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I'm working on a program where I need to draw spheres. The method I'm using is creating an octahedron

enter image description here And using tessellation shaders to subdivide it into a sphere, however there is a patch of the sphere that is not being rendered, and I don't know what's causing it. Keep in mind, this is the same when I stop normalizing the UVW coordinates through the evaluation shader

enter image description here

enter image description here Here is my tessellation evaluation shader

#version 450 core

// from control shader
layout(triangles, equal_spacing, cw) in;

// input from control shader
in vec3 vertex_coord[];
// output vec
out vec3 vert;

// allows for object transformations and for movement
uniform mat4 model;
uniform mat4 view;
uniform mat4 projection;

void main()
{
    // get / make the barycentric coordinates of the octahedron
    vec3 u = gl_TessCoord.x * vertex_coord[0];
    vec3 v = gl_TessCoord.y * vertex_coord[1];
    vec3 w = gl_TessCoord.z * vertex_coord[2];
    // normalizing the positions of the triangles allows for every point to be
    // the same distance away from the center, forming a sphere
    vec3 pos = normalize(u + v + w);
    // output patch point position in clip space
    gl_Position = projection * view * model * vec4(pos, 1.0);
}

Here is my tessellation control shader

#version 450 core

// specify control points per output per patch
// control size of input and output arrays
layout(vertices=3) out;
// input from vertex shader
in vec3 vert_coord[];
// output to evaluation shader
out vec3 vertex_coord[];

// for tessellation
uniform mat4 view;
uniform mat4 model;

void main()
{
    // pass attributes through
    gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;
    vertex_coord[gl_InvocationID] = vert_coord[gl_InvocationID];

    // control tessellation
    if(gl_InvocationID==0)
    {
        // dynamic LOD step by step
        // first: define rendering constants to control tessellation
        const float MIN_TESS_LEVEL = 4;
        const float MAX_TESS_LEVEL = 64;
        const float MIN_DISTANCE = 20;
        const float MAX_DISTANCE = 800;
        // second: transform each vertex into each eye
        vec4 eye_space_pos_1 = view * model * gl_in[0].gl_Position;
        vec4 eye_space_pos_2 = view * model * gl_in[1].gl_Position;
        vec4 eye_space_pos_3 = view * model * gl_in[2].gl_Position;
        // third: distance from camera scaled between 0 and 1
        float distance_1 = clamp((abs(eye_space_pos_1.z)-MIN_DISTANCE)/(MAX_DISTANCE-MIN_DISTANCE), 0.0, 1.0);
        float distance_2 = clamp((abs(eye_space_pos_2.z)-MIN_DISTANCE)/(MAX_DISTANCE-MIN_DISTANCE), 0.0, 1.0);
        float distance_3 = clamp((abs(eye_space_pos_3.z)-MIN_DISTANCE)/(MAX_DISTANCE-MIN_DISTANCE), 0.0, 1.0);
        // fourth: interpolate edge tessellation level based on closer vertex
        float tess_level_1 = mix(MAX_TESS_LEVEL, MIN_TESS_LEVEL, min(distance_3, distance_1));
        float tess_level_2 = mix(MAX_TESS_LEVEL, MIN_TESS_LEVEL, min(distance_1, distance_2));
        float tess_level_3 = mix(MAX_TESS_LEVEL, MIN_TESS_LEVEL, min(distance_2, distance_1));
        // fifth: set the corresponding outer tessellation levels
        gl_TessLevelOuter[0] = tess_level_1;
        gl_TessLevelOuter[1] = tess_level_2;
        gl_TessLevelOuter[2] = tess_level_3;
        // sixth: set the inner tessellation levels
        gl_TessLevelInner[0] = tess_level_1;
        gl_TessLevelInner[1] = tess_level_2;
    }
}

Here is my vertex shader

#version 450 core

// position of the object
layout (location = 0) in vec3 pos;
layout (location = 1) in vec3 vert;

// vertices to make the sphere
out vec3 vert_coord;

void main()
{
    // position object
    gl_Position = vec4(pos, 1.0f);
    // put the vertices into a different vector
    vert_coord = vert;
}

And finally, here are my octahedron vertices and buffers and draw call

    float vertices[] = {
    //top-north-east
     0.0f, -1.0f,  0.0f,
     0.0f,  0.0f,  1.0f,
     1.0f,  0.0f,  0.0f,

    //top-north-west
     0.0f,  1.0f,  0.0f,
    -1.0f,  0.0f,  0.0f,
     0.0f,  0.0f,  1.0f,

    //top-south-west
     0.0f,  1.0f,  0.0f,
     0.0f,  0.0f, -1.0f,
    -1.0f,  0.0f,  0.0f,

    //top-south-east
     0.0f,  -1.0f,  0.0f,
     1.0f,  0.0f,  0.0f,
     0.0f,  0.0f, -1.0f,

    //bottom-north-east
     0.0f, -1.0f,  0.0f,
     1.0f,  0.0f,  0.0f,
     0.0f,  0.0f,  1.0f,

    //bottom-north-west
     0.0f, -1.0f,  0.0f,
     0.0f,  0.0f,  1.0f,
    -1.0f,  0.0f,  0.0f,

    //bottom-south-west
     0.0f, -1.0f,  0.0f,
    -1.0f,  0.0f,  0.0f,
     0.0f,  0.0f, -1.0f,

    //bottom-south-east
     0.0f, 1.0f,  0.0f,
     0.0f,  0.0f, -1.0f,
     1.0f,  0.0f,  0.0f
};

unsigned int indices[] = {
    // first triangle
    0, 1, 2,
    // second triangle
    3, 4, 5,
    // third triangle
    6, 7, 8,
    // fourth triangle
    9, 10, 11,
    // fifth triangle
    12, 13, 14,
    // sixth triangle
    15, 16, 17,
    // seventh triangle
    18, 19, 20,
    // eighth triangle
    21, 22, 23
};

unsigned int vbo, vao, ebo;
glGenVertexArrays(1, &vao);
glGenBuffers(1, &vbo);
glGenBuffers(1, &ebo);

glBindVertexArray(vao);

// upload vertex data to gpu
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices) * sizeof(float), &vertices[0], GL_STATIC_DRAW);

// upload index data to gpu
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices) * sizeof(unsigned int), &indices[0], GL_STATIC_DRAW);

// position attribute
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
glEnableVertexAttribArray(0);

// normal attribute
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)(3 * sizeof(float)));
glEnableVertexAttribArray(1);

// max tessellation points / patches
glPatchParameteri(GL_PATCH_VERTICES, 3);
glDrawElements(GL_PATCHES, 24, GL_UNSIGNED_INT, 0);

As a bonus, if it's helpful, this is where the tutorial where I adapted my code from. The guy making it uses an icosahedron, which did not work when I tried it https://prideout.net/blog/old/blog/index.html@p=48.html

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  • $\begingroup$ there are a few things which look a bit strange to me... 1: you are defining 3 vec3 values for each vertex but only use 2 vertexAttributePointers. 2: you are using GL_ELEMENT_ARRAY_BUFFER with glDrawArrays command instead of glDrawElements. Are there 3 vertices in your vertices list per block? have you tried to disable culling glDisable(GL_CULL_FACE);? maybe one of the patches is oriented the wrong way $\endgroup$
    – Thomas
    Commented Mar 31, 2023 at 6:39
  • $\begingroup$ @pmw1234 my max is 64 $\endgroup$
    – Chillzy
    Commented Mar 31, 2023 at 13:26
  • $\begingroup$ @Thomas i'll try that right now and going back to you $\endgroup$
    – Chillzy
    Commented Mar 31, 2023 at 13:27
  • $\begingroup$ @pmw1234 yeah i'll get that fixed real quick $\endgroup$
    – Chillzy
    Commented Mar 31, 2023 at 13:28
  • 1
    $\begingroup$ There are multiple bugs in the code, fixing one is just causing another to become obvious. Trying a change that is clearly a bug isn't the way to go about fixing it. Identify bugs like the one I just pointed, fix it, then find the next one. $\endgroup$
    – pmw1234
    Commented Apr 1, 2023 at 14:25

1 Answer 1

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So I went onto a different forum and I asked this question, and somebody helped me solve it. It turns out that the coordinates for the octahedron were wrong. The problematic ones were top-north-east and bottom-south-east. I'll give a list of the new and improved coordinates as well as the output of it.

    float vertices[] = {
    //top-north-east
     0.0f, 1.0f,  0.0f,
     0.0f,  0.0f,  1.0f,
     1.0f,  0.0f,  0.0f,

    //top-north-west
     0.0f,  1.0f,  0.0f,
    -1.0f,  0.0f,  0.0f,
     0.0f,  0.0f,  1.0f,

    //top-south-west
     0.0f,  1.0f,  0.0f,
     0.0f,  0.0f, -1.0f,
    -1.0f,  0.0f,  0.0f,

    //top-south-east
     0.0f, -1.0f,  0.0f,
     1.0f,  0.0f,  0.0f,
     0.0f,  0.0f, -1.0f,

    //bottom-north-east
     0.0f, -1.0f,  0.0f,
     1.0f,  0.0f,  0.0f,
     0.0f,  0.0f,  1.0f,

    //bottom-north-west
     0.0f, -1.0f,  0.0f,
     0.0f,  0.0f,  1.0f,
    -1.0f,  0.0f,  0.0f,

    //bottom-south-west
     0.0f, -1.0f,  0.0f,
    -1.0f,  0.0f,  0.0f,
     0.0f,  0.0f, -1.0f,

    //bottom-south-east
     0.0f, 1.0f,  0.0f,
     0.0f,  0.0f, -1.0f,
     1.0f,  0.0f,  0.0f
};

enter image description here

Finally solved after two months...

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