I have a very basic OpenGL instanced rendering setup, which is compiling and running, however it is super slow, and even though I spent days of asking and reading how to fix it, I still have no clue, what causes the problem..

Overlapping small window

What does slow mean? At the moment as you can see, it draws

16 000 instances (48 000 vertices) @ 512*512px resolution / 38-43 FPS

But if I start scaling the window, up to the actual size of my monitor (2560 * 1440) the FPS drops down to 1. I expect at least half a million vertices rendered at 60FPS, that would be the goal.

The setup is very simple, I use GLFW to create the window, GLEW to setup OpenGL properly. So it looks something like this:

int main(void)
    // ... init window and context
    void *resources = setup();

    // ... start of event loop
        // ... clear, get buff data, viewport
        // ... swap buffs, poll events

    // ... clean up everything
    return 0;

Now, the functions from the above pseudo snippet are here, they are in the instrender.c file. This is where the actual drawing happening:

#include <stdlib.h>             // srand(), rand()
#include <jemalloc/jemalloc.h>  // malloc(), free()

#include <time.h>               // time()
#include <stdio.h>              // fprintf()

#include <GL/glew.h>            // GL*

typedef struct resources
    GLuint vs_id;
    GLuint fs_id;
    GLuint program_id;

    GLuint coords_pos;
    GLuint offset_pos;
    GLuint colour_pos;

    GLuint colour_buffer_id;
    GLuint offset_buffer_id;

} Resources;

const char *vert_shader = "                                 \
#version 150 core                                           \n\
in  vec2 coords;                                            \n\
in  vec2 offset;                                            \n\
in  vec3 colors;                                            \n\
out vec3 color;                                             \n\
void main()                                                 \n\
{                                                           \n\
    gl_Position = vec4(coords.x + offset.x,                 \n\
                       coords.y + offset.y, 0.0, 1.0);      \n\
    color = colors;                                         \n\
}                                                           \n";

const char *frag_shader = "                                 \
#version 150 core                                           \n\
in  vec3 color;                                             \n\
out vec4 fragment;                                          \n\
void main()                                                 \n\
{                                                           \n\
    fragment = vec4(color, 1.0);                            \n\
}                                                           \n";

load_and_compile_shader(GLuint *shader_id, const char *buffer, GLenum type)
    // Get shader_id, pass shader source, compile it
    *shader_id = glCreateShader(type);
    glShaderSource(*shader_id, 1, (const GLchar **)&buffer, NULL);

    // If there was a problem during the compilation
    GLint is_compiled;
    glGetShaderiv(*shader_id, GL_COMPILE_STATUS, &is_compiled);
    if (!is_compiled)
        fprintf(stderr, "Shader failed to compile\n");
        // Get debugging information
        GLint info_log_length;
        glGetShaderiv(*shader_id, GL_INFO_LOG_LENGTH, &info_log_length);
        GLchar *info_log = malloc(info_log_length*sizeof(GLchar));
        glGetShaderInfoLog(*shader_id, info_log_length, NULL, info_log);
        fprintf(stderr, "%s\n", info_log);
        *shader_id = 0;

#define IR_OFFSET_COUNT 16000
void *
    Resources *rsc = malloc(sizeof(Resources));
    if (!rsc)
        fprintf(stderr, "Failed to allocate space for resources\n");
        return (void *)NULL;
    load_and_compile_shader(&rsc->vs_id, vert_shader, GL_VERTEX_SHADER);
    load_and_compile_shader(&rsc->fs_id, frag_shader, GL_FRAGMENT_SHADER);

    // Create new program and get program ID
    rsc->program_id = glCreateProgram();

    // Attach shaders
    glAttachShader(rsc->program_id, rsc->vs_id);
    glAttachShader(rsc->program_id, rsc->fs_id);

    // Vertex coordinates
    GLfloat vertices[] = {  -.95f, -.95f,
                            -.95f, +.00f,
                            -.70f, -.95f   };

    // Vertex indices
    GLushort indices[] = {0, 1, 2};

    // Instance offsets
    GLfloat offset[IR_OFFSET_COUNT];
    for (int i=0; i<IR_OFFSET_COUNT; i++)
        offset[i] = (GLfloat)(rand() % 200) / 100.f;

    // Color values
    GLfloat colors[IR_COLOUR_COUNT];
    for (int i=0; i<IR_COLOUR_COUNT; i++)
        colors[i] =  (GLfloat)rand() / (GLfloat)RAND_MAX;

    // Shader layout position
    int pos_index = 0;

    // Setup VAO
    GLuint vertex_array_id;
    glGenVertexArrays(1, &vertex_array_id);

    // Setup coordinates VBO
    GLuint vertex_buffer_id;
    glGenBuffers(1, &vertex_buffer_id);
    glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer_id);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    rsc->coords_pos = pos_index;
    glBindAttribLocation(rsc->program_id, pos_index++, "coords");
                          (GLvoid *)NULL);

    // Setup offsets VBO
    glGenBuffers(1, &rsc->offset_buffer_id);
    glBindBuffer(GL_ARRAY_BUFFER, rsc->offset_buffer_id);
    glBufferData(GL_ARRAY_BUFFER, sizeof(offset), offset, GL_STATIC_DRAW);
    rsc->offset_pos = pos_index;
    glBindAttribLocation(rsc->program_id, pos_index++, "offset");
                          (GLvoid *)NULL);
    glVertexAttribDivisor(rsc->offset_pos, 1);

    // Setup colors VBO
    glGenBuffers(1, &rsc->colour_buffer_id);
    glBindBuffer(GL_ARRAY_BUFFER, rsc->colour_buffer_id);
    glBufferData(GL_ARRAY_BUFFER, sizeof(colors), colors, GL_STATIC_DRAW);
    rsc->colour_pos = pos_index;
    glBindAttribLocation(rsc->program_id, pos_index++, "colors");
                          (GLvoid *)NULL);
    glVertexAttribDivisor(rsc->colour_pos, 1);

    // Link shader program

    // If there was a problem during the linking
    GLint is_linked;
    glGetProgramiv(rsc->program_id, GL_LINK_STATUS, &is_linked);
    if (!is_linked)
        fprintf(stderr, "Shader program linking failed.\n");
        // Get debugging informations
        GLint info_log_length;
        glGetProgramiv(rsc->program_id, GL_INFO_LOG_LENGTH, &info_log_length);
        GLchar *info_log = malloc(info_log_length*sizeof(GLchar));
        glGetProgramInfoLog(rsc->program_id, info_log_length, NULL, info_log);
        fprintf(stderr, "%s\n", info_log);
        // Clean up
        glDetachShader(rsc->program_id, rsc->vs);
        glDetachShader(rsc->program_id, rsc->fs);
        rsc->program_id = 0;

    // Setup indices VBO
    GLuint index_array_buffer_id;
    glGenBuffers(1, &index_array_buffer_id);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index_array_buffer_id);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    // Set fragment output
    glBindFragDataLocation(rsc->program_id, 0, "fragment");

    // Set basic GL options
    glClearColor(.46f, .71f, .67f, 1.f);

    // Start using program

    return rsc;

draw(void *resources)
    Resources *rsc = (Resources *)resources;
/* num of elems to draw */  3,
   /* index value types */  GL_UNSIGNED_SHORT,
  /* pointer to indices */  0,
/* num of items to draw */  IR_OFFSET_COUNT);

cleanup(void *resources)
    glBlendFunc(GL_ONE, GL_ZERO);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    Resources *rsc = (Resources *)resources;





Video Card:
    NVidia GeForce 9600M GT 512MB
    Mac OS X 10.9.3 / Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)


Based on a friendly advice, which was about "over-drawing", I created a version, where the triangles only have a very limited overlapping in one direction:

Non-overlapping small window

Now this produces a constant 48-50FPS, however when I scale the window up to 2560*1440 this number drops down to 22-26FPS (which is of course way better than the previous 1FPS but still not the one I'm looking for):

Non-overlapping full screen

So I guess, the main problem is not over-drawing/overlapping.


Here is a time profile I created:

Instrument Profile

As you can see, 75.7% of the time is spent to call/execute the glDrawElementsInstanced function, and its subfunction calls.


During the tests of the code with @syb0rg, another interesting bug appeared: on every 10-15-20th (basically at absolutely random) running the program produces this, and then crashes:

Noise Animation

  • \$\begingroup\$ Don't pass around void* unless you have to (e.g. because it can be any kind of pointer). In your case you know what it is so you can use the proper type. \$\endgroup\$ Commented May 24, 2014 at 21:05
  • \$\begingroup\$ that's exactly the case, why I pass void *s in the first place. Ofc the above example is a bit simplified version of the actual code, but in real life, it has to be a void * since the caller does not know what pointer that is. \$\endgroup\$
    – Peter Varo
    Commented May 24, 2014 at 21:06
  • \$\begingroup\$ I don't know about your performance issue, but there's no reason to use indexed draw calls if your index buffer is {0, 1, 2}. That's just a triangle list. \$\endgroup\$
    – Trillian
    Commented May 24, 2014 at 23:16
  • \$\begingroup\$ @Trillian okay, thanks for your comment -- but as I mentioned earlier, this is a simplified version of what I am doing. In my real code I'm not drawing single triangles, but more complex meshes, where the indexing is absolutely necessary. The above code is modeling that situation. \$\endgroup\$
    – Peter Varo
    Commented May 25, 2014 at 11:35
  • 1
    \$\begingroup\$ @syb0rg here we go: ZIP and then make && ./build/main -- and don't press space, ifdef OVERLAPPING_OFF. thank you very much for the effort! \$\endgroup\$
    – Peter Varo
    Commented May 27, 2014 at 14:19

1 Answer 1


Disclaimer: This is the first time I have ever really looked into using OpenGL

Keep in mind that my review may contain code that is not fully included in the question (such as the simplified main() function).


