[OpenGL] Draw 3D and Texture with BMP image using OpenGL Part I

It has been more than half of year not posting any article in my blogger and that makes me a little bit embarrassed. Well, for breaking this situation, I just quickly explain a simple concept about OpenGL coordinate.

Before taking an adventure to OpenGL, we have to know the coordinate in OpenGL first. Please check out the following graph. As we can see, the perspective of z position is pointed to us and it's so different from OpenCV.

If we take a look closer, the following OpenGL code can be explained in the picture below:


glBegin(GL_QUADS) # Start Drawing The Cube

# Front Face (note that the texture's corners have to match the quad's corners)
glTexCoord2f(1.0, 0.0); glVertex3f(-1.0, -1.0, 1.0) # Bottom Left Of The Texture and Quad
glTexCoord2f(1.0, 1.0); glVertex3f( 1.0, -1.0, 1.0) # Bottom Right Of The Texture and Quad
glTexCoord2f(0.0, 1.0); glVertex3f( 1.0, 1.0, 1.0) # Top Right Of The Texture and Quad
glTexCoord2f(0.0, 0.0); glVertex3f(-1.0, 1.0, 1.0) # Top Left Of The Texture and Quad

# Back Face
glTexCoord2f(1.0, 0.0); glVertex3f(-1.0, -1.0, -1.0) # Bottom Right Of The Texture and Quad
glTexCoord2f(1.0, 1.0); glVertex3f(-1.0, 1.0, -1.0) # Top Right Of The Texture and Quad
glTexCoord2f(0.0, 1.0); glVertex3f( 1.0, 1.0, -1.0) # Top Left Of The Texture and Quad
glTexCoord2f(0.0, 0.0); glVertex3f( 1.0, -1.0, -1.0) # Bottom Left Of The Texture and Quad

# Top Face
glTexCoord2f(0.0, 1.0); glVertex3f(-1.0, 1.0, -1.0) # Top Left Of The Texture and Quad
glTexCoord2f(0.0, 0.0); glVertex3f(-1.0, 1.0, 1.0) # Bottom Left Of The Texture and Quad
glTexCoord2f(1.0, 0.0); glVertex3f( 1.0, 1.0, 1.0) # Bottom Right Of The Texture and Quad
glTexCoord2f(1.0, 1.0); glVertex3f( 1.0, 1.0, -1.0) # Top Right Of The Texture and Quad

# Bottom Face
glTexCoord2f(1.0, 1.0); glVertex3f(-1.0, -1.0, -1.0) # Top Right Of The Texture and Quad
glTexCoord2f(0.0, 1.0); glVertex3f( 1.0, -1.0, -1.0) # Top Left Of The Texture and Quad
glTexCoord2f(0.0, 0.0); glVertex3f( 1.0, -1.0, 1.0) # Bottom Left Of The Texture and Quad
glTexCoord2f(1.0, 0.0); glVertex3f(-1.0, -1.0, 1.0) # Bottom Right Of The Texture and Quad

# Right face
glTexCoord2f(1.0, 0.0); glVertex3f( 1.0, -1.0, -1.0) # Bottom Right Of The Texture and Quad
glTexCoord2f(1.0, 1.0); glVertex3f( 1.0, 1.0, -1.0) # Top Right Of The Texture and Quad
glTexCoord2f(0.0, 1.0); glVertex3f( 1.0, 1.0, 1.0) # Top Left Of The Texture and Quad
glTexCoord2f(0.0, 0.0); glVertex3f( 1.0, -1.0, 1.0) # Bottom Left Of The Texture and Quad

# Left Face
glTexCoord2f(0.0, 0.0); glVertex3f(-1.0, -1.0, -1.0) # Bottom Left Of The Texture and Quad
glTexCoord2f(1.0, 0.0); glVertex3f(-1.0, -1.0, 1.0) # Bottom Right Of The Texture and Quad
glTexCoord2f(1.0, 1.0); glVertex3f(-1.0, 1.0, 1.0) # Top Right Of The Texture and Quad
glTexCoord2f(0.0, 1.0); glVertex3f(-1.0, 1.0, -1.0) # Top Left Of The Texture and Quad

glEnd(); # Done Drawing The Cube



So, right now, if we just look at the first section of the code as follows, it represents the blue quadrilateral for the front face.

And, the texture coordinate represents the direction of the image.

In sum, we can see the result just like this: