STISIM Drive - Graphics Tab Box

Choosing the "Graphics" tab box option allows you to set various parameters that directly affect how the roadway display information will be presented to the driver. This includes the simulation frame rate, how many display screens will be used and the sizing of each individual screen. When this option is selected, a dialog box similar to the following will be displayed:

As you can see by the figure displayed above, the graphics tab box contains a wide array of varied options. The first option is the simulation frame rate. Since STISIM Drive is a real time driver in the loop simulation, it is affected by the amount of time it takes the computer system to process a single frame of the simulation run. Since it takes a finite amount of time to process each frame, both time and distance are not directly controllable by the program. In general, STISIM Drive tries to run the simulation at the constant update rate that the user specifies (we generally recommend 20 times a second). Unfortunately, very complex display scenes require more processing time and this frame rate may slip and become longer on certain frames. As long as the frame time does not become outrageous (less than say 10 frames be second), the display scene will look smooth and the driver may not even notice. You probably think great, if the driver can’t tell then there is no problem. Unfortunately, that is not the case. The problem arises when you are trying to collect data at discreet time or distance intervals. Because the time may be slipping, the timing interval may slip also. When the program collects data it checks to see if the criteria (time or distance) has been met or exceeded and then saves the desired data. If the frame time is jumping around, then the data collection time will jump around also. Additionally, if you are collecting data based on distance, it is impossible to collect data at even intervals because the distance that the vehicle has traveled during a single frame will change based on the speed that the vehicle is traveling. There is no exact way to avoid these data collection problems, but you may wish to keep them in mind when analyzing your data.

The frame rate determines how smoothly the roadway display images will be displayed to the driver. If the frame rate is too small, the simulation will seem to pause between each frame or image, causing an uneven jerky display that will be objectionable to the driver. However, if you set the frame rate too large and then display a really complex scene, the display system will be unable to keep up with the specified frame rate and the timing will not be consistent. Unfortunately there is no way to tell if you have a consistent frame rate until you actually drive your simulation. In the experimenter's display window there is a variable that shows you the current frame rate, paying attention to this will give you an idea of where your simulation slows down, and what your maximum frame time is.

To specify the frame rate, we recommend you set it as fast as possible. This is generally at 20 frames per second, however if you have a real simple scenario that is not displaying a lot of objects in the display scene a frame rate of 30 will be fine. If you want to see what your minimum frame rate is, set the frame rate to as fast as possible and then watch the frame rate as the simulation progresses. The lowest value you see will be your minimum frame rate.

Below the desired frame rate option is the "Screen Resolution" option. STISIM Drive tries to make a determination of the maximum resolution your graphics can produce and then enables/disables the screen resolution accordingly. In general you should always try and use the maximum resolution available. This will provide the best looking roadway scene and will generally get rid of some visual anomalies that occur at some of the lower resolutions. Reasons for using a lower resolution include your monitor not being able to handle the higher resolutions and timing issues. Since the higher resolutions modes must activate more pixels and this takes time, you can sometimes improve the graphics performance by reducing the screen resolution. Therefore in rare cases you may have to make a trade off between visual anomalies and performance.

The screen sizing button will be skipped until the end of this section because it involves an additional dialog window. This brings us to the "Monitor startup delay time" parameter. This parameter allows you to set a delay time for the roadway display monitors. Present day monitors are all energy efficient and therefore shut themselves off if there is not a signal currently being fed to them. This means that when a signal is finally fed to them, they must turn back on and this takes a finite amount of time. If you do not have a monitor delay time specified, then as soon as the simulation is ready to go, if you tell it to start, the monitors may not be ready and you will have a blank screen for several seconds. If you use the monitor delay time, then the system will begin feeding a signal to the monitors and wait the specified number of seconds before allowing you to begin the run. This way, when the run is started the monitors are warmed up and will display the image immediately. Different monitors vary in the amount of time it takes them to warm back up. We have seen anywhere from 0 to 5 seconds. You will have to play with this parameter until you are satisfied with the results.

The "Display Options" option allows you to select how the roadway will be displayed to the driver. There are three different ways to display the roadway scene, using a single monitor, using a wide field of view system with 3 monitors or using a head mounted display (HMD). The single monitor option is the simplest and most common configuration. It requires a single computer sending the roadway graphics out to a single display source (monitor or projector). The wide field of view option is more complex because it requires 3 separate computers to be networked together to form the wide field of view simulation system. When this option is used, you need 3 computers, 3 display sources, network cards, a network hub and network cables (refer to the section on hardware configuration). The resulting display scene provides a 135 degree field of view which allows the driver to see much more of the driving scene especially when handling turns and traversing intersections.

The most complex of the display system options is the HMD. In this configuration, the driver essentially wears a pair of goggles that contain little monitors that project the image. Attached to these goggles is a sensor unit that tracks the motion of the driver's head and displays the roadway scene based on the direction that the driver's head is pointed. This option offers some distinct advantages over the other systems, but alas it has many disadvantages as well. On the plus side, the HMD provides a true 360 degree field of regard. This means that no matter where the driver looks, they will see the scene in that direction. For example with the HMD, a driver can truly look down the road at an intersection to look for oncoming traffic or they can look over their shoulder when changing lanes. Since one display unit is used for the entire 360 degrees, only a single computer is required and this helps keep the costs down. However, good HMDs are very expensive and they have a small field of view (approximately 40 degrees). This means that even though the driver has the potential to see 360 degrees they can only see 40 degrees of it at a time. The tracking that is done with the HMD is done on the head and not the eye, so when most people would just avert their eyes in order to change their view, in this case the driver must move their entire head. Another drawback to the HMD is the sensor. Since a sensor is needed in order to keep track of the head position, there is some delay added to the system as the driver moves their head and the sensors reads the data and communicates with the computer. If this lag becomes too large it can make the simulator undrivable and lead to simulation sickness. Therefore we have added a handful of options that you can use to fine tune the HMD.

If the "Head mounted display system" option is chosen, the "Head Mounted Display Options" will become active. The first thing you will have to do is stipulate where the data will be coming into the computer from the HMD. Currently the only HMDs that STISIM Drive supports use RS-232 serial communication to get the data from the sensor to the computer and therefore you will have to choose a communication port to use. Next, you are allowed to choose which motion axis will be active. You may selectively choose any or all of the axis systems. Pitch refers to the head's up and down movement like nodding yes. Roll refers to tilting your head side to side as if try to touch your ear to your shoulder. Finally, yaw refers to the side to side motion of your head as if nodding no. You can selectively choose if this motion will be represented in the display scene because the different motions can bother some drivers. In all cases you will want to use the yaw axis, in some cases the pitch axis, and generally you will never need the roll axis.

The final 3 parameters try to stabilize the system so that the delay that was mentioned earlier is minimized and the sensor performs at optimal levels. First is the filter time constant and this is generally set to a value of .05 to .1.This parameter tries to filter out any noise in the signal coming from the sensor. Next is the washout coefficient. Some sensors have a tendency to drift so that if you keep them in a certain position for too long they are not pointing in the exact same direction. The washout coefficient tries to wash out this effect so that the sensor is always looking in the direction expected. Finally, there is a deadband parameter. Once again this parameter helps to eliminate noise and small resolution anomalies by not allowing the direction of the roadway scene to change unless a minimum (or deadband) threshold has been met.

Setting up using a head mounted display is not for the faint of heart. We recommend that you contact STI before venturing off in this direction. For one thing, you need to know what sensor we support because we so conveniently left that part out.

This brings us to the "Roadway display far clipping plane distance". This is a graphics term referring to the absolute limit that the driver can see ahead of them in the simulator. Everything past this distance is clipped out of the scene and not rendered, meaning that the end of the world appears at this point. The far clipping plane is important for a couple of reasons. First it takes a finite amount of time to compute all of the polygons for everything you see in the roadway display and the faster these are computed the faster the frame time and the better the simulation will run. If the computer has to compute the polygons for objects that are so far ahead that the driver will not notice them, then this is a waste of processor time. Therefore, we set the far clipping plane so that the graphics will not render any objects outside of this distance. You may say what about the mountains or other horizon scenes? These are handled a little different so that they will remain visible.

This brings us to the final option in the "Graphics" tab box. Earlier we said we would get back to the "Screen Sizing" option button, and here we are. If you click on this button, a dialog window similar to the following will appear:

This dialog window allows you to specify the size and location of the roadway display scene on the display source. This actually allows you to set it up so that the driving simulation only appears in a specified section of the screen if that is desired. As you can see from the sample window, the screen size can be set for each display screen (assuming you have a wide field of view system otherwise only the center screen is enabled). Setting the left and top parameters will set the position of the scene on the display device while setting the right and bottom will specify the size (refer to screen sizing for details). If you change the settings and then don't like them, you can simply click on the "Default Settings" button and it will restore the screen size to full screen.