glsl Tutorial => Changing the geometry with tessellation shaders in...

Example

A simple OGL 4.0 GLSL shader program that shows that shows how to add details with tessellation shader to the geometry. The program is executed with a python script. To run the script, PyOpenGL and NumPy must be installed.

The basic mesh in this example is an icosahedron that consists of 20 triangles. The tessellation control shader defines how each triangle is divided into a set of many small parts. When tessellating a triangle, the generated data are barycentric coordinates based on the original triangle. The tessellation evaluation shader generates new geometry from the data obtained in this way. In this example, each triangle gets a peak in the middle, which rises outward from the center of the icosader. In this way a much more complex geometry is generated than the original icosahedron.

Vertex shader

tess.vert

layout (location = 0) in vec3 inPos;
layout (location = 1) in vec3 inNV;

out TVertexData
{
    vec3 pos;
    vec3 nv;
} outData;

uniform mat4 u_projectionMat44;
uniform mat4 u_modelViewMat44;
uniform mat3 u_normalMat33;

void main()
{
    vec4 viewPos = u_modelViewMat44 * vec4( inPos, 1.0 );
    
    outData.pos = viewPos.xyz / viewPos.w;
    outData.nv  = u_normalMat33 * normalize( inNV );
    
    gl_Position = u_projectionMat44 * viewPos;
}

Tessellation control shader

tess.tctrl

#version 400

layout( vertices=3 ) out;

in TVertexData
{
    vec3 pos;
    vec3 nv;
} inData[];

out TVertexData
{
    vec3 pos;
    vec3 nv;
} outData[];

void main()
{
    outData[gl_InvocationID].pos = inData[gl_InvocationID].pos;
    outData[gl_InvocationID].nv  = inData[gl_InvocationID].nv;
  
    if ( gl_InvocationID == 0 )
    {
        gl_TessLevelOuter[0] = 10.0;
        gl_TessLevelOuter[1] = 10.0;
        gl_TessLevelOuter[2] = 10.0;
        gl_TessLevelInner[0] = 10.0;
    }
}

Tessellation evaluation shader

tess.teval

#version 400

layout(triangles, equal_spacing, ccw) in;

in TVertexData
{
    vec3 pos;
    vec3 nv;
} inData[];

out TTessData
{
    vec3  pos;
    vec3  nv;
    float height;
} outData;

uniform mat4 u_projectionMat44;

void main()
{
    float sideLen[3] = float[3]
    (
        length( inData[1].pos - inData[0].pos ),
        length( inData[2].pos - inData[1].pos ),
        length( inData[0].pos - inData[2].pos )
    );
    float s = ( sideLen[0] + sideLen[1] + sideLen[2] ) / 2.0;
    float rad = sqrt( (s - sideLen[0]) * (s - sideLen[1]) * (s - sideLen[2]) / s );

    vec3 cpt = ( inData[0].pos + inData[1].pos + inData[2].pos ) / 3.0;
    vec3 pos = inData[0].pos * gl_TessCoord.x + inData[1].pos * gl_TessCoord.y + inData[2].pos * gl_TessCoord.z;
    vec3 nv  = normalize( inData[0].nv * gl_TessCoord.x + inData[1].nv * gl_TessCoord.y + inData[2].nv * gl_TessCoord.z );

    float cptDist      = length( cpt - pos );
    float sizeRelation = 1.0 - min( rad, cptDist ) / rad; 
    float height       = pow( sizeRelation, 2.0 );

    outData.pos    = pos + nv * height * rad;
    outData.nv     = mix( nv, normalize( pos - cpt ), height );
    outData.height = height;

    gl_Position = u_projectionMat44 * vec4( outData.pos, 1.0 );
}

Fragment shader

tess.frag

#version 400

in TTessData
{
    vec3  pos;
    vec3  nv;
    float height;
} inData;

out vec4 fragColor;

uniform sampler2D u_texture;

uniform UB_material
{
    float u_roughness;
    float u_fresnel0;
    vec4  u_color;
    vec4  u_specularTint;
};

struct TLightSource
{
    vec4 ambient;
    vec4 diffuse;
    vec4 specular;
    vec4 dir;
};

uniform UB_lightSource
{
    TLightSource u_lightSource;
};

float Fresnel_Schlick( in float theta );
vec3 LightModel( in vec3 esPt, in vec3 esPtNV, in vec3 col, in vec4 specularTint, in float roughness, in float fresnel0 );

