Question

I am working on adding a phong shader to my working program. Basically, after I implemented my new shaders, my code gets a “Segmentation Fault: 11” during:

glDrawArrays(GL_TRIANGLES, 0, mCubes.getArrayNumberOfElements());

I know the number of elements is correct because it worked for my previous, simple shader.

Here is my Vertex Shader:

// vertex shader
attribute vec4 vPosition;
attribute vec3 vNormal;
varying vec4 color; //vertex shader
// light and material properties
uniform vec4 AmbientProduct, DiffuseProduct, SpecularProduct; 
uniform mat4 ModelView;
//uniform mat4 Projection;
uniform vec4 LightPosition;
uniform float Shininess;
vec3 L, H, N, pos, E;
vec4 diffuse, specular, ambient;
float Kd, Ks;

void main()
{
// Transform vertex position into eye coordinates 
pos = (ModelView * vPosition).xyz; 
L = normalize( LightPosition.xyz - pos ); 
E = normalize( -pos );
H = normalize( L + E );
// Transform vertex normal into eye coordinates
N = normalize( ModelView*vec4(vNormal, 0.0) ).xyz;
// Compute terms in the illumination equation 
ambient = AmbientProduct;
Kd = max( dot(L, N), 0.0 );
diffuse = Kd*DiffuseProduct;
Ks = pow( max(dot(N, H), 0.0), Shininess );
specular = Ks * SpecularProduct;
if( dot(L, N) < 0.0 ) 
specular = vec4(0.0, 0.0, 0.0, 1.0); 

gl_Position = ModelView * vPosition;
color = ambient + diffuse + specular;
color.a = 1.0; 
        }

Here is my display function in which the code ends up getting the fault:

void display(void) {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

vector<float> cell = mCubes.getCell();

mat4 matrix = rot * scale(1.0/cell[0], 1.0/cell[1], 1.0/cell[2]) * translate(-cell[0]/2.0, -cell[1]/2.0, -cell[2]/2.0);
glUniformMatrix4fv(vShaderModelView, 1, GL_TRUE, matrix);

glDrawArrays(GL_TRIANGLES, 0, mCubes.getArrayNumberOfElements());


glutSwapBuffers();
glFlush();
}

And here is my init function that mostly sets up and interacts with the shaders:

void init() {
// Create a vertex array object
GLuint vao;
#ifdef __APPLE__
glGenVertexArraysAPPLE( 1, &vao );
glBindVertexArrayAPPLE( vao );
#else
glGenVertexArrays( 1, &vao );
glBindVertexArray( vao );
#endif

// Create and initialize a buffer object
GLuint buffer;

glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glBufferData( GL_ARRAY_BUFFER,
             mCubes.getDisplayArraySize() + mCubes.getDisplayArraySize()*3,                NULL, GL_STATIC_DRAW );
GLintptr offset = 0;
glBufferSubData(GL_ARRAY_BUFFER, offset, mCubes.getDisplayArraySize(), mCubes.getDisplayArray());
offset+= mCubes.getDisplayArraySize();

glBufferSubData(GL_ARRAY_BUFFER, offset, mCubes.getDisplayArraySize(), mCubes.getNormalVector());

// Load shaders and use the resulting shader program
string evname = "PROTCAD3DIR";
string path = PCGeneralIO::getEnvironmentVariable(evname);

path += "/data/shaders/";
#ifdef __APPLE__
string vshadername = path + "kw_vshader1_mac.glsl";
string fshadername = path + "kw_fshader1_mac.glsl";
//#else
//  string vshadername = path + "kw_vshader1.glsl";
//  string fshadername = path + "kw_fshader1.glsl";
#endif

GLuint program = InitShader( vshadername.c_str(), fshadername.c_str() );

glUseProgram(program);
// Initialize the vertex position attribute from the vertex shader    
GLuint vShaderPosition = glGetAttribLocation(program, "vPosition");
glEnableVertexAttribArray(vShaderPosition);
glVertexAttribPointer(vShaderPosition, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0));

GLuint vShaderNormal = glGetAttribLocation(program, "vNormal");
glEnableVertexAttribArray(vShaderNormal);
//glVertexAttribPointer(vShaderPosition, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(offset)); //this was the ORIGINAL PROBLEM, now commented out and below is solution
  glVertexAttribPointer(vShaderNormal, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(offset));


vShaderModelView = glGetUniformLocation(program, "ModelView");
vShaderLightPosition = glGetUniformLocation(program, "LightPosition");

vShaderAmbientProduct = glGetUniformLocation(program, "AmbientProduct");
vShaderDiffuseProduct = glGetUniformLocation(program, "DiffuseProduct");
vShaderSpecularProduct = glGetUniformLocation(program, "SpecularProduct");
vShaderShininess = glGetUniformLocation(program, "SpecularProduct");
glEnable( GL_DEPTH_TEST );

vec4 light = vec4(0.5,1.5,1.0,0.0);
glUniform4fv(vShaderLightPosition, 1, light);
vec4 amb = vec4(1.0f,0.0f,0.20f,1.0f);
glUniform4fv(vShaderAmbientProduct, 1, amb);
vec4 diff = vec4(0.5f,0.5f,0.5f,1.0f);
glUniform4fv(vShaderDiffuseProduct, 1, diff);
vec4 spec = vec4(0.80f,0.80f,0.80f,1.0f);
glUniform4fv(vShaderSpecularProduct, 1, spec);
float shin = 6.0f;
glUniform1f(vShaderShininess,shin);


glClearColor(.2, .2, .2, 1); /* Grey background */
}

If you have any question, feel free to ask and I will elaborate. I feel that either the vertex shader itself has a problem, or the way I interact with the shader is doing something wonky. Any help or suggestions are accepted!

