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I'm capturing a picture from a webcam with openCV. Then the frame should be transformed into an openGL texture and be shown on the screen. I got the following code but the window remains black. I'm very new to openGL and have no more ideas why it doesn't work.

int main ()
{
    int w = 800,h=600;

    glfwInit();

    //configure glfw 
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
    glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);


    GLFWwindow* window = glfwCreateWindow(w, h, "OpenGL", NULL, nullptr); // windowed
    glfwMakeContextCurrent(window);

    glewExperimental = GL_TRUE;
    glewInit();

    initializeCapturing();

    //init GL
    glViewport(0, 0, w, h); // use a screen size of WIDTH x HEIGHT
    glEnable(GL_TEXTURE_2D);     // Enable 2D texturing

    glMatrixMode(GL_PROJECTION);     // Make a simple 2D projection on the entire window
    glLoadIdentity();
    glOrtho(0.0, w, h, 0.0, 0.0, 100.0);

    glMatrixMode(GL_MODELVIEW);    // Set the matrix mode to object modeling

    glClearColor(0.0f, 0.0f, 0.0f, 0.0f); 
    glClearDepth(0.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clear the window

    cv::Mat frame;
    captureFromWebcam(frame,capture0);


    /* OpenGL texture binding of the image loaded by DevIL  */
    GLuint texid;
    glGenTextures(1, &texid); /* Texture name generation */
    glBindTexture(GL_TEXTURE_2D, texid); /* Binding of texture name */
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); /* We will use linear interpolation for magnification filter */
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); /* We will use linear interpolation for minifying filter */
    glTexImage2D(GL_TEXTURE_2D, 0,3, frame.size().width, frame.size().height, 0, GL_RGB, GL_UNSIGNED_BYTE, 0); /* Texture specification */


    while(!glfwWindowShouldClose(window))
    {
        glfwPollEvents();

        if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
            glfwSetWindowShouldClose(window, GL_TRUE);

        // Clear color and depth buffers
       glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
       glBindTexture(GL_TEXTURE_2D,texid);
       glMatrixMode(GL_MODELVIEW);     // Operate on model-view matrix

        /* Draw a quad */
       glBegin(GL_QUADS);
           glTexCoord2i(0, 0); glVertex2i(0,   0);
           glTexCoord2i(0, 1); glVertex2i(0,   h);
           glTexCoord2i(1, 1); glVertex2i(w, h);
           glTexCoord2i(1, 0); glVertex2i(w, 0);
       glEnd();
       glFlush();
       glfwSwapBuffers(window);
    }


    releaseCapturing();
    glfwTerminate();

    return 1;
}

and the other procedures

cv::VideoCapture capture0;
cv::VideoCapture capture1;

void captureFromWebcam(cv::Mat &frame, cv::VideoCapture &capture)
{
    capture.read(frame);
}

bool initializeCapturing()
{
    capture0.open(0);
    capture1.open(1);

    if(!capture0.isOpened() | !capture1.isOpened())
    {
        std::cout << "Ein oder mehrere VideoCaptures konnten nicht geöffnet werden" << std::endl;

        if(!capture0.isOpened())
            capture0.release(); 
        if(!capture1.isOpened())
            capture1.release();
        return false;
    }

    return true;
}

void releaseCapturing()
{
    capture0.release();
    capture1.release();
}
Benedikt Bock
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2 Answers2

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It looks like you mixed several code fragments you found at different places. You are rquesting an OpenGL 3.2 core profile. But the drawing code you are using is immediate mode with the fixed-function pipeline, which is not available in a core profile. You basically requested a modern GL context, but the drawing code is completely outdated and not supported any more by the GL version you selected.

As a quick fix, you could simply remove the following lines:

glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);

Doing so should provide you with some legacy GL context OR a compatibility profile of some modern context, depending on the operation system an GL implementation you use. However, you can expect from that to get at least OpenGL 2.1 that way (except on very old hardware not supporting GL2 at all, but even that would be OK for your code). The rest of the code should work in such a context.

I still suggest that you learn mordern GL instead of the old, deprecated legacy stuff you are using here.

derhass
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  • Do you know a goog place to learn the modern way (websites, books,...)? The most tutorials and How To's I found were the old way. Am I right that primarily the glBegin() and glEnd() is the old code? Or is the glMatrixMode() and glViewport also deprecated? – Benedikt Bock Jan 18 '15 at 17:45
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    `glMatrixMode` is also deprecated, `glViewport` isn't. As for modern OpenGL, have a look at the following sites: [arcsynthesis.org/gltut/](http://arcsynthesis.org/gltut/), [open.gl](http://open.gl) and [opengl-tutorial.org](http://www.opengl-tutorial.org/). – derhass Jan 18 '15 at 18:07
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You did not set the pointer from the frame.

glTexImage2D(GL_TEXTURE_2D, 0,3, frame.size().width, frame.size().height, 0, GL_RGB, GL_UNSIGNED_BYTE, (void*)frame.data()); /* Texture specification */

And with this code, you only are going to get the first frame.

Put "captureFromWebcam" and load the texture inside the while.

(Sorry for my english)

Mateo Miccino
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