Bind different textures for each object

Question

I am a bit confused about "loading" or binding different textures for seperate objects in my scene.

This is the way I set the texture (loading the texture from harddrive and binding):

setTexture ( const std::string& t_filename )
{
int width, height, nrChannels;

glBindTexture( GL_TEXTURE_2D, m_TEX );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );

unsigned char* image = stbi_load( t_filename.c_str(), &width, &height, &nrChannels, 0 );

if ( image )
{
    glTexImage2D( GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, image );
    glGenerateMipmap( GL_TEXTURE_2D );
}
else
{
    throw std::string( "Texture could not get loaded. Abort" );
}
stbi_image_free( image );

glBindTexture( GL_TEXTURE_2D, 0 );
}

The function belongs to a class called Rectangle .

I have member variables for the VAO, VBO and the texture object:

GLuint m_VAO{ 0 };
GLuint m_VBO{ 0 };
GLuint m_VEBO{ 0 };
GLuint m_TEX{ 0 };

So every instance of Rectangle has one texture object m_Tex .

Everytime I draw my object in the main function for drawing the frame I use this way:

glBindVertexArray( m_VAO );
glBindTexture( GL_TEXTURE_2D, m_TEX );
glDrawElements( GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr );
glBindVertexArray( 0 );

The problem: All of my objects (let's say I have 3 Rectangle instances) use the same texture although all of them bind there own texture object before drawing. Why is that?

Edit: Okay my fault! I forgot to generate the texture object in my setTexture function like that:

glGenTextures(1, &m_TEX);

Now I can load my textures and "switch" between them as expected!

Answer 1

How is it possible to load every texture for theoretically infinite objects at the beginning and then just "switching" between them?

By just creating several texture objects , and binding them when you want to use them. Texture units have nothing to do with that.

// at init time
GLuint texA, texB;

// create texture A
glGenTextures(1, &texA);
glBindTexture(GL_TEXTURE_2D, texA);
glTexParameter(...); glTexImage(...);

// create texture B
glGenTextures(1, &texB);
glBindTexture(GL_TEXTURE_2D, texB);
glTexParameter(...); glTexImage(...);


// at draw time
glBindTexture(GL_TEXTURE_2D, texA);
glDraw*(...); // draw with texture A
glDraw*(...); // still draw with texture A

glBindTexture(GL_TEXTURE_2D, texB);
glDraw*(...); // now draw with texture B

It is not clear what m_TEX in your code is, but it looks like some member variable of a class. However, you do not show which classes your code fragments belong to, so it is hard to guess what exactly you're doing. To me, it looks like you only have a single m_TEX variable, meaning you are able to remember only one texture object name, and hence re-use the same texture over and over again. Ie, your loader code does not generate a new texture object, but just re-uses m_TEX .

Texture units are for multitexturing , to be able to bind more than one texture at the same time (meaning during a single draw call):

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texA);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, texB);
glDraw*(...)  // draw with both texA and texB

This will of course require a shader which makes use of both textures, and how to combine them (or use them for different kinds of data) is totally up to you.