Why does my Perlin Noise look "blocky"?
The interpolation looks fine. The main problem here is that the hash function you're using isn't very good. If I look at just one octave, and visualize the hash result by outputting hash(PT).x, I get something like this:



This is supposed to be completely random per grid square, but you can see that it has a lot of diagonal line patterns in it (it almost looks like a checkerboard), so it's not a very random hash, and those patterns will show up in the noise produced by it.

The other problem is that your hash only returns gradient vectors in [0, 1], while they should be in [&minus;1, 1] to get gradients in all directions. That part's easy to fix by remapping.

To fix those problems, I switched the code to use this hash function (which I learned from Mikkel Gjoel, and is probably due to a paper by W.J.J. Rey):

vec2 hash(vec2 co) {
    float m = dot(co, vec2(12.9898, 78.233));
    return fract(vec2(sin(m),cos(m))* 43758.5453) * 2. - 1.;
}


Note that due to the trig functions it's going to be a bit more expensive than your version. However, it considerably improves the appearance of the resulting noise: