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Asked: 2026-06-15T07:47:14+00:00

I’m trying to draw a simple texture in opengl. I made a simple class

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I’m trying to draw a simple texture in opengl. I made a simple class Texture:

class Texture{
public:
    unsigned int id;
    unsigned char image[256*256*3];
    int level;
    int border;
    int width;
    int height;    

    Texture (int level =0, int border = 0) : level(level), border(border) {
        glGenTextures(1, &id);
        width = 256, height = 256;
        glTexImage2D(GL_TEXTURE_2D, level, GL_RGB, width, height, border, GL_RGB, GL_UNSIGNED_BYTE, &image[0]);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
        glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE); 


        for (int i= 0; i<width*height*3; i+=3){
            image[i]=1;//i%255;
            image[i+1] =1;// 255-i%255;
            image[i+2] =1;// i%128;
        }
    }

    void useIt(){
        glBindTexture( GL_TEXTURE_2D, id );
    }
};

It creates an unsigned char array and fill it with some random data. I’m trying to use it this way:

glEnable(GL_TEXTURE_2D);
texture->useIt();
glBegin(GL_TRIANGLES);

glNormal3d(0, 1, 0);
glTexCoord2d(0.0,0.0);
glVertex3f(width/-2.f,height/2.f,depth/2.f);
glTexCoord2d(1.0,1.0);  
glVertex3f(width/2.f,height/2.f,depth/2.f);
glTexCoord2d(1.0,0.0);
glVertex3f(width/2.f,height/2.f,depth/-2.f);
glTexCoord2d(0.0,0.0);
glVertex3f(width/-2.f,height/2.f,depth/2.f);
glTexCoord2d(1.0,1.0);
glVertex3f(width/2.f,height/2.f,depth/-2.f);
glTexCoord2d(0.0,1.0);
glVertex3f(width/-2.f,height/2.f,depth/-2.f);

glEnd();
glDisable(GL_TEXTURE_2D);

It draws the plane, but withouht texture (draws with the previously used material). what am i doing wrong?

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1 Answer

  1. Editorial Team

    Three possible issues with your code. Brett Hale already told you, that you need to bind a texture object before uploading data to it with glTexImage.

    glTexImage creates copy of the data you supply to it (this is different to the glVertex…Pointer functions, which only take a pointer or offset into a buffer object). However you’re filling the image array with data after you copied it’s contents to the texture. Also you may safely delete the image array after copying the data to the texture.

    Last but not least: Those operations are found in a constructor. If you have the texture class instance in a scope that’s initialized before a OpenGL context has been created, nothing will happen at all, because there’s no OpenGL context. So either make sure, the texture object is created only after a OpenGL context is available, or put the texture creation and upload code into a separate method, that you call once a OpenGL context is available.

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