  • I found that when I tried to resize the window, I would always crash the program.

    enter image description here

    That wasn't very fun, so I set out to fix that first and foremost. It was rather easy to fix, all I had to do was add a frame buffer sizing callback, and set it to the window with glfwSetFramebufferSizeCallback().

    The frame buffer callback function:

    static void framebuffer_size_callback(GLFWwindow* window, int width, int height)
        glViewport(0, 0, width, height); // reset the viewport
        glMatrixMode(GL_PROJECTION); // modify the projection matrix
        glLoadIdentity();            // load an identity matrix into the projection matrix
        glOrtho(0, width, 0, height, -1.0, 1.0); // create new projection matrix
        /// Important!!! You need to switch back to the model-view matrix
        /// or else your OpenGL calls are modifying the projection matrix!
        glMatrixMode(GL_MODELVIEW); // return to the model matrix
        glLoadIdentity();           // load an identity matrix into the model-view matrix
        // OpenGL has now compensated for the resized window, and is ready to draw again.

    The modified run() function:

    run(GLFWwindow *window)
        void *resources = setup();
        glfwSetWindowUserPointer(window, resources);
    #ifdef MEASURE
        int viewport_width, viewport_height;
        // set for proper resizing of window and viewport
        glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
        while (!glfwWindowShouldClose(window))
            glfwGetFramebufferSize(window, &viewport_width, &viewport_height);
            framebuffer_size_callback(window, viewport_width, viewport_height);
    #ifdef MEASURE

    This doesn't fully fix the crashing, occasionally the program will still misbehave; however, at least I am able to scale the window now more consistently.

  • When I undefined OVERLAPPING_OFF to benchmark the code on my computer, I got some errors of an unknown type name offset. Looking at your code, you defined offset only if OVERLAPPING_OFF is defined but then try to use it if it isn't defined as well. The fix was simple enough, I just moved the declaration of offset outside of the #ifdef OVERLAPPING_OFF.

        // Instance offsets
        int i;
        GLfloat offset[OFFSET_COUNT];
        int j, idx=0;
        GLfloat x, y;
        for (i=0; i<DIMi; i++)
            y = (GLfloat)i * STEP;
            for (j=0; j<DIMi; j+=2)
                offset[idx++] = x = (GLfloat)j * STEP;   // x
                offset[idx++] = y;                       // y
        for (i=0; i<OFFSET_COUNT; i++)
            offset[i] = genc();


  • Running my own profiling tests for a longer duration, I came up with the following data.

    enter image description here

    As we can see, glDrawElementsInstanced() now only takes up 2% of the total run time. The big time hogs are CGLFlushDrawable and _glfwPlatformPollEvents.

  • There are two ways to process pending events. glfwPollEvents() processes only those events that have already been received and then returns immediately. This is the best choice when rendering continually, like most games do.

    If instead you only need to update your rendering once you have received new input, glfwWaitEvents() is a better choice. It waits until at least one event has been received, putting the thread to sleep in the meantime, and then processes all received events just like glfwPollEvents() does. This saves a great deal of CPU cycles and is useful for, for example, many kinds of editing tools.

  • \$\begingroup\$ thanks, I completely forgot about the frame buffer size callback -- nice catch! The offset was a typo, but thanks for pointing that too! \$\endgroup\$
    – Peter Varo
    Commented May 27, 2014 at 17:12
  • \$\begingroup\$ @PeterVaro Oops, I accidentally rejected your edit when I made the edit of my own! If you re-submit it I'll approve. \$\endgroup\$
    – syb0rg
    Commented May 27, 2014 at 17:22
  • \$\begingroup\$ okay, np, I updated again \$\endgroup\$
    – Peter Varo
    Commented May 27, 2014 at 17:24
  • \$\begingroup\$ btw: if glfwSetFramebufferSizeCallback is called -- so a callback function set -- then calling the glGetFramebufferSize is absolutely unnecessary. \$\endgroup\$
    – Peter Varo
    Commented May 27, 2014 at 17:30
  • \$\begingroup\$ @PeterVaro glfwGetFramebufferSize directly retrieves the current size of the framebuffer of a window, and stores it in viewport_width and viewport_height. It is still a necessary function call (you can see this when you remove the function call and run the program). \$\endgroup\$
    – syb0rg
    Commented May 27, 2014 at 17:34

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