Pro Android with Kotlin

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CHAPTER 9: User Interface
The new fragment shader takes the interpolated positions and normals from the vertex 
shader, adds a uniform variable for the light’s position, and in our case uses the Phong 
shading model to apply light.
val fragmentShaderCode = """
precision mediump float;
varying vec4 fColor;
varying vec3 N;
varying vec3 v;
uniform vec4 lightPos;
void main() {
vec3 L = normalize(lightPos.xyz - v);
vec3 E = normalize(-v); // eye coordinates!
vec3 R = normalize(-reflect(L,N));
//calculate Ambient Term:
vec4 Iamb = vec4(0.0, 0.1, 0.1, 1.0);
//calculate Diffuse Term:
vec4 Idiff = vec4(0.0, 0.0, 1.0, 1.0) *
max(dot(N,L), 0.0);
Idiff = clamp(Idiff, 0.0, 1.0);
// calculate Specular Term:
vec4 Ispec = vec4(1.0, 1.0, 0.5, 1.0) *
pow(max(dot(R,E),0.0),
/*shininess=*/5.0);
Ispec = clamp(Ispec, 0.0, 1.0);
// write Total Color:
gl_FragColor = Iamb + Idiff + Ispec;
//gl_FragColor = fColor; // use vertex color instead
}
""".trimIndent()
Don’t forget to add a handle for the light’s position inside the renderer’s onDrawFrame() 
function.
// The light position
val lightPosHandle = GLES20.glGetUniformLocation(
program!!, "lightPos");
GLES20.glUniform4f(lightPosHandle,
lightPos[0],lightPos[1],lightPos[2],lightPos[3])
The vectors used in the Phong shading algorithm are depicted in Figure 
9-2
. You will find the 
same vector names used in the shader code.

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