Image Interlacing is simply combining two images by layering one over the other with a certain opacity (usually 50%).  The intention is to use the image in video, so that two images are displayed within one single frame, thereby doubling the frames-per-second without doubling the actual, physical amount of frames.  Wikipedia provides a fairly decent explanation of this type of video technology: http://en.wikipedia.org/wiki/Interlaced_video .

Best Description :
http://www.100fps.com/

Some sample code :

<?php

            // Author: [email protected]
    
        // Imagick Type
        // ---------------------------------------------

    $imagick_type = new Imagick();
    
        // Open File
        // ---------------------------------------------
        
    $file_to_grab = "image_workshop_directory/test.jpg";
    
    $file_handle_for_viewing_image_file = fopen($file_to_grab, 'a+');
    
        // Grab File
        // ---------------------------------------------

    $imagick_type->readImageFile($file_handle_for_viewing_image_file);
    
        // Get Rendering Intent Values
        // ---------------------------------------------

    $imagick_rendering_intent = $imagick_type->getImageRenderingIntent();
    
    switch($imagick_rendering_intent)
    {
        case '0':
            $image_rendering_intent_evaluated = "Undefined";
            break;
            
        case '1':
            $image_rendering_intent_evaluated = "Saturation";
            break;
            
        case '2':
            $image_rendering_intent_evaluated = "Perceptual";
            break;
            
        case '3':
            $image_rendering_intent_evaluated = "Absolute";
            break;
            
        case '4':
            $image_rendering_intent_evaluated = "Relative";
            break;
    }
    
        // Print Rendering Intent Values
        // ---------------------------------------------
    
    print("# $imagick_rendering_intent - $image_rendering_intent_evaluated");

?>