Similarity or Relation between Walsh Hadamard Transform and Slant Transform
On a 2-sample vector, or $2\times2$ pixel images, Haar, Walsh-Hadamard, or Slant are exactly the same. They are all separable, orthogonal transformations with fast $N\log N$ algorithms. According to Slant Transform Image Coding, 1974, it is superior in appearance to the above, only inferior to Karhunen-Loève,  which has no fast enough algorithms.