That trick to cull backfaces won't work in all cases with vertex normals because the interpolated normal isn't consistent with the planar geometry. I notice similar artifacts when using that trick for 2-sided lighting. You will need to use face normals, since these are consistent with the underlying geometry, or just tolerate the artifacts.
You can get face normals using the derivatives of the interpolated surface position:
varying vec3 lerpPosition; // surface position, interpolated from vertex positions
void main(void)
{
vec3 faceNormal = normalize( cross( dFdx( lerpPosition ), dFdy( lerpPosition ) ))
}
I have tried to use the face normal but the result is very bad all the faces are culled.
here is how i use it
#version 100
precision highp int;
precision highp float;
attribute vec4 vertex;
attribute vec3 normal;
uniform mat4 normalMatrix;
uniform mat4 modelViewProjectionMatrix;
uniform mat4 modelView;
uniform vec3 camera_world_position;
varying vec3 ec_pos;
varying vec3 camPos;
void main()
{
gl_Position = modelViewProjectionMatrix * vertex;
vec3 norm = normal;
//norm *=-1.0;
ec_pos = vec3(gl_Position.x, gl_Position.y, gl_Position.z);
camPos = camera_world_position;
//lightDiffuse = dot(normalize(vec3(norm.x, norm.y, norm.z)), normalize(camera_world_position - vec3(gl_Position.x, gl_Position.y, gl_Position.z)));
#version 100
precision highp int;
precision highp float;
uniform float time;
uniform float touchX;
uniform float touchY;
uniform float touchZ;
uniform float line;
varying vec3 ec_pos;
varying vec3 camPos;
void main()
{
vec3 ec_normal = normalize(cross(dFdx(ec_pos), dFdy(ec_pos)));
float lightDiffuse = dot( ec_normal, normalize(camPos));
float rampLight =lightDiffuse;
float light = (1.0 - rampLight) * 1.0;
vec4 lightColor = vec4(1.0,1.0,1.0, 1.0);
vec4 diffuseColor = lightColor * light;
if(rampLight <0.0)
{
discard;
}
diffuseColor = smoothstep(vec4(0.0, 0.0, 0.0, 0.0), vec4(0.7, 0.7, 0.7, 0.7), vec4(diffuseColor));
gl_FragColor = diffuseColor;
}
}
