sin_ksv = monitor->keystone_vertical/sqrt(1.0f+monitor->keystone_vertical*monitor->keystone_vertical);
cos_ksv = sqrt(1.0f-sin_ksv*sin_ksv);
distance = monitor->perspective+sin_ksv*((sin_ksv>0)-monitor->vertical_center)/aspect;
sin_ksv = monitor->keystone_vertical/sqrt(1.0f+monitor->keystone_vertical*monitor->keystone_vertical);
cos_ksv = sqrt(1.0f-sin_ksv*sin_ksv);
distance = monitor->perspective+sin_ksv*((sin_ksv>0)-monitor->vertical_center)/aspect;