void main()
{
    vec3 col = mix( u_color.rgb, vec3( 1.0, 1.0, 1.0 ), inData.height );
    vec3 lightCol = LightModel( inData.pos, inData.nv, col, u_specularTint, u_roughness, u_fresnel0 );
    fragColor = vec4( clamp( lightCol, 0.0, 1.0 ), 1.0 );
}

float Fresnel_Schlick( in float theta )
{
    float m = clamp( 1.0 - theta, 0.0, 1.0 );
    float m2 = m * m;
    return m2 * m2 * m; // pow( m, 5.0 )
}

vec3 LightModel( in vec3 esPt, in vec3 esPtNV, in vec3 col, in vec4 specularTint, in float roughness, in float fresnel0 )
{
  vec3  esVLight      = normalize( -u_lightSource.dir.xyz );
  vec3  esVEye        = normalize( -esPt );
  vec3  halfVector    = normalize( esVEye + esVLight );
  float HdotL         = dot( halfVector, esVLight );
  float NdotL         = dot( esPtNV, esVLight );
  float NdotV         = dot( esPtNV, esVEye );
  float NdotH         = dot( esPtNV, halfVector );
  float NdotH2        = NdotH * NdotH;
  float NdotL_clamped = max( NdotL, 0.0 );
  float NdotV_clamped = max( NdotV, 0.0 );
  float m2            = roughness * roughness;
  
  // Lambertian diffuse
  float k_diffuse = NdotL_clamped;
  // Schlick approximation
  float fresnel = fresnel0 + ( 1.0 - fresnel0 ) * Fresnel_Schlick( HdotL );
  // Beckmann distribution
  float distribution = max( 0.0, exp( ( NdotH2 - 1.0 ) / ( m2 * NdotH2 ) ) / ( 3.14159265 * m2 * NdotH2 * NdotH2 ) );
  // Torrance-Sparrow geometric term
  float geometric_att = min( 1.0, min( 2.0 * NdotH * NdotV_clamped / HdotL, 2.0 * NdotH * NdotL_clamped / HdotL ) );
  // Microfacet bidirectional reflectance distribution function 
  float k_specular = fresnel * distribution * geometric_att / ( 4.0 * NdotL_clamped * NdotV_clamped );
  
  vec3 lightColor = col.rgb * u_lightSource.ambient.rgb +
                    max( 0.0, k_diffuse ) * col.rgb * u_lightSource.diffuse.rgb +
                    max( 0.0, k_specular ) * mix( col.rgb, specularTint.rgb, specularTint.a ) * u_lightSource.specular.rgb;
  return lightColor;
}

Python script

from OpenGL.GL import *
from OpenGL.GLUT import *
from OpenGL.GLU import *
import numpy as np
from time import time
import math
import sys

sin120 = 0.8660254
rotateCamera = False

# draw event
def OnDraw():
    dist = 3.0
    currentTime = time()
    comeraRotAng = CalcAng( currentTime, 10.0 ) 
    # set up projection matrix
    prjMat = Perspective(90.0, wndW/wndH, 0.5, 100.0) 
    # set up view matrix
    viewMat = np.matrix(np.identity(4), copy=False, dtype='float32')
    viewMat = Translate( viewMat, np.array( [0.0, 0.0, -12.0] ) )
    viewMat = RotateView( viewMat, [30.0, comeraRotAng if rotateCamera else 0.0, 0.0] )

    # set up light source
    lightSourceBuffer.BindDataFloat(b'u_lightSource.dir', TransformVec4([-1.0, -1.0, -5.0, 0.0], viewMat) )
    
    # set up icosahedron model matrix
    icoModelMat = np.matrix(np.identity(4), copy=False, dtype='float32')
    if not rotateCamera: icoModelMat = RotateY( icoModelMat, comeraRotAng ) 
    icoModelMat = Scale( icoModelMat, np.repeat( 5, 3 ) )
    icoModelMat = RotateY( icoModelMat, CalcAng( currentTime, 17.0 ) )
    icoModelMat = RotateX( icoModelMat, CalcAng( currentTime, 13.0 ) )

    # set up attributes and shader program
    glEnable( GL_DEPTH_TEST )
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT )
    glUseProgram( shaderProgram )
    glUniformMatrix4fv( projectionMatLocation, 1, GL_FALSE, prjMat )
    lightSourceBuffer.BindToTarget()
    
