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Asked: 2026-05-30T01:29:44+00:00

I’m trying to understand how to load a texture in OpenGL and I wrote

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I’m trying to understand how to load a texture in OpenGL and I wrote this very simple code:

GLuint texture;

void loadTexture() {

    GLubyte data[] = {  255,0,0,
                        0,255,0,
                        0,255,0,
                        255,0,0    };

    glGenTextures(1, &texture);

    glBindTexture(GL_TEXTURE_2D, texture);          

    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);   
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);   

    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
    glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );

    int chk = gluBuild2DMipmaps( GL_TEXTURE_2D, GL_RGB, 2, 2, GL_RGB, GL_UNSIGNED_BYTE, data );

    if (chk!=0) 
        printf("error code = %d\n",chk);
    else 
        printf("success\n");  

}

I use loadTexture() to load the texture in memory. The texture, in this sample, is extremely simple, but at the moment it doesn’t matter.

void drawTexturedSquare() {

    glEnable( GL_TEXTURE_2D );
    glBegin (GL_QUADS);

    glTexCoord2f (0.0, 0.0);
    glNormal3f(0, 0, 1);
    glVertex3f (0.0, 0.0, 0.0);

    glTexCoord2f (1.0, 0.0);
    glNormal3f(0, 0, 1);
    glVertex3f (10.0, 0.0, 0.0);

    glTexCoord2f (1.0, 1.0);
    glNormal3f(0, 0, 1);
    glVertex3f (10, 10, 0.0);

    glTexCoord2f (0.0, 1.0);
    glNormal3f(0, 0, 1);
    glVertex3f (0.0, 10, 0.0);

    glEnd ();
    glDisable( GL_TEXTURE_2D);

}

I would like to apply this simple texture to a square. I call function drawTexturedSquare() from inside a draw() function where I already called 

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

The problem is that this is the result I get

enter image description here

while I expected the square to be green in main diagonal (upper-left to lower-right) and red is secondary diagonal (upper-right to lower-left). May someone explain me why?

Besides, every time I run the program I get a different result:

enter image description here enter image description here enter image description here enter image description here

I do not understand where this blue comes out…May someone help me?

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1 Answer

  1. Editorial Team

    Each row of your data needs to be 4-byte aligned. Either pad each row with 0 0, or use a RGBA texture.

    If you don’t want to do the aforementioned, you can use:

    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

    The reason your color is changing is that since you don’t pad your rows correctly, OpenGL reads past the end of the array.

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