SIFT は特許あり（使用にちゅういが必要）

HOG などにも興味あり

see
<a href="http://www.robots.ox.ac.uk/~vgg/publications/2012/Arandjelovic12/">
Three things everyone should know to improve object retrieval
</a> 


            float sum = 0.0f;
            for (float f : desc)
                sum += f;
            if (sum != 0) {
                for (i = 0; i < desc.length; i++) {
                    if (desc[i] < 0) desc[i] = (float) -Math.sqrt(-desc[i] / sum);
                    else desc[i] = (float) Math.sqrt(desc[i] / sum);

                }
            }