Downsample

/**
 * Gaussian downsampling of an image with ImageJ on-board tools.
 *
 * Motivation:
 * Sound downsampling of an image requires the elimination of image frequencies
 * higher than half the sampling frequency in the result image (see the
 * Nyquist-Shannon sampling theorem).  The exclusive tool for this is Gaussian
 * convolution.
 *
 * This script calculates the required Gaussian kernel for a given target size,
 * smoothes the image and resamples it.
 *
 * Furthermore, you can define the "intrinsic" Gaussian kernel of the source and
 * target images.  An optimal sampler is identified by sigma=0.5.  If your
 * source image was blurred already, you may set a higher source sigma for a
 * sharper result.  Setting target sigma to values smaller than 0.5 makes the
 * result appear sharper and therefore eventually aliased.
 */
var imp = WindowManager.getCurrentImage();
var width = 0;
var height = 0;
var sourceSigma = 0.5;
var targetSigma = 0.5;
var widthField;
var heightField;
var fieldWithFocus;

var textListener = new java.awt.event.TextListener(
  {
    textValueChanged : function( e )
    {
      var source = e.getSource();
      var newWidth = Math.round( widthField.getText() );
      var newHeight = Math.round( heightField.getText() );
      
      if ( source == widthField && fieldWithFocus == widthField && newWidth )
      {
        newHeight = Math.round( newWidth * imp.getHeight() / imp.getWidth() );
        heightField.setText( newHeight );
      }
      else if ( source == heightField && fieldWithFocus == heightField && newHeight )
      {
        newWidth = Math.round( newHeight * imp.getWidth() / imp.getHeight() );
        widthField.setText( newWidth );
      }
    } 
  } );

var focusListener = new java.awt.event.FocusListener(
  {
    focusGained : function ( e )
    {
      fieldWithFocus = e.getSource();
    },
    focusLost : function( e ){} 
  } );

if ( imp )
{
  width = imp.getWidth();
  height = imp.getHeight();
  
  gd = new GenericDialog( "Downsample" );
  gd.addNumericField( "width :", width, 0 );
  gd.addNumericField( "height :", height, 0 );
  gd.addNumericField( "source sigma :", sourceSigma, 2 );
  gd.addNumericField( "target sigma :", targetSigma, 2 );
  gd.addCheckbox( "keep source image", true );
  var fields = gd.getNumericFields();
  
  widthField = fields.get( 0 );
  heightField = fields.get( 1 );
  fieldWithFocus = widthField;
  
  widthField.addFocusListener( focusListener );
  widthField.addTextListener( textListener );
  heightField.addFocusListener( focusListener );
  heightField.addTextListener( textListener );
    
  gd.showDialog();
  if ( gd.wasOKed() )
  {
    width = gd.getNextNumber();
    height = gd.getNextNumber();
    sourceSigma = gd.getNextNumber();
    targetSigma = gd.getNextNumber();
    keepSource = gd.getNextBoolean();
    
    if ( width <= imp.getWidth() )
    {
      var s;
      if ( fieldWithFocus == widthField )
        s = targetSigma * imp.getWidth() / width;
      else
        s = targetSigma * imp.getHeight() / height;
      
      if ( keepSource )
        IJ.run( "Duplicate...", "title=" + imp.getTitle() + " duplicate" );
      IJ.run( "Gaussian Blur...", "sigma=" + Math.sqrt( s * s - sourceSigma * sourceSigma ) + " stack" );
      IJ.run( "Scale...", "x=- y=- width=" + width + " height=" + height + " process title=- interpolation=None" );
      IJ.run( "Canvas Size...", "width=" + width + " height=" + height + " position=Center" );
    }
    else
      IJ.showMessage( "You try to upsample the image.  You need an interpolator for that not a downsampler." );
  }
}
else
  IJ.showMessage( "You should have at least one image open." );