📚 BROWSE.TXT
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BROWSE.TXT
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55 lines***** File BROWSE.TXