this is the code I wrote: int InitGL ( GLsizei Width, GLsizei Height ) { glClearColor(1.0f, 1.0f, 1.0f, 0.0f); // This Will Clear The Background Color To Black glClearDepth(1.0); // Enables Clearing Of The Depth Buffer glShadeModel(GL_SMOOTH); // Enables Smooth Color Shading glViewport ( 0, Width, 0, Height ); glMatrixMode(GL_PROJECTION); glLoadIdentity(); // Reset The Projection Matrix gluOrtho2D ( 0, Width, 0, Height ); glMatrixMode(GL_MODELVIEW); } GLvoid ReSizeGLScene ( GLsizei Width, GLsizei Height ) { if ( Height == 0 ) Height = 1; glViewport ( 0, 0, Width, Height ); glMatrixMode ( GL_PROJECTION ); glLoadIdentity (); // gluOrtho2D ( 0, Width, 0, Height ); gluPerspective ( 0.0f, (GLfloat) Width / (GLfloat) Height, 0.1f, 100.0f ); glMatrixMode ( GL_MODELVIEW ); } GLvoid DrawGLScene ( GLvoid ) { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clear The Screen And The Depth Buffer glLoadIdentity(); // Reset The View glBegin(GL_POLYGON); // start drawing a polygon glColor3f(1.0f,0.0f,0.0f); // Set The Color To Red glVertex3f( 0.0f, 0.1f, 0.0f); // Top glColor3f(0.0f,1.0f,0.0f); // Set The Color To Green glVertex3f( -0.1f,-0.1f, 0.0f); // Bottom Right glColor3f(0.0f,0.0f,1.0f); // Set The Color To Blue glVertex3f(0.1f,-0.1f, 0.0f); // Bottom Left glEnd(); glutSwapBuffers (); } The triangle I draw is like this : one side from the middle point of the top of the window to the right bottom point of the window, one side from the middle point to the left bottom point of the window, one side is from right bottom point to left bottom point of the window. I donot understand why I get this result. If I want to draw a triangle in pixels , how can I do? thanks a lot. ( confusing )

actually, I want to draw in pixels ,

I found code below on opengl resources webpage which said I can use to change unit to pixel :

glViewport(0, 0, screen_width, screen_height);
glMatrixMode ( GL_PROJECTION );
glLoadIdentity ();
gluOrtho2D ( 0, screen_width, 0, screen_height );


But after I added this in DrawGLScreen(), it didnot work.

thanks, I will try your code.

one more question: Does how to explain opengl function depend on different operation system? say, for windows, linux, unix,..., it is possible they they will show different figures for the same code?