Resize an image based on the area of another image while maintaining the same ratio as before.

resize.js

/**
 * Resize an image based on the area of another image 
 * while maintaining the same ratio as before.
 * 
 * @param width    {Number} the width of the image not to be resized
 * @param height   {Number} the height of the image not to be resized
 * @param rWidth   {Number} the width of the image to be resized
 * @param rHeight  {Number} the height of the image to be resized
 * @param resize   {Float}  decimal percentage of how much to resize
 */
function calc(width, height, rWidth, rHeight, resize) {
    // Calculate the comparison image area
    var area = width * height;
    
    // The total area of the resized imaged
    var targetArea = area * resize;

    // Get the ratio of the picture
    var ratio = (rWidth > rHeight) ? rWidth / rHeight : rHeight / rWidth;

    // If horizontal make the width larger than the height
    if(rWidth > rHeight) {
        return {
            width:  Math.sqrt(targetArea) * Math.sqrt(ratio),
            height: Math.sqrt(targetArea) / Math.sqrt(ratio),
        }
    }

    // If vertical or square make the height larger than the width
    return { 
        width:  Math.sqrt(targetArea) / Math.sqrt(ratio),
        height: Math.sqrt(targetArea) * Math.sqrt(ratio),
    }
}