Introduction to Shaders: Specular Highlights, Indexed Geometry, Directional Lighting

#! /usr/bin/env python

Specular Highlights, Indexed Geometry, Directional Lighting

This tutorial builds on earlier tutorials by adding:
  • specular lighting (Phong Lighting)
  • specular lighting (Blinn-Phong Lighting)
  • per-fragment lighting
  • Sphere geometry object (indexed rendering)
This tutorial completes the Phong Shading rendering code that we started in the last tutorial by adding "specular" highlights to the material. Specular highlights are basically "shininess", that is, the tendancy of a material to re-emit light *in a particular direction* based on the angle of incidence of the light ray.
from OpenGLContext import testingcontext BaseContext = testingcontext.getInteractive() from OpenGL.GL import * from OpenGL.arrays import vbo from OpenGLContext.arrays import * from OpenGL.GL import shaders
This is our only new import. The Sphere geometry node generates a "compiled" piece of sphere geometry as a pair of two VBOs and a count that tells us how many vertices need to be rendered from the VBOs to render the geometry. The first VBO contains the per-vertex data which is to be rendered, while the second contains indices into that first VBO which allows triangles to be generated which share the vertex records. In "smooth" geometry vertices tend to be shared, so this rendering method tends to be more efficient than using expanded arrays of vertices.
from OpenGLContext.scenegraph.basenodes import Sphere class TestContext( BaseContext ): """Demonstrates use of attribute types in GLSL """ def OnInit( self ): """Initialize the context"""

Phong and Blinn Reflectance

A shiny surface will tend to have a "bright spot" at the point on the surface where the angle of incidence for the reflected light ray and the viewer's ray are (close to) equal. A perfect mirror would have the brights spot solely when the two vectors are exactly equal, while a perfect Lambertian surface would have the "bright spot" spread across the entire surface.
The Phong rendering process models this as a setting, traditionally called material "shininess" in Legacy OpenGL. This setting acts as a power which raises the cosine (dot product) of the angle between the reflected ray and the eye. The calculation of the cosine (dot product) of the two angles requires that we do a dot product of the two angles once for each vertex/fragment for which we wish to calculate the specular reflectance, we also have to find the angle of reflectance before we can do the calculation:
""" L_dir = (V_pos-L_pos) R = 2N*(dot( N, L_dir))-L_dir // Note: in eye-coordinate system, Eye_pos == (0,0,0) Spec_factor = pow( dot( R, V_pos-Eye_pos ), shininess) """
which, as we can see, involves the vertex position in a number of stages of the operation, so requires recalculation all through the rendering operation.
There is, however, a simplified version of Phong Lighting called Blinn-Phong which notes that if we were to do all of our calculations in "eye space", and were to assume that (as is normal), the eye and light coordinates will not change for a rendering pass, (note: this limits us to directional lights!) we can use a pre-calculated value which is the bisecting angle between the light-vector and the view-vector, called the "half vector" to perform approximately the same calculation. With this value:
""" // note that in Eye coordinates, Eye_EC_dir == 0,0,-1 H = normalize( Eye_EC_dir + Light_EC_dir ) Spec_factor = pow( dot( H, N ), shininess ) """
Note: however, that the resulting Spec_factor is not *precisely* the same value as the original calculation, so the "shininess" exponent must be slightly lower to approximate the value that Phong rendering would achieve. The value is, however, considered close to "real world" materials, so the Blinn method is generally preferred to Phong.
Traditionally, n_dot_pos would be cut off at 0.0, but that would create extremely hard-edged cut-offs for specular color. Here we "fudge" the result by 0.05
phong_weightCalc = """ vec2 phong_weightCalc( in vec3 light_pos, // light position in vec3 half_light, // half-way vector between light and view in vec3 frag_normal, // geometry normal in float shininess ) { // returns vec2( ambientMult, diffuseMult ) float n_dot_pos = max( 0.0, dot( frag_normal, light_pos )); float n_dot_half = 0.0; if (n_dot_pos > -.05) { n_dot_half = pow(max(0.0,dot( half_light, frag_normal )), shininess); } return vec2( n_dot_pos, n_dot_half); } """
We are going to use per-fragment rendering. As a result, our vertex shader becomes very simple, just arranging for the Normals to be varied across the surface.
vertex = shaders.compileShader( """ attribute vec3 Vertex_position; attribute vec3 Vertex_normal; varying vec3 baseNormal; void main() { gl_Position = gl_ModelViewProjectionMatrix * vec4( Vertex_position, 1.0 ); baseNormal = gl_NormalMatrix * normalize(Vertex_normal); }""", GL_VERTEX_SHADER)
Our fragment shader looks much like our previous tutorial's vertex shader. As before, we have lots of uniform values, but now we also calculate the light's half-vector (in eye-space coordinates). The phong_weightCalc function does the core Blinn calculation, and we simply use the resulting factor to add to the colour value for the fragment.
Note the use of the eye-coordinate-space to simplify the half-vector calculation, the eye-space eye-vector is always the same value (pointing down the negative Z axis), and the eye-space eye-coordinate is always (0,0,0), so the eye-to-vertex vector is always the eye-space vector position.
fragment = shaders.compileShader( phong_weightCalc + """ uniform vec4 Global_ambient; uniform vec4 Light_ambient; uniform vec4 Light_diffuse; uniform vec4 Light_specular; uniform vec3 Light_location; uniform float Material_shininess; uniform vec4 Material_specular; uniform vec4 Material_ambient; uniform vec4 Material_diffuse; varying vec3 baseNormal; void main() { // normalized eye-coordinate Light location vec3 EC_Light_location = normalize( gl_NormalMatrix * Light_location ); // half-vector calculation vec3 Light_half = normalize( EC_Light_location - vec3( 0,0,-1 ) ); vec2 weights = phong_weightCalc( EC_Light_location, Light_half, baseNormal, Material_shininess ); gl_FragColor = clamp( ( (Global_ambient * Material_ambient) + (Light_ambient * Material_ambient) + (Light_diffuse * Material_diffuse * weights.x) // material's shininess is the only change here... + (Light_specular * Material_specular * weights.y) ), 0.0, 1.0); } """, GL_FRAGMENT_SHADER) self.shader = shaders.compileProgram(vertex,fragment)
Here's the call that creates the two VBOs and the count of records to render from them. If you're curious you can read through the source code of the OpenGLContext.scenegraph.quadrics module to read the mechanism that generates the values.
The sphere is a simple rendering mechanism, as for a unit-sphere at the origin, the sphere's normals are the same as the sphere's vertex coordinate. The complexity comes primarily in generating the triangle indices that link the points generated.
self.coords,self.indices,self.count = Sphere( radius = 1 ).compile()
We have a few more uniforms to control the specular components. Real-world coding would also calculate the light's half-vector and provide it as a uniform (so that it would only need to be calculated once), but we are going to do the half-vector calculation in the shader to make it obvious what is going on. The legacy OpenGL pipeline provides the value pre-calculated as part of the light structure in GLSL.
for uniform in ( 'Global_ambient', 'Light_ambient','Light_diffuse','Light_location', 'Light_specular', 'Material_ambient','Material_diffuse', 'Material_shininess','Material_specular', ): location = glGetUniformLocation( self.shader, uniform ) if location in (None,-1): print 'Warning, no uniform: %s'%( uniform ) setattr( self, uniform+ '_loc', location ) for attribute in ( 'Vertex_position','Vertex_normal', ): location = glGetAttribLocation( self.shader, attribute ) if location in (None,-1): print 'Warning, no attribute: %s'%( uniform ) setattr( self, attribute+ '_loc', location ) def Render( self, mode = None): """Render the geometry for the scene.""" BaseContext.Render( self, mode ) glUseProgram(self.shader) try:

Indexed VBO Rendering

You'll notice here that we are binding two different VBO objects. As we mentioned above, the Sphere renderer generated both VBOs, but doesn't the second binding replace the first binding? That is, why doesn't OpenGL try to read the Vertex data out of the indices VBO?
OpenGL defines multiple binding "targets" for VBOs, the first VBO (vertices) was bound to the GL_ARRAY_BUFFER target (the default for the class), which is used for reading per-vertex data arrays, while the indices buffer was defined as targetting the GL_ELEMENT_ARRAY_BUFFER, which is used solely for reading indices.
Each target can be bound to a different VBO, and thus we can bind both VBOs at the same time without confusion.
self.coords.bind() self.indices.bind()
Here, being lazy, we use the numpy array's nbytes value to specify the stride between records. The VBO object has a "data" value which is the data-set which was initially passed to the VBO constructor. The first element in this array is a single vertex record. This array happens to have 8 floating-point values (24 bytes), the first three being the vertex position, the next two being the texture coordinate and the last three being the vertex normal. We'll ignore the texture coordinate for now.
stride =[0].nbytes try: glUniform4f( self.Global_ambient_loc, .05,.05,.05,.1 ) glUniform4f( self.Light_ambient_loc, .1,.1,.1, 1.0 ) glUniform4f( self.Light_diffuse_loc, .25,.25,.25,1 )
We set up a yellow-ish specular component in the light and move it to rest "just over our right shoulder" in relation to the initial camera.
glUniform4f( self.Light_specular_loc, 0.0,1.0,0,1 ) glUniform3f( self.Light_location_loc, 6,2,4 ) glUniform4f( self.Material_ambient_loc, .1,.1,.1, 1.0 ) glUniform4f( self.Material_diffuse_loc, .15,.15,.15, 1 )
We make the material have a bright specular white colour and an extremely "shiny" surface. The shininess value has the effect of reducing the area of the highlight, as the cos of the angle is raised to the power of the (fractional) shininess.
glUniform4f( self.Material_specular_loc, 1.0,1.0,1.0, 1.0 ) glUniform1f( self.Material_shininess_loc, .95) glEnableVertexAttribArray( self.Vertex_position_loc ) glEnableVertexAttribArray( self.Vertex_normal_loc ) glVertexAttribPointer( self.Vertex_position_loc, 3, GL_FLOAT,False, stride, self.coords ) glVertexAttribPointer( self.Vertex_normal_loc, 3, GL_FLOAT,False, stride, self.coords+(5*4) )
Here we introduce the OpenGL call which renders via an index-array rather than just rendering vertices in definition order. The last two arguments tell OpenGL what data-type we've used for the indices (the Sphere renderer uses shorts). The indices VBO is actually just passing the value c_void_p( 0 ) (i.e. a null pointer), which causes OpenGL to use the currently bound VBO for the GL_ELEMENT_ARRAY_BUFFER target.
glDrawElements( GL_TRIANGLES, self.count, GL_UNSIGNED_SHORT, self.indices ) finally:
Note the need to unbind *both* VBOs, we have to free *both* VBO targets to avoid any other rendering operation from trying to access the VBOs.
self.coords.unbind() self.indices.unbind() glDisableVertexAttribArray( self.Vertex_position_loc ) glDisableVertexAttribArray( self.Vertex_normal_loc ) finally: glUseProgram( 0 ) if __name__ == "__main__": TestContext.ContextMainLoop()
Our per-fragment Blinn-Phong rendering engine is a very simplistic model of real-world lighting, and is currently limited to a single directional light. Our next tutorial will begin to reduce these "simplifying assumptions".