Figure 1: The video client user interface.
Figure 1: PS 478 Kb
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J. W. Percival
Space Astronomy Laboratory, University of Wisconsin,
Madison, WI 53706
R. L. White
Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218
A Tk-based GUI allows the user to control image quality and frame rate, select image size, set maximum bandwidth, save frames to disk, and change channels remotely. This system is in use at the WIYN Observatory.
The WIYN Telescope Control System was designed to be a modern, distributed client/server system with special attention paid to conservation of bandwidth on network connections. This emphasis was intended to promote the use of remote observing (including eavesdropping) for both professional and educational users of the telescope. We felt that low bandwidth network tools were essential to any remote observing effort due to the remoteness of most observatories, the competition for bandwidth on public networks, and the high cost of dedicated private networks.
We focussed on three communications problems related to remote observing: engineering data (Percival 1995a), science data (Percival & White 1993), and observatory video signals, the subject of this paper. For each of these three areas, we decided to target the regime of ultra-low bandwidth, which we define as phone-grade connections of 28.8 kbps or less (for a discussion of remote observing in the regime of ultra-low bandwidth, see Percival 1995b).
For the WIYN video transmission system, we specifically exclude the problem of person-to-person videoconferencing. Many programs exist for videoconferencing, all seem to require bandwidths of 100 kbps or higher, and they offer tiny images with significant degradation to maintain frame rate at the low end of their bandwidth range. Rather, we targeted video signals with significant scientific content such as wavefront sensors, guide star cameras, and all-sky monitors, whose time rate of change is far below that of normal video signals, and for which the quantitative fidelity of individual frames is more important to the user than interframe variations. In other words, we designed a video frame browser rather than a real-time videoconferencing tool.
The problem of sending large-format digital images over ultra-low bandwidth connections is well understood (Percival & White 1993). We transform an image using the 2-D Haar wavelet transform (White 1993; White & Percival 1994), encode and compress each bitplane from MSB to LSB, and send the compressed bitplanes to the client. As the client receives each bitplane, it inverts the Haar transform and displays the partially received image. The 2-D Haar transform lends itself especially well to this method of decomposition and reconstruction because it is fast, compact, precisely invertible, and behaves well in the regime of high compression, when only a few bits per pixel or less have been received by the client. The Progressive Image Transmission system home page gives more detail, examples, and further references.
The Progressive Video Transmission system uses the same protocol as the Progressive Image Transmission system, the only modification to the system being the ability of the server to operate a frame grabber rather than reading static files from a disk.
Our protocol is interactive at all times during a client-server session, unlike FTP, for example, which blocks during a file transfer. The user can grab frames, select image size, change the baud rate on the fly, pause, resume, stop, and save partially received images to disk.
Three configurable features in the server improve the frame rate over what one would normally expect.
The first is a simple change in frame size caused by rebinning the image at the server. The user can monitor the frames rebinned 2x2 or 4x4 and enjoy a 4 or 16 times increase in frame rate, and then change back to a full size image on the fly when something of interest pops up.
The second feature is the simple compression resulting from sending difference frames, a technique used in all videoconferencing and real-time video products. The received images can still be lossless, and in lossy modes the user can request a new baseline frame at any time to cure the slow degradation in fidelity that results from a succession of lossy difference frames.
The third feature is the automatic detection of the noise level, and an automatic skip to the next frame when this level has been reached. In this mode, the server monitors the compressibility of each bitplane as it is extracted, encoded, compressed, and sent. When a bitplane is not compressible, the assumption is made that the distribution of bits in this and subsequent bitplanes is random and will contribute little structure to the image being reconstructed by the client. It turns out that for many images, most of the time is spent transmitting these dense and uninteresting bitplanes, so a significant increase in frame rate results from this simple scheme.
Figure 1 shows the user's Graphical User Interface.
Figure 1: The video client user interface.
Figure 1: PS 478 Kb
Our user interface is an X11 program built using the Tk graphical scripting language. The GUI is used to name and connect to a video server, start, pause, resume, save files to disk, and so on.
The ``etc'' button hides esoteric controls, such as FITS header skipping and suppression, automatic display, and the verbosity of progress messages.
Special video controls include selecting video source, resetting the sequence of difference frames, pausing at the end of a frame, selecting automatic frame skipping, forcing a frame skip, and changing frame size.
The two progress bars along the bottom indicate the progress within the whole frame, and the progress withing the current bitplane. The image is updated at the end of each bitplane. At the left is elapsed time, and at the right is the time to go.
The ``more'' button pops up a window with numerical metrics such as number of bytes received, number of bytes to go, total number of bytes, the effective baud rate, and the current level of compression.
The Progressive Video Transmission system is suitable for sending full size video frames over ultra-low bandwidth connections. The user is given many ways to control image quality and frame rate on the fly using a simple but effective interactive TCP-based protocol. A Tk-based GUI allows easy reconfiguration of the controls to suit different types of users.
We targeted the WIYN Telescope video network as the primary application of this product, but it is suitable for any low-bandwidth video transmission situation.
This work was supported by NASA contract NAG5-2694.
Percival, J. W. 1995b, WGAS session on Remote Observing, 185th meeting of the AAS, ``Low Bandwidth Techniques for Remote Observing''
Percival, J. W., & White, R. L. 1993, in Astronomical Data Analysis Software and Systems II, ASP Conf. Ser., Vol. 52, eds. R. J. Hanisch, R. J. V. Brissenden, & J. Barnes (San Francisco, ASP), p. 321
White, R. L. 1993, in Space and Earth Science Data Compression Workshop, ed. James C. Tilton, NASA Conference Publication 3183, 117
White, R. L., & Percival, J. W. 1994, SPIE Technical Conference 2199