EDIT::: (code edited to reflect solution)The problem was in the second:

glVertexAttribPointer(vShaderPosition, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(offset));

which should have read:

glVertexAttribPointer(vShaderNormal, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(offset));

And was a stupid copy/paste mistake. However, the finished product still does not look correct:

![at Rotation 0 it seems to be fully colored][1]

https://i.stack.imgur.com/CKJ3f.png

![Rotation of a little bit reveals some odd behavior][2]

https://i.stack.imgur.com/kyRfI.png

![Even more rotation leads you to pull your hair out][3]

i.stack.imgur.com/lYOzK.png

![Then it whites out and you know i screwed up!!][4]

i.stack.imgur.com/FZcqF.png

So, as you rotate the color gets screwed up and turns white, black, patterned and everything, but this is obviously incorrect.

Edit::: This is my attempt to “Correct” the issue of passing the wrong amount of values with vNormal:

void init() {
// Create a vertex array object
GLuint vao;
#ifdef __APPLE__
glGenVertexArraysAPPLE( 1, &vao );
glBindVertexArrayAPPLE( vao );
#else
glGenVertexArrays( 1, &vao );
glBindVertexArray( vao );
#endif

// Create and initialize a buffer object
GLuint buffer;
realVec *normArray = new realVec[mCubes.getNormalArraySize()];//vec4 array compared to vec3 array     
normArray = mCubes.getNormalVector(); // new array of normals

for(int i=0; i<mCubes.getArrayNumberOfElements();i++){
    printf("Normal at %d  is %f \n",i,normArray[i][0]); //to print normals
    printf("Normal at %d  is %f \n",i,normArray[i][1]); //to print normals
    printf("Normal at %d  is %f \n",i,normArray[i][2]); //to print normals
}

glGenBuffers(1, &buffer);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
glBufferData( GL_ARRAY_BUFFER,
             mCubes.getDisplayArraySize() + mCubes.getNormalArraySize(),                NULL, GL_STATIC_DRAW ); //Changed size for vec3 array of normals
GLintptr offset = 0;
glBufferSubData(GL_ARRAY_BUFFER, offset, mCubes.getDisplayArraySize(), mCubes.getDisplayArray());
offset+= mCubes.getDisplayArraySize();

glBufferSubData(GL_ARRAY_BUFFER, offset, mCubes.getNormalArraySize(), normArray);

// Load shaders and use the resulting shader program
string evname = "PROTCAD3DIR";
string path = PCGeneralIO::getEnvironmentVariable(evname);

path += "/data/shaders/";
#ifdef __APPLE__
string vshadername = path + "kw_vshader1_mac.glsl";
string fshadername = path + "kw_fshader1_mac.glsl";
//#else
//  string vshadername = path + "kw_vshader1.glsl";
//  string fshadername = path + "kw_fshader1.glsl";
#endif

GLuint program = InitShader( vshadername.c_str(), fshadername.c_str() );

glUseProgram(program);
//offset =0;
// Initialize the vertex position attribute from the vertex shader    
GLuint vShaderPosition = glGetAttribLocation(program, "vPosition");
glEnableVertexAttribArray(vShaderPosition);
glVertexAttribPointer(vShaderPosition, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0));

GLuint vShaderNormal = glGetAttribLocation(program, "vNormal");
glEnableVertexAttribArray(vShaderNormal);
glVertexAttribPointer(vShaderNormal, 3, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(offset));


//vShaderMatrix = glGetUniformLocation(program, "vMatrix");
//vShaderColor = glGetUniformLocation(program, "vColor")
vShaderModelView = glGetUniformLocation(program, "ModelView");
vShaderLightPosition = glGetUniformLocation(program, "LightPosition");
//vShaderProjection = glGetUniformLocation(program, "Projection");

vShaderAmbientProduct = glGetUniformLocation(program, "AmbientProduct");
vShaderDiffuseProduct = glGetUniformLocation(program, "DiffuseProduct");
vShaderSpecularProduct = glGetUniformLocation(program, "SpecularProduct");
vShaderShininess = glGetUniformLocation(program, "SpecularProduct");
glEnable( GL_DEPTH_TEST );

vec4 light = vec4(0.5,1.5,1.0,0.0);
glUniform4fv(vShaderLightPosition, 1, light);
vec4 amb = vec4(1.0f,0.0f,0.20f,1.0f);
glUniform4fv(vShaderAmbientProduct, 1, amb);
vec4 diff = vec4(0.5f,0.5f,0.5f,1.0f);
glUniform4fv(vShaderDiffuseProduct, 1, diff);
vec4 spec = vec4(0.80f,0.80f,0.80f,1.0f);
glUniform4fv(vShaderSpecularProduct, 1, spec);
float shin = 6.0f;
glUniform1f(vShaderShininess,shin);


glClearColor(.2, .2, .2, 1); /* Grey background */
}

Should I maybe change the light, ambient, specular, and diffuse properties? I am not sure what the problem is.

Answer 1

You pass your vNormal attribute data using the following code

glVertexAttribPointer(vShaderNormal, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(offset));

This indicates that your normal have 4 components, whereas in your vertex shader you declare it as

attribute vec3 vNormal;

This mismatch may be related to your problem if the normals are misinterpreted.