Question

So I am trying to render my program in landscape mode… however I am coming across something that I am having trouble wrapping my mind around. In order to move down on my map I have to have a negative offset. If I keep the current positive yOffset of 100 then the map scrolls down instead of up. Why is this? I’m not happy having to put negative y coords to see the map. Is there a way I can reverse this so the map scrolls in the opposite direction when yOffset is positive?

        float yOffset = 100;
    float xOffset = 0;
    glOrthof(0 - yOffset,
             320 - yOffset,
             480 + xOffset,
             0 + xOffset,
             -10000.0f,10000.0f);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();       
    glTranslatef(320.0f, 0.0f, 0.0f );
    glRotatef(90.0, 0.0, 0.0, 1.0);

    static const GLfloat squareVertices[] = {
    0.0f,  0.0f,
    1024.0f,  0.0f,
    0.0f,   1024.0f,
    1024.0f,   1024.0f
};



static const GLfloat texCoords[] = {
    0.0, 1.0,
    1.0, 1.0,
    0.0, 0.0,
    1.0, 0.0
};


glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_TEXTURE_2D);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);    

glTexCoordPointer(2, GL_FLOAT, 0, texCoords);   
glVertexPointer(2, GL_FLOAT, 0, squareVertices);        


glBindTexture(GL_TEXTURE_2D, mapTexture);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);


glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisable(GL_TEXTURE_2D);
glDisable(GL_BLEND);

Answer 1

OpenGL uses a “right-handed” coordinate system. To switch your y direction, you can do something like:

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glScalef (1.0f, -1.0f, 1.0f);
// multiply view transforms as usual... 
// multiply model transforms as usual...