    # draw icosahedron
    icoMaterialBuffer.BindToTarget()
    modelViewMat = Multiply(viewMat, icoModelMat)
    glUniformMatrix4fv( modelViewMatLocation, 1, GL_FALSE, modelViewMat )
    glUniformMatrix3fv( normalMatLocation, 1, GL_FALSE, ToMat33(modelViewMat) )
    glBindVertexArray( icoVAObj )
    glPatchParameteri( GL_PATCH_VERTICES, 3 )
    glDrawArrays( GL_PATCHES, 0, len(icoPosData) )
    
    glutSwapBuffers()

def Fract(val): return val - math.trunc(val)
def CalcAng(currentTime, intervall): return Fract( (currentTime - startTime) / intervall ) * 360.0
def CalcMove(currentTime, intervall, range):
    pos = Fract( (currentTime - startTime) / intervall ) * 2.0
    pos = pos if pos < 1.0 else (2.0-pos)
    return range[0] + (range[1] - range[0]) * pos
    
# read shader program and compile shader
def CompileShader( sourceFileName, shaderStage ):
    with open( sourceFileName, 'r' ) as sourceFile:
        sourceCode = sourceFile.read()
    nameMap = { GL_VERTEX_SHADER: 'vertex', GL_GEOMETRY_SHADER: 'geometry', GL_FRAGMENT_SHADER: 'fragment' }    
    print( '\n%s shader code:' % nameMap.get(shaderStage, '') )
    print( sourceCode )
    shaderObj = glCreateShader( shaderStage )
    glShaderSource( shaderObj, sourceCode )
    glCompileShader( shaderObj )
    result = glGetShaderiv( shaderObj, GL_COMPILE_STATUS )
    if not (result):
        print( glGetShaderInfoLog( shaderObj ) )
        sys.exit()
    return shaderObj

# link shader objects to shader program
def LinkProgram( shaderObjs ):
    shaderProgram = glCreateProgram()
    for shObj in shaderObjs:
        glAttachShader( shaderProgram, shObj )
    glLinkProgram( shaderProgram )
    result = glGetProgramiv( shaderProgram, GL_LINK_STATUS )
    if not (result):
        print( 'link error:' )
        print( glGetProgramInfoLog( shaderProgram ) )
        sys.exit()
    return shaderProgram

# create vertex array object
def CreateVAO( dataArrays ):
    noOfBuffers = len(dataArrays)
    buffers = glGenBuffers(noOfBuffers)
    newVAObj = glGenVertexArrays( 1 )
    glBindVertexArray( newVAObj )
    for inx in range(0, noOfBuffers):
        vertexSize, dataArr = dataArrays[inx]
        arr = np.array( dataArr, dtype='float32' )
        glBindBuffer( GL_ARRAY_BUFFER, buffers[inx] )
        glBufferData( GL_ARRAY_BUFFER, arr, GL_STATIC_DRAW )
        glEnableVertexAttribArray( inx )
        glVertexAttribPointer( inx, vertexSize, GL_FLOAT, GL_FALSE, 0, None )
    return newVAObj 

# representation of a uniform block
class UniformBlock:
     def __init__(self, shaderProg, name):
        self.shaderProg = shaderProg 
        self.name = name
     def Link(self, bindingPoint):
        self.bindingPoint = bindingPoint
        self.noOfUniforms = glGetProgramiv(self.shaderProg, GL_ACTIVE_UNIFORMS)
        self.maxUniformNameLen = glGetProgramiv(self.shaderProg, GL_ACTIVE_UNIFORM_MAX_LENGTH)
        self.index = glGetUniformBlockIndex(self.shaderProg, self.name)
        intData = np.zeros(1, dtype=int)
        glGetActiveUniformBlockiv(self.shaderProg, self.index, GL_UNIFORM_BLOCK_ACTIVE_UNIFORMS, intData)
        self.count = intData[0]
        self.indices = np.zeros(self.count, dtype=int)
        glGetActiveUniformBlockiv(self.shaderProg, self.index, GL_UNIFORM_BLOCK_ACTIVE_UNIFORM_INDICES, self.indices)
        self.offsets = np.zeros(self.count, dtype=int)
        glGetActiveUniformsiv(self.shaderProg, self.count, self.indices, GL_UNIFORM_OFFSET, self.offsets)
        strLengthData = np.zeros(1, dtype=int)
        arraysizeData = np.zeros(1, dtype=int)
        typeData = np.zeros(1, dtype='uint32')
        nameData = np.chararray(self.maxUniformNameLen+1)
        self.namemap = {}
        self.dataSize = 0 
        for inx in range(0, len(self.indices)):
            glGetActiveUniform( self.shaderProg, self.indices[inx], self.maxUniformNameLen, strLengthData, arraysizeData,     typeData, nameData.data )
            name = nameData.tostring()[:strLengthData[0]]
            self.namemap[name] = inx
            self.dataSize = max(self.dataSize, self.offsets[inx] + arraysizeData * 16) 
        glUniformBlockBinding(self.shaderProg, self.index, self.bindingPoint)
        print('\nuniform block %s size:%4d' % (self.name, self.dataSize))
        for uName in self.namemap:
            print( '    %-40s index:%2d    offset:%4d' % (uName, self.indices[self.namemap[uName]], self.offsets[self.namemap    [uName]]) ) 

