These drawing modes can be selected in the Drawing Mode submenu of the Display menu, or the Dialog Bar - Left.
The OpenGL modes use the OpenGL system software for a more "realistic" appearance. Since this type of drawing automatically handles the interpenetration and intersection of solid objects (using a depth buffer), it allows the depiction of symmetry elements, that is rotation axes and mirror planes. See 3D Drawing modes for more details. Settings for this mode are in the 3D Parameters dialog in the Input2 menu.
The OpenGL Stereopair mode shows two images, as in the 2D Stereopair mode, but they are drawn using the 3D method.
Although a fully-shaded SHAPE drawing in the 2D Drawing modes has an excellent 3-dimensional appearance, the term "3D drawing" is used to denote a method of drawing which is different in several respects. SHAPE uses the OpenGL software package for Windows, Linux and Macintosh. 3D Drawing modes give a more realistic appearance in full color, especially on screen, but are not generally suitable for schematic drawings, as there are no boundary lines. See below and in Description of the Calculations for details on the difference between 2D and 3D Drawing modes.
Stereopairs. Selecting 3D Stereopair causes two separate drawings to be made, which are identical except for the Stereopair Rotation Angle. Many people cannot view a stereopair directly because of the difficulty of getting the left and right eyes to focus on separate points at a short distance. However, various devices are available for such viewing, using lenses, mirrors or prisms.
Stereo viewing works best when Perspective is selected.
In order to show non-separated stereo views with OpenGL, you must have an appropriate graphics card and viewing hardware, and you must select the OpenGL Quad Stereo Drawing Mode
Some people can view stereopairs by crossing their eyes, without artificial aids. For this, the Stereopair Rotation Angle should be negative.
Older stereo drawing drivers from nVidia, which worked with shutter glasses but only on Cathode-Ray-Tube monitors, not flat-screen LCD monitors, supported OpenGL without quad-buffering. If you are using such a system, you do not need to select the SHAPE 3D Quad Stereo Mode; stereo images can be viewed in the 3D Single Drawing mode. However, stereo will be shown only in Fullscreen display, not in a window. The stereo effect is turned on with a hot key or other switch for the hardware.
Difference Between 2D and 3D Drawing Modes.
In 3D drawing, the surfaces of three dimensional objects such as spheres and cylinders are converted to an assemblage of planar polygons. Then each polygon is drawn essentially independently. The critical difference from 2D Drawing modes is that a depth buffer is used in 3D imaging. This is an array of integers, one for each pixel in the display or output (or that portion which is currently being drawn). Each element, representing a pixel, holds the relative x coordinate (in the SHAPE observer coordinate system) of the foremost object or polygon. The color for this object is retained in the color buffer, which is a similar array representing pixels; this array is actually the image itself. Whenever a polygon is drawn, each pixel which it contains is compared against the depth buffer; if the x coordinate of the pixel is greater, or closer to the observer than what is in that element of the depth buffer, the color for this pixel in the new polygon replaces the value in the color buffer. That is, the depth buffer keeps track of the front surface of the drawing, and ensures that only this front surface (not any hidden surfaces) are kept in the color buffer or image itself. It is also possible to have transparent or translucent objects, through which hidden surfaces may show partially, by mixing the color of the latest object with the color which is already in the buffer.
Actually, the "double buffer" method is normally used, the color buffer being kept in an area of memory and then copied to the screen memory when the drawing is completed. This is usually faster than drawing directly to the screen.
The 2D Drawing modes of SHAPE do not use a depth buffer. When necessary, the intersections of objects are solved analytically and only the required portions of each are drawn.
Using a depth buffer has the advantages that it is not necessary to solve analytically for intersections, nor to sort the objects with respect to depth (unless some objects are translucent). This may save considerable time, especially for complex drawings, since the time for sorting tends to increase exponentially with the number of objects. Since analytical solution of intersections is not necessary, it is possible to place essentially any objects into the drawing. in any location. This overcomes the problem in SHAPE 2D Drawing modes of sometimes-incorrect drawing of crystal edges. The 3D method allows more complex shading and lighting effects, such as specular highlights and multiple light sources. For a completely 3-dimensional image, on a computer which has sufficient memory, the 3D Drawing mode is superior to the SHAPE 2D Drawing mode.
However, there are disadvantages to the 3D method. The depth buffer may result in excessive memory requirements even for the screen (but some 3D accelerator cards may have special memory for the depth buffer). Because printers have much higher resolution than the screen, this causes even higher memory requirements, and for reasonable sized printed drawings it is usually necessary to do the printing in bands or segments.