Selecting frame rate on the basis of known target's positions and velocities
Selecting frame rate on the basis of known target's positions and velocities |
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Selecting frame rate on the basis of known target's positions and velocities |
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The frame rate greatly affects the required bandwidth of communication channels, the capacity of the archives, computing power, and hence the total cost of video surveillance systems.
At the same time, choosing optimal frame rate - not an easy task and criteria for selection are not always unambiguous. When the frame rate is too low, omission of important events are possible, too high frame rate leads to an unjustified rise in price of video surveillance systems.
VideoCAD offers a tool for choosing frame rates based on the desired frequency of getting in the frame objects with the known positions and velocity. Additionally, you can create animated images with the specified frame rate.
Problem
Place of camera installation, the probable locations and velocity of moving objects are known. We should choose an optimal frame rate to ensure that any moving object gets into the frame at least twice.
Order of work
1. Construct 3D model of the scene, place the camera. See Example.
You can use the Shadow tool to take into account camera control area with shadows from obstacles.
2. Place the 3D models at the expected locations of moving objects.
3. Specify velocity vectors for the 3D models according to expected speeds and directions of moving objects.
| • | double click on the model to switch it to editing state; |
| • | on the appeared Current construction parameter panel type a nonzero velocity value (in meter(foot) per second) into the Speed box. |
You can edit the velocity vector of 3D model in editing state by moving its terminus.
Bring the cursor to the terminus of the velocity vector, click the left mouse button and drag the terminus. At the same time the speed value and direction of movement is being edited simultaneously. Click for the second time to specify new velocity and finish editing. |
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4. Remove selections from the 3D models if there are selected ones.
5. Open the 3D Video window. Enable image processing,if it is disabled. By right clicking open the Image parameters panel. Chose Camera tab.
6. Try to change Frame rate value, watching the velocity vectors of 3D models in the Graphics window.
If 3D model is in the normal state, the length of the velocity vector equals to the distance which the 3D model passes for the period between successive frames of the active camera.
7. Select the Frame rate in order that all velocity vectors fit in the view area (control area) projection at least twice.
8. Choose the View tab on the Image parameters panel. Check Animation box. Set the Number of frames =3 or more.
In several seconds animated image with specified frame rate will appear in the 3D Video. In the image there will be 3D models with specified velocity.
External link: "The principles of CCTV design in VideoCAD. Part 5. Video surveillance of moving objects"(*.pdf)
