WebGL Normal calculations from position texture - glsl

Question

Iam trying to create a procedural water puddle in webGL with "water ripples" by vertex displacement.
The problem I'm having is that I get a noise I can't explain.
Below is the first pass vertex shader where I calculate the vertex positions that i later render to a texture that i then use in the second pass.
void main() {
float damping = 0.5;
vNormal = normal;
// wave radius
float timemod = 0.55;
float ttime = mod(time , timemod);
float frequency = 2.0*PI/waveWidth;
float phase = frequency * 0.21;
vec4 v = vec4(position,1.0);
// Loop through array of start positions
for(int i = 0; i < 200; i++){
float cCenterX = ripplePos[i].x;
float cCenterY = ripplePos[i].y;
vec2 center = vec2(cCenterX, cCenterY) ;
if(center.x == 0.0 && center.y == 0.0)
center = normalize(center);
// wave width
float tolerance = 0.005;
radius = sqrt(pow( uv.x - center.x , 2.0) + pow( uv.y -center.y, 2.0));
// Creating a ripple
float w_height = (tolerance - (min(tolerance,pow(ripplePos[i].z-radius*10.0,2.0)) )) * (1.0-ripplePos[i].z/timemod) *5.82;
// -2.07 in the end to keep plane at right height. Trial and error solution
v.z += waveHeight*(1.0+w_height/tolerance) / 2.0 - 2.07;
vNormal = normal+v.z;
}
vPosition = v.xyz;
gl_Position = projectionMatrix * modelViewMatrix * v;
}
And the first pass fragment shader that writes to the texture:
void main()
{
vec3 p = normalize(vPosition);
p.x = (p.x+1.0)*0.5;
p.y = (p.y+1.0)*0.5;
gl_FragColor = vec4( normalize(p), 1.0);
}
The second vertex shader is a standard passthrough.
Second pass fragmentshader is where I try to calculate the normals to be used for light calculations.
void main() {
float w = 1.0 / 200.0;
float h = 1.0 / 200.0;
// Nearest Nieghbours
vec3 p0 = texture2D(rttTexture, vUV).xyz;
vec3 p1 = texture2D(rttTexture, vUV + vec2(-w, 0)).xyz;
vec3 p2 = texture2D(rttTexture, vUV + vec2( w, 0)).xyz;
vec3 p3 = texture2D(rttTexture, vUV + vec2( 0, h)).xyz;
vec3 p4 = texture2D(rttTexture, vUV + vec2( 0, -h)).xyz;
vec3 nVec1 = p2 - p0;
vec3 nVec2 = p3 - p0;
vec3 vNormal = cross(nVec1, nVec2);
vec3 N = normalize(vNormal);
float theZ = texture2D(rttTexture, vUV).r;
//gl_FragColor = vec4(1.,.0,1.,1.);
//gl_FragColor = texture2D(tDiffuse, vUV);
gl_FragColor = vec4(vec3(N), 1.0);
}
The result is this:
The image displays the normalmap and the noise I'm refering to is the inconsistency of the blue.
Here is a live demonstration:
http://oskarhavsvik.se/jonasgerling_water_ripple/waterRTT-clean.html
I appreciate any tips and pointers, not only fixes for this problem. But the code in genereal, I'm here to learn.

Answer 1

After a brief look it seems like your problem is in storing x/y positions.
gl_FragColor = vec4(vec3(p0*0.5+0.5), 1.0);
You don't need to store them anyway, because the texel position implicitly gives the x/y value. Just change your normal points to something like this...
vec3 p2 = vec3(1, 0, texture2D(rttTexture, vUV + vec2(w, 0)).z);
Rather than 1, 0 you will want to use a scale appropriate to the size of your displayed quad relative to the wave height. Anyway, the result now looks like this.
The height/z seems to be scaled by distance from the centre, so I went looking for a normalize() and removed it...
vec3 p = vPosition;
gl_FragColor = vec4(p*0.5+0.5, 1.0);
The normals now look like this...