# representation of a uniform block buffer
class UniformBlockBuffer:
    def __init__(self, ub):
        self.namemap = ub.namemap
        self.offsets = ub.offsets
        self.bindingPoint = ub.bindingPoint
        self.object = glGenBuffers(1)
        self.dataSize = ub.dataSize
        glBindBuffer(GL_UNIFORM_BUFFER, self.object)
        dataArray = np.zeros(self.dataSize//4, dtype='float32')
        glBufferData(GL_UNIFORM_BUFFER, self.dataSize, dataArray, GL_DYNAMIC_DRAW)
    def BindToTarget(self):
        glBindBuffer(GL_UNIFORM_BUFFER, self.object)
        glBindBufferBase(GL_UNIFORM_BUFFER, self.bindingPoint, self.object)
    def BindDataFloat(self, name, dataArr):
        glBindBuffer(GL_UNIFORM_BUFFER, self.object)
        dataArray = np.array(dataArr, dtype='float32')
        glBufferSubData(GL_UNIFORM_BUFFER, self.offsets[self.namemap[name]], len(dataArr)*4, dataArray)

def Translate(matA, trans):
    matB = np.copy(matA)
    for i in range(0, 4): matB[3,i] = matA[0,i] * trans[0] + matA[1,i] * trans[1] + matA[2,i] * trans[2] + matA[3,i] 
    return matB

def Scale(matA, s):
    matB = np.copy(matA)
    for i0 in range(0, 3):
        for i1 in range(0, 4): matB[i0,i1] = matA[i0,i1] * s[i0] 
    return matB

def RotateHlp(matA, angDeg, a0, a1):
    matB = np.copy(matA)
    ang = math.radians(angDeg)
    sinAng, cosAng = math.sin(ang), math.cos(ang)
    for i in range(0, 4):
        matB[a0,i] = matA[a0,i] * cosAng + matA[a1,i] * sinAng
        matB[a1,i] = matA[a0,i] * -sinAng + matA[a1,i] * cosAng
    return matB

def RotateX(matA, angDeg): return RotateHlp(matA, angDeg, 1, 2)
def RotateY(matA, angDeg): return RotateHlp(matA, angDeg, 2, 0)
def RotateZ(matA, angDeg): return RotateHlp(matA, angDeg, 0, 1)
def RotateView(matA, angDeg): return RotateZ(RotateY(RotateX(matA, angDeg[0]), angDeg[1]), angDeg[2])

def Multiply(matA, matB):
    matC = np.copy(matA)
    for i0 in range(0, 4):
        for i1 in range(0, 4):
            matC[i0,i1] = matB[i0,0] * matA[0,i1] + matB[i0,1] * matA[1,i1] + matB[i0,2] * matA[2,i1] + matB[i0,3] * matA[3,i1]        
    return matC

def ToMat33(mat44):
    mat33 = np.matrix(np.identity(3), copy=False, dtype='float32')
    for i0 in range(0, 3):
        for i1 in range(0, 3): mat33[i0, i1] = mat44[i0, i1]
    return mat33

def TransformVec4(vecA,mat44):
    vecB = np.zeros(4, dtype='float32')
    for i0 in range(0, 4):
        vecB[i0] = vecA[0] * mat44[0,i0] + vecA[1] * mat44[1,i0] + vecA[2] * mat44[2,i0]  + vecA[3] * mat44[3,i0]
    return vecB

def Perspective(fov, aspectRatio, near, far):
    fn, f_n = far + near, far - near
    r, t = aspectRatio, 1.0 / math.tan( math.radians(fov) / 2.0 )
    return np.matrix( [ [t/r,0,0,0], [0,t,0,0], [0,0,-fn/f_n,-2.0*far*near/f_n], [0,0,-1,0] ] )

def AddToBuffer( buffer, data, count=1 ): 
    for inx_c in range(0, count):
        for inx_s in range(0, len(data)): buffer.append( data[inx_s] ) 

# initialize glut
glutInit()

# create window
wndW, wndH = 800, 600
glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_ALPHA | GLUT_DEPTH)
glutInitWindowPosition(0, 0)
glutInitWindowSize(wndW, wndH)
wndID = glutCreateWindow(b'OGL window') 
glutDisplayFunc(OnDraw) 
glutIdleFunc(OnDraw)

# define icosahedron vertex array opject
icoPts = [
    ( 0.000,  0.000,  1.000), ( 0.894,  0.000,  0.447), ( 0.276,  0.851,  0.447), (-0.724,  0.526,  0.447),
    (-0.724, -0.526,  0.447), ( 0.276, -0.851,  0.447), ( 0.724,  0.526, -0.447), (-0.276,  0.851, -0.447), 
    (-0.894,  0.000, -0.447), (-0.276, -0.851, -0.447), ( 0.724, -0.526, -0.447), ( 0.000,  0.000, -1.000) ]
icoCol = [ [1.0, 0.0, 0.0], [0.0, 0.0, 1.0], [1.0, 1.0, 0.0], [0.0, 1.0, 0.0], [1.0, 0.5, 0.0], [1.0, 0.0, 1.0] ]
icoIndices = [
  2,  0,  1,  3,  0,  2,  4,  0,  3,  5,  0,  4,  1,  0,  5, 11,  7,  6, 11,  8,  7, 11,  9,  8, 11, 10,  9, 11,  6, 10, 
  1,  6,  2,  2,  7,  3,  3,  8,  4,  4,  9,  5,  5, 10,  1,  2,  6,  7,  3,  7,  8,  4,  8,  9,  5,  9, 10,  1, 10,  6  ]
icoPosData = []
for inx in icoIndices: AddToBuffer( icoPosData, icoPts[inx] )
icoNVData = []
for inx_nv in range(0, len(icoIndices) // 3):
    nv = [0.0, 0.0, 0.0]
    for inx_p in range(0, 3): 
        for inx_s in range(0, 3): nv[inx_s] += icoPts[ icoIndices[inx_nv*3 + inx_p] ][inx_s]
    AddToBuffer( icoNVData, nv, 3 )
icoVAObj = CreateVAO( [ (3, icoPosData), (3, icoNVData) ] )

# load, compile and link shader
shaderProgram = LinkProgram( [
        CompileShader( 'tess.vert', GL_VERTEX_SHADER ),
        CompileShader( 'tess.tctrl', GL_TESS_CONTROL_SHADER ),
        CompileShader( 'tess.teval', GL_TESS_EVALUATION_SHADER ), 
        CompileShader( 'tess.frag', GL_FRAGMENT_SHADER )
    ] )
# get unifor locations
projectionMatLocation = glGetUniformLocation(shaderProgram, "u_projectionMat44")
modelViewMatLocation  = glGetUniformLocation(shaderProgram, "u_modelViewMat44")
normalMatLocation     = glGetUniformLocation(shaderProgram, "u_normalMat33")
# linke uniform blocks
ubMaterial = UniformBlock(shaderProgram, "UB_material")
ubLightSource = UniformBlock(shaderProgram, "UB_lightSource")
ubMaterial.Link(1)
ubLightSource.Link(2)

# create uniform block buffers
lightSourceBuffer = UniformBlockBuffer(ubLightSource)
lightSourceBuffer.BindDataFloat(b'u_lightSource.ambient', [0.2, 0.2, 0.2, 1.0])
lightSourceBuffer.BindDataFloat(b'u_lightSource.diffuse', [0.2, 0.2, 0.2, 1.0])
lightSourceBuffer.BindDataFloat(b'u_lightSource.specular', [1.0, 1.0, 1.0, 1.0])

icoMaterialBuffer = UniformBlockBuffer(ubMaterial)
icoMaterialBuffer.BindDataFloat(b'u_roughness', [0.45])
icoMaterialBuffer.BindDataFloat(b'u_fresnel0', [0.4])
icoMaterialBuffer.BindDataFloat(b'u_color', [0.6, 0.5, 0.8, 1.0])
icoMaterialBuffer.BindDataFloat(b'u_specularTint',[1.0, 0.5, 0.5, 0.8])

# start main loop
startTime = time()
glutMainLoop()

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