Texture mapping with OpenGL and QT - C++

https://stackoverflow.com/questions/22182781

05-06-2023
|

Question

I'm having trouble getting my texture to map to a quad using OpenGL and Qt. I've looked at several other SO threads, but a lot of the function calls have to be used slightly differently for me to compile (Qt Verison 4.8.6). Here's my relevant code, right now all that happens is a window is displayed with a black background, but nothing else.

void LoadGLTextures( const char * name )
{
    QImage img;

    if(!img.load("resources/Green_Dragon.bmp")){
        std::cerr << "ERROR in loading image" << std::endl;
    }

    QImage t = QGLWidget::convertToGLFormat(img);

    glGenTextures(1, &texture[0]);
    glBindTexture(GL_TEXTURE_2D, texture[0]);
    glTexImage2D(GL_TEXTURE_2D, 0, 3, t.width(), t.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, t.bits());

    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
    glBindTexture( GL_TEXTURE_2D, 0 );
}



void GLWidget::initializeGL()
{


    qglClearColor(qtBlack.dark());
    glEnable(GL_CULL_FACE);
    glShadeModel(GL_SMOOTH);
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glEnable(GL_MULTISAMPLE);
    static GLfloat lightPosition[4] = { 0.5, 5.0, 7.0, 1.0 };
    glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);
    cameraPos = 0;
    glEnable(GL_TEXTURE_2D);
    LoadGLTextures("resources/Green_Dragon.jpeg");

}

void GLWidget::paintGL()
{
    glClearColor( 0.0f, 0.0f, 0.0f, 0.0f);
    glClear( GL_COLOR_BUFFER_BIT );


    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    glShadeModel( GL_FLAT );
    glEnable(GL_LIGHTING);
    glEnable(GL_LIGHT0);
    glEnable(GL_DEPTH_TEST);
    glEnable(GL_TEXTURE_2D);
    glColor3f(0.5, 0.5, 0);
    glBindTexture(GL_TEXTURE_2D, texture[0]);

    glBegin(GL_QUADS);
    glTexCoord2f(0.0f, 1.0f); glVertex2f(-0.5f, 0.5f);  // vertex 1
    glTexCoord2f(0.0f, 0.0f); glVertex2f(-0.5f, -0.5f); // vertex 2
    glTexCoord2f(1.0f, 0.0f); glVertex2f(0.5f, -0.5f); // vertex 3
    glTexCoord2f(1.0f, 1.0f); glVertex2f(0.5f, 0.5f); // vertex 4
    glEnd();
    glDisable(GL_TEXTURE_2D);

    glFlush();
}

Solution

Looking at your comment, the answer is fairly clear now, sorry that I missed it in the question comments.

You are seeing a black screen because you are enabling depth testing, but you are not clearing the depth buffer between frames. Therefore, the previous frame's depth buffer values remain in place, and the depth tests fail for all subsequent frames (note that the default depth function is GL_LESS).

You may leave depth testing enabled. The correct solution is to clear your depth buffer in addition to your color buffer before each render. You have:

glClear( GL_COLOR_BUFFER_BIT );

But you need:

glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );