Axis, Perspective vs Otho

OpenGL OpenGL: Basic Coding
1

Hi,I am tying OpenGL and setup two projections: perspective and orthographic.
What I can not understand is why is the asix Y (green) inverted in ortho view.
View is from Z axis. What am I missing?
Code:

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity;

  //for perpective:
  gluPerspective(45.0,  (Right-Left) / (Bottom-Top), 1, 1000);
  gluLookAt(0, 0, 300, 0, 0, 0, 0, 1, 0);

//for orthographic
 glOrtho(Left, Right, Bottom, Top, 1, 1000);
gluLookAt(0, 0, 100, 0, 0, 0, 0, 1, 0);

glMatrixMode(GL_MODELVIEW);
glClear(GL_DEPTH_BUFFER_BIT or GL_COLOR_BUFFER_BIT);

axis
axis871×1057 13.6 KB

2

I’m guessing that you have Bottom>Top (i.e. top-left origin, with Y increasing downward). OpenGL’s fixed coordinate systems (clip coordinates, NDC and window coordinates) have the Y axis upward. gluPerspective will preserve the orientation for any field-of-view angle less than 180°, while glOrtho will flip it if top<bottom (the resulting projection matrix will map Yeye=bottom to YNDC=-1 and Yeye=top to YNDC=1).

3

Hi,
You were absolutly right.

Thank You very, very much for help!