Question

I have a 3d scene drawn by OpenGL to a resizeable window. Now, when the window gets resized, I do not want to scale the viewport, rather I want to keep the scene at a fixed size and show a larger portion of it (or crop the scene image). This is my current code:

GLfloat ratio;

// Protect against a divide by zero
if ( height == 0 )
    height = 1;

ratio = ( GLfloat )width / ( GLfloat )height;
// Setup our viewport.
glViewport( 0, 0, ( GLint )width, ( GLint )height );

// change to the projection matrix and set our viewing volume.
glMatrixMode( GL_PROJECTION );
glLoadIdentity( );

gluPerspective( 60.0f, ratio, 0.1f, 1000.0f );
// Switch back to the modelview
glMatrixMode( GL_MODELVIEW );

If I keep the ratio fixed, then the scene image simply gets scaled, but I want to keep it at fixed size and simply show a wider view. Any ideas on this?

Answer 1

Adjust the fov parameters. Technically what you want to do is easier if done using glFrustum instead of gluPerspective.

// Protect against a divide by zero
if ( height == 0 )
    height = 1;

// Setup our viewport.
glViewport( 0, 0, ( GLint )width, ( GLint )height );

// change to the projection matrix and set our viewing volume.
glMatrixMode( GL_PROJECTION );
glLoadIdentity( );

// supply some sensefull value for this; ideally let the user adjust it somehow
exterm float Zoom;

// near far should tightly wrap the actually visible set of objects. Hardcoded values
// like 0.1 ... 1000.f are problematic. Also your choosen value range slices your viewport
// into 10000 depth slices. Say you get only a 16 bit depth buffer already in the lineary
// slicing ortho projection a OpenGL length units in depth would recieve only about 6 
// slices. In perspective mode the slice density follows a 1/depth law. So already at depth
// 10 you'll run into depth resolution problems.
glFrustum(-Zoom * width, Zoom * width, -Zoom * height, Zoom * height, near, far);

// Switch back to the modelview
glMatrixMode( GL_MODELVIEW );

Take note that this code belongs into the display function. Any tutorial that sets viewport and projection in a window reshape handler is very bad style; don’t follow it.