idGameLocal::CalcFov from dhewm3/neo/game/Game_local.cpp - modified with additional assertions and debug-prints and some comments on how [https://github.com/dhewm/dhewm3/issues/70] shouldn't happen (my guess is that renderSystem->GetScreenWidth() and/or renderSystem->GetScreenHeight() return 0 or a negative value)

gistfile1.cpp

void idGameLocal::CalcFov( float base_fov, float &fov_x, float &fov_y ) const {
	float	x;
	float	y;
	float	ratio_x;
	float	ratio_y;

	assert(base_fov > 0);

	// first, calculate the vertical fov based on a 640x480 view
	// XXX: will be positive because tan() is positive if the given value is positive as asserted above
	x = 640.0f / tan( base_fov / 360.0f * idMath::PI );
	y = atan2( 480.0f, x ); // XXX: == arctan(480/x) with x > 0 => will be > 0
	fov_y = y * 360.0f / idMath::PI;

	// FIXME: somehow, this is happening occasionally
	assert( fov_y > 0 ); // XXX: this *really* shouldn't be able to happen
	if ( fov_y <= 0 ) {
		Error( "idGameLocal::CalcFov: bad result" );
	}

	switch( r_aspectRatio.GetInteger() ) {
	default :
	case -1 :
		// auto mode => use aspect ratio from resolution, assuming screen's pixels are squares
		ratio_x = renderSystem->GetScreenWidth();
		ratio_y = renderSystem->GetScreenHeight();
		if(!(ratio_x > 0 && ratio_y > 0))
			Printf("## ratio_x = %f, ratio_y = %f\n", ratio_x, ratio_y);
		break;
	case 0 :
		// 4:3
		fov_x = base_fov;
		return;
		break;

	case 1 :
		// 16:9
		ratio_x = 16.0f;
		ratio_y = 9.0f;
		break;

	case 2 :
		// 16:10
		ratio_x = 16.0f;
		ratio_y = 10.0f;
		break;
	}

	assert(ratio_x > 0 && ratio_y > 0);

	y = ratio_y / tan( fov_y / 360.0f * idMath::PI ); // XXX: will be > 0, because ratio_y > 0 and fov_y > 0
	// XXX: atan2(ratio_x, y) == arctan(ratio_x / y) => will be > 0 because ratio_x and y are > 0
	fov_x = atan2( ratio_x, y ) * 360.0f / idMath::PI; // XXX: thus fov_x will be > 0

	if ( fov_x < base_fov ) {
		fov_x = base_fov;
		x = ratio_x / tan( fov_x / 360.0f * idMath::PI ); // XXX: will be > 0 because everything is > 0
		fov_y = atan2( ratio_y, x ) * 360.0f / idMath::PI; // XXX: as x and ratio_y are > 0, this should be > 0
	}

	// FIXME: somehow, this is happening occasionally
	assert( ( fov_x > 0 ) && ( fov_y > 0 ) ); // XXX: so this can't happen :-P
	if ( ( fov_y <= 0 ) || ( fov_x <= 0 ) ) {
		Error( "idGameLocal::CalcFov: bad result" );
	}
}