Per fragment lighting on heightmap and generating normals

Question

I am trying to implement per framgent lighting on a heightmap. I am uploading the height map to the shader as a texture and adjusting vertex heights according to respective pixels. To generate the normals I take values of four neighbouring pixels, make vectors of them and compute the cross product like so:

vec3 offset = vec3(-1.0/mapSize, 0, 1.0/mapSize);
float s1 = texture2D(sampler, texCoord + offset.xy).x;
float s2 = texture2D(sampler, texCoord + offset.zy).x;
float s3 = texture2D(sampler, texCoord + offset.yx).x;
float s4 = texture2D(sampler, texCoord + offset.yz).x;
vec3 va = normalize(vec3(1.0, 0.0, s2 - s1));
vec3 vb = normalize(vec3(0.0, 1.0, s3 - s4));
vec3 n = normalize(cross(va, vb));

and heres my lighting function

vec4 directional(Light light){
    vec4 ret = vec4(0.0);
    vec3 lPos = (V * vec4(light.position, 0.0)).xyz;
    vec3 normal = normalize(vNormal);
    vec3 lightDir = normalize(lPos);
    vec3 reflectDir = reflect(-lightDir, normal);    
    vec3 viewDir = normalize(-vPosition);

    float lambertTerm = max(dot(lightDir, normal), 0.0);
    float specular = 0.0;

    if(lambertTerm > 0.0){
        float specAngle = max(dot(reflectDir, viewDir), 0.0);
        specular = pow(specAngle, material.shininess);
    }
    ret = vec4(light.ambient * material.ambient + light.diffuse * material.diffuse * lambertTerm + light.specular * material.specular * specular, 1.0);
    return ret;
} 

This kinda works. Only the y and z axis seem to by flipped ie. if I move the light along the y axis it looks like its moving along the z axis and vice versa.

I should also point out that the function works perfectly on regular 3D models, so I assume the problem is in the generation of normals.

Answer 1

If you're using a y-up coordinate system then you want to be doing your deltas on the y component, not the z-component.

vec3 va = normalize(vec3(1.0, s2 - s1, 0.0));
vec3 vb = normalize(vec3(0.0, s4 - s3, 0.0));

Also you should confirm whether it's s3 - s4 or s4 - s3.