University of Salford, UK Centre for Virtual Environments and Future Media. Carl Moore, Toby Duckworth and David Roberts

Transcription

1 University of Salford, UK Centre for Virtual Environments and Future Media Carl Moore, Toby Duckworth and David Roberts

2 Introduc)on Our 3D telepresence system uses multiple camera silhouettes to reconstruct a 3D volume If images are not captured at the same time, the person may be in different positions This can lead to missing features or distortion Hardware triggers normally used to overcome this, we investigate software triggers as a possible cheaper alternative

3 Related Work Our previous work (DS- RT 2010) showed possibility of using unsynchronised frames for 3D reconstruction This used simulated models and movement Prior to this no one seemed to have investigated its necessity Other reconstruction systems appear to assume hardware synchronisation as necessary Matsumaya et al 2004, Origami (Grau et al 2004), Blue- C (Gross et al 2003) and Towles et al 2003

4 Introduc)on We focus here on an acquisition trigger Acquisition Starts the exposure cycle for a frame Grab Retrieval of an already acquired frame Differences in acquisition start times between cameras results in sub- frame differences Controlled by hardware acquisition trigger Using images grabbed from different frame sets results in larger multi- frame differences Controlled by software buffer management

5 Introduc)on Push Acquisition triggered, continues at regular intervals No delay between captured frames Pull Acquires single frame and waits for next trigger Decreased frame rate and latency due to waiting for exposure Pull Approach 2 Acquisition triggered, continues at regular intervals Frame returned when requested

6 Approach Project a millisecond clock onto the floor, visible by all cameras 102Hz display gives us granularity of 9.8ms Capture a number of frame sets using software trigger with UDP broadcast Compare digits in returned frames to discover level of synchronisation Exposures reduced (1ms) to allow clear capture of digits Gain increased to compensate for low light

7 Projected Clock

8 System We used 6 cameras connected to 3 capture nodes Capture nodes connected to server through gigabit network and router 102Hz projector for clock projection Application to broadcast UDP to capture nodes Software at capture node to trigger acquisition and save

9 Results 48 frame sets captured in total 42 sets showing exact clock digits Must have been captured within 10ms frame 5 sets with two clock frames, some blur between two Captured during clock update 1 set contains two clearly different clock frames Could mean acquisition start difference of 1-20ms

10 Timing a Pull System Used the same software with automated trigger Trigger sent after previous set received Exposure times increased for normal capture mode Added compression to reflect real data transfer sizes Timers in system When request sent and received When frames send and when full set of frames received

11 Results Total round trip time for requesting and receiving a full set of frames was 62ms 45ms spent acquiring and encoding 17ms spent sending and receiving Time a bit high, early implementation Expect around 12ms for 12- camera system

12 Discussion 17ms would cut FPS by nearly 45%, 12ms by about 35% Pull requests could be managed to reduce acquisition delays Overlapping frame receive and subsequent acquisition removes a large portion of delay

13 Discussion Provides easy synchronisation of grabbed frames Exposure does no start until request is made, adds delay in grabbing, equal to exposure (21ms) Simple form less acceptable for real- time communication than other two approaches mentioned Better request management could mask latency in some systems

14 Real Test Test in our system demonstrated good results Fast moving ball captured in juggler sequence using only software acquisition trigger All reconstructions captured at Salford have used software acquisition trigger

15 Future Work 10ms granularity of timing could be refined Same approach using a higher frequency display device would achieve this (LED Sequencer) Millisecond and possibly sub- millisecond achievable Compare output with model generated from hardware synchronised cameras

16 Conclusion Software triggering of image acquisition produces good results, as seen in juggler, with fast moving object In a system like ours, software acquisition triggers would likely provide an acceptable level of synchronisation for 3D telepresence

17 Ques)ons Thanks for listening