			ErrorCodes.IMAGE_NOT_GRAY8IMAGE,
                			image.toString(),
                			null,
                			null);
        }
        Gray8Image gray = (Gray8Image) image;
        int nSum = 0, nSumSq = 0;
        byte[] data = gray.getData();
        for (int i=0; i<gray.getHeight(); i++) {
            for (int j=0; j<gray.getWidth(); j++) {
                int pixel = (data[i*image.getWidth()+j]) - Byte.MIN_VALUE;
                nSum += pixel;
                nSumSq += pixel*pixel;
            }
        }
        /** Compute mean and variance. Both are scaled by 256 for accuracy.
         */
        int nCount = image.getHeight() * image.getWidth();
        this.nMean = 256 * nSum / nCount;
        // expanded form of variance computation
        // note order of multiplications and divisions. we're trying to
        // avoid overflow here.
        this.nVariance =  
                (nSumSq / (nCount - 1) - 
                    nSum / nCount * nSum  / (nCount - 1)) << 8;
    }
    
    /** Return computed mean, times 256.
     *
     * @return the mean value, times 256.
     */
    public int getMean() {
        return this.nMean;
    }
    
    /**
     * Return standard deviation, times 256 using Newton's iteration.
     * @return the standard deviation, times 256.
     * @throws jjil.core.Error if the variance computed in push() is less than zero.
     */
    public int getStdDev() throws jjil.core.Error {
        // n = variance * 256 * 256 (for accuracy)
        int n = getVariance() << 8; // getVariance() already is * 256
        if (n < 0) throw new Error(
            			Error.PACKAGE.ALGORITHM,
            			ErrorCodes.STATISTICS_VARIANCE_LESS_THAN_ZERO,
            			new Integer(n).toString(),
            			null,
            			null);
        // return standard deviation * 256 = sqrt(variance * 256 * 256)
        return MathPlus.sqrt(n);
    }
    
    /** Return computed variance, times 256.
     *
     * @return the computed variance value.
     */
    public int getVariance() {
        return this.nVariance;
    }
}