Barrel distortion shader

Hi,

I am trying to implement barrel distortion in a vertex shader. I'm using a simplified version of Brown's model.
Here's my vertex shader code:

Code (CSharp):

1.             v2f vert(appdata_full v) {
2.                 v2f o;
3.  
4.                 o.vertex = UnityObjectToClipPos(v.vertex);
5.  
6.                 float2 h = o.vertex.xy / o.vertex.w;
7.              
8.                 float r = sqrt(h.x * h.x + h.y * h.y);
9.                 float f = r + _c1 * pow(r, 3) + _c2 * pow(r, 5);
10.  
11.                 h = f * _scale * h;
12.  
13.                 o.vertex.xy = h.xy * o.vertex.w;
14.  
15.                 o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
16.  
17.                 return o;
18.             }

However this doesn’t seem to work properly, as I'm getting something that looks like a pincushion distortion, regardless what values I use for _c1 and _c2.

Any suggestions welcome, thanks