Height based custom material - fragment shader.

uniform vec3 m_region1;
uniform vec3 m_region2;
uniform vec3 m_region3;
uniform vec3 m_region4;
uniform vec4 g_LightDirection;
uniform vec4 g_LightPosition;

uniform sampler2D m_region1ColorMap;
uniform sampler2D m_region2ColorMap;
uniform sampler2D m_region3ColorMap;
uniform sampler2D m_region4ColorMap;
uniform sampler2D m_slopeColorMap;

uniform float m_minSlopeAngle;
uniform float m_maxSlopeAngle;

uniform float m_slopeTileFactor;
uniform float m_terrainSize;

varying vec3 normal ;
varying vec4 position;

vec4 GenerateTerrainColor() {

    float height = position.y;
    vec4 p = position / m_terrainSize;
    // This angles can be changed to match user

    // The minimum and maximum angles at which slope texture appears
    float minAngle = 30;
    float maxAngle = 60;

    float angleRange = m_maxSlopeAngle - m_minSlopeAngle;

    vec3 toUp = vec3(0.0, 1.0, 0.0);
    float angle = degrees (acos (  dot(toUp, normal) ))  ;

    // The amount the current angle is above the minimum angle the slope tex appears
    float deltaAngle = angle - minAngle;

    float angleImpact = clamp (deltaAngle / angleRange,0.00001 , 1 );

    vec4 slopeColor = texture2D(m_slopeColorMap, p.xz * m_slopeTileFactor);

    vec4 terrainColor = vec4(0.0, 0.0, 0.0, 1.0);

    float m_regionMin = 0.0;
    float m_regionMax = 0.0;
    float m_regionRange = 0.0;
    float m_regionWeight = 0.0;


    // Terrain m_region 1.
    m_regionMin = m_region1.x;
    m_regionMax = m_region1.y;
    m_regionRange = m_regionMax - m_regionMin;
    m_regionWeight = (m_regionRange - abs(height - m_regionMax)) / m_regionRange;
    m_regionWeight = max(0.0, m_regionWeight);
    terrainColor += m_regionWeight * texture2D(m_region1ColorMap, p.xz * m_region1.z);

    // Terrain m_region 2.
    m_regionMin = m_region2.x;
    m_regionMax = m_region2.y;
    m_regionRange = m_regionMax - m_regionMin;
    m_regionWeight = (m_regionRange - abs(height - m_regionMax)) / m_regionRange;
    m_regionWeight = max(0.0, m_regionWeight);
    terrainColor += m_regionWeight * (texture2D(m_region2ColorMap, p.xz * m_region2.z));

    // Terrain m_region 3.
    m_regionMin = m_region3.x;
    m_regionMax = m_region3.y;
    m_regionRange = m_regionMax - m_regionMin;
    m_regionWeight = (m_regionRange - abs(height - m_regionMax)) / m_regionRange;
    m_regionWeight = max(0.0, m_regionWeight);
    terrainColor += m_regionWeight * texture2D(m_region3ColorMap, p.xz * m_region3.z);

    // Terrain m_region 4.
    m_regionMin = m_region4.x;
    m_regionMax = m_region4.y;
    m_regionRange = m_regionMax - m_regionMin;
    m_regionWeight = (m_regionRange - abs(height - m_regionMax)) / m_regionRange;
    m_regionWeight = max(0.0, m_regionWeight);
    terrainColor += m_regionWeight * texture2D(m_region4ColorMap, p.xz * m_region4.z);



    return (mix(terrainColor, slopeColor, angleImpact ));
}

void main() {
    vec4 color = GenerateTerrainColor();

        vec3 lightDir = vec3(1.0, 3.0, 2.0);

        float lambert = dot(-g_LightPosition.xyz,normal);
        vec4 darkColor = vec4(0.0, 0.0, 0.0, 1.0);

        float rgb = 1;
        color = mix(darkColor,color , lambert);
        color = clamp(color, vec4(0,0,0,0), vec4(rgb,rgb,rgb,1));
        color = color * 0.8 ;
        gl_FragColor = color;

}