Render an array of triangle struct references in OpenGL

Question

I have a std::vector<Triangle*> that I am trying to render in OpenGL. My vector is defined as:

#pragma once

#include <glm\glm.hpp>

struct Triangle
{
    glm::vec3 _vertex0, _vertex1, _vertex2;
    Triangle(glm::vec3 vertex0, glm::vec3 vertex1, glm::vec3 vertex2)
    {
        _vertex0 = vertex0;
        _vertex1 = vertex1;
        _vertex2 = vertex2;
    }

    Triangle(glm::vec4 vertex0, glm::vec4 vertex1, glm::vec4 vertex2)
    {
        _vertex0 = glm::vec3(vertex0.x, vertex0.y, vertex0.z);
        _vertex1 = glm::vec3(vertex1.x, vertex1.y, vertex1.z);
        _vertex2 = glm::vec3(vertex2.x, vertex2.y, vertex2.z);
    }
};

My Asset struct is:

struct Asset
{
    Asset() { }
    Asset(std::string assetOBJFile)
    {
        glGenVertexArrays(1, &vertexArrayID);
        glBindVertexArray(vertexArrayID);

        programID = LoadShaders("vertexShader.txt", "fragmentShader.txt");

        // Read our .obj file
        loadOBJ(assetOBJFile.c_str(), originalTriangles, vertices, faces);

        boundingSphere = BoundingSphere(vertices);

        // Load it into a VBO
        glGenBuffers(1, &vertexbuffer);
        glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
        glBufferData(GL_ARRAY_BUFFER, originalTriangles.size() * sizeof(Triangle), &originalTriangles[0], GL_STATIC_DRAW);

        //velocity = glm::vec3(0.0, 1.0, 1.0);
        velocity = glm::vec3(0.0, 0.0, 0.0);
        position = glm::vec3(0.0, 0.0, 0.0);

        lastTime = glfwGetTime();
    }

    GLuint vertexArrayID;
    GLuint programID;

    GLuint vertexbuffer;

    std::vector<GLuint> faces;
    std::vector<glm::vec3> vertices;
    std::vector<Triangle*> originalTriangles;
    std::vector<Triangle*> triangles;

    BoundingSphere boundingSphere;

    glm::vec3 velocity;

    double lastTime;
    glm::vec3 position;
};

and my code to render an asset is:

void renderAsset(Asset asset)
{   
    glUseProgram(asset.programID);

    GLuint cameraID = glGetUniformLocation(asset.programID, "camera");
    glUniformMatrix4fv(cameraID, 1, GL_FALSE, &camera[0][0]);

    GLuint positionID = glGetUniformLocation(asset.programID, "position");
    glUniformMatrix4fv(positionID, 1, GL_FALSE, &glm::translate(glm::mat4(), asset.position)[0][0]);

    glEnableVertexAttribArray(0);

    glBindBuffer(GL_ARRAY_BUFFER, asset.vertexbuffer);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
    glDrawArrays(GL_TRIANGLES, 0, asset.originalTriangles.size()* 9);

    glDisableVertexAttribArray(0);
}

However, when I try to render an asset I just end up with a bunch of gibberish on the screen. If I change the vector of Triangle* to Triangle everything renders fine, but I need pointers to construct my spatial hash.

think about it, what does &originalTriangles[0] point to? A list of pointers to Triangle, the contents of the Triangle are ... somewhere. Could you expand why "But I need pointers to construct my spatial hash" is relevant? OpenGL needs those triangles in a consecutive piece of memory, that's just the way it is. — PeterT
– PeterT, Commented Nov 28, 2014 at 21:59

ratchet freak · Accepted Answer · 2014-11-28 21:58:46Z

You will need to flatten the triangle vector. You are now passing a array of pointers to glBufferData, instead you want the actual data.

Triangle flattenedTriangles[] = new Triangle[ originalTriangles.size() ];
for(int i = 0; i<originalTriangles.size(); i++){
    flattenedTriangles[i] = *(originalTriangles[i]);
}

glBufferData(GL_ARRAY_BUFFER, originalTriangles.size() * sizeof(Triangle), &flattenedTriangles[0], GL_STATIC_DRAW);
delete[] flattenedTriangles;

or if you don't want to allocate the array instead map the buffer and construct in place:

glBufferData(GL_ARRAY_BUFFER, originalTriangles.size() * sizeof(Triangle), 0, GL_STATIC_DRAW);//allocate
do{
Triangle *flattenedTriangles = (flattenedTriangles*)glMapBuffer(GL_ARRAY_BUFFER,GL_WRITE_ONLY);
for(int i = 0; i<originalTriangles.size(); i++){
        flattenedTriangles[i] = *(originalTriangles[i]);
    }
}while(glUnmapBuffer(GL_ARRAY_BUFFER));//repeat if map got invalidated

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