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Chapter 10. Onboard Software: Image Compression
The major functions of the flight software are command processing, instrument control, image processing and compression, status monitoring, and telemetry control. In this chapter we describe the image processing and compression options available for LASCO data.
The low spacecraft telemetry rate (4.2 Kbps) would result in a transmission time of about 60 minutes for a full 1024x1024, 2 bytes per pixel image. (We note that the individual pixel intensities from the camera analog-to-digital converters are actually represented by only 14 bits, leaving a factor of 4 headroom in the 16 bits which are reserved in the data format.) Thus, image compression is desirable. A number of image processing techniques have been included in the flight software, from simple square root through transform encoding. The types of processing and compression utilized will be determined by ground command and executed through stored sequences, with parameters stored in tables. After a camera image is stored in a 2 Mbyte image buffer, the appropriate algorithms are applied. Image columns that are known to be bad will be replaced by the average intensities of the adjacent, or nearest good, columns. The locations of the bad columns (for each CCD) will be stored in a bitmap located in RAM, which can be updated as necessary.
Two general classes of techniques are included in the flight software. The first class consists of image processing techniques which, although by themselves they do not directly compress the telemetry required to transmit an image, do transform the image into a format which is more suitable for true image compression. The second class are true image compression techniques which directly reduce the telemetry load. These consist of both geometrical techniques which reduce the total number of pixels composing an image, and coding techniques which reduce the number of bits necessary to transmit an image, which may already have had other techniques applied previously. Transform encoding yields the highest compression ratio, but is very computationally intensive.
Additional compression is obtained by transmitting only the pixels that are not obscured by the occulting disk or the aperture stop. For some situations, the microprocessor will compute intensities only along a radial spoke, thereby saving up to 89% of the telemetry for a full image. Time resolution can be traded against field coverage to further reduce the data download requirement.
10.1 Image Processing Techniques
These techniques prepare the image for true compression by transforming the original image into an new image whose intensity histogram distribution has more intensity values falling in fewer histogram bins than did the original intensity histogram distribution:
10.3 Image Compression Techniques
The geometric data compression techniques for LASCO are:
The coding compression techniques can be divided into two categories, lossless or lossy, depending upon whether the image can be reconstructed on the ground with no loss of information, or with some (small or negligible) loss. The LASCO options are:
These compression schemes may be combined to achieve a higher compression. The average compression factor is expected to be about 10. Thus on average, the readout time should be about 6 minutes, which would allow around 200 images each day to be transmitted.
Figure 10-1 illustrates at top left an eclipse image, which is compressed by a factor of 10.5 and then reconstructed using three different compression techniques. The top right image uses the ADCT technique described above, while the bottom two images use techniques (Adaptive Hadamard Technique and Block Truncation Coding) which were finally not implemented for the LASCO software. The image quality losses for all three techniques are minimal, as seen by the small values of the normalized mean square errors of the three reconstructed images from the original.