OpenGL: Unexpected Vertex Positions of a Specified Triangle

Question

I specified a set of vertices in the points array to draw a triangle which is supposedly starting from the top middle of the window - (+0.0f, +1.0f, +0.0f) - going through the bottom left corner - (-1.0f, -1.0f, +0.0f) - and finally ending in the bottom right corner of the window- +1.0f, -1.0f, +0.0f. The triangle, however, isn't rendered as expected above, since it looks like it starts from the top middle of the window, then goes on to the top left corner, and finally ends up right in the middle of the window. I included two screenshots to aid my problem description below.

Unexpected rendering

Expected triangle hinted in red

#include <cstdio>
#include <cstdlib>

#include <GL/glew.h>
#include  <GLFW\glfw3.h>

int main() {
    // GLFW
    if (!glfwInit()) {
        fprintf(stderr, "ERROR: could not start GLFW3\n");
        return EXIT_FAILURE;
    }

    // Create window
    GLFWwindow* window = glfwCreateWindow(800, 600, "OpenGL Test", NULL, NULL);
    if (!window) {
        fprintf(stderr, "ERROR: could not open window with GLFW3\n");
        glfwTerminate();
        return EXIT_FAILURE;
    }
    glfwMakeContextCurrent(window);

    // GLEW 
    glewExperimental = GL_TRUE;
    glewInit();

    // Get version info
    const GLubyte* renderer = glGetString(GL_RENDERER);
    const GLubyte* version = glGetString(GL_VERSION);
    printf("Renderer: %s\n", renderer);
    printf("Supported OpenGL version: %s\n", version);

    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LESS);

    /* Put code here */

    // Vertices to be rendered
    float points[] = {
        +0.0f, +1.0f, +0.0f
        -1.0f, -1.0f, +0.0f
        +1.0f, -1.0f, +0.0f
    };

    // Vertex buffer object
    GLuint bufferId = 0;
    glGenBuffers(1, &bufferId);
    glBindBuffer(GL_ARRAY_BUFFER, bufferId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(points), points, GL_STATIC_DRAW);

    // Vertex attribute object
    GLuint attributeId = 0;
    glGenVertexArrays(1, &attributeId);
    glBindVertexArray(attributeId);
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, bufferId);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0);

    // Shaders
    const char* vertexShaderSource =
        "#version 400\n"
        "in vec3 position;"
        "void main () {"
        "  gl_Position = vec4(position.x, position.y, position.z, 1.0);"
        "}";

    const char* fragmentShaderSource =
        "#version 400\n"
        "out vec4 outColor;"
        "void main () {"
        "  outColor = vec4 (0.0, 0.0, 1.0, 1.0);"
        "}";

    // Compile shaders
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    // Link shaders
    GLuint shaderPorgram = glCreateProgram();
    glAttachShader(shaderPorgram, fragmentShader);
    glAttachShader(shaderPorgram, vertexShader);
    glLinkProgram(shaderPorgram);

    // Render loop
    while (!glfwWindowShouldClose(window)) {
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glUseProgram(shaderPorgram);
        glBindVertexArray(attributeId);
        glDrawArrays(GL_TRIANGLES, 0, 3);
        glfwPollEvents();
        glfwSwapBuffers(window);
    }

    glfwTerminate();
    return EXIT_SUCCESS;
}

Wow, a complete, yet short piece of code? A concise description of the problem with example pictures? Might as well be giving us free cake! — Dietrich Epp

Dietrich Epp Dietrich Epp · Accepted Answer · 2016-06-14T00:11:14

Missing commas.

float points[] = {
    +0.0f, +1.0f, +0.0f
    -1.0f, -1.0f, +0.0f
    +1.0f, -1.0f, +0.0f
};

Should be:

float points[] = {
    +0.0f, +1.0f, +0.0f,
    -1.0f, -1.0f, +0.0f,
    +1.0f, -1.0f, +0.0f,
};

The last comma is not necessary but it is permitted and I prefer it as a matter of style.

OpenGL: Unexpected Vertex Positions of a Specified Triangle

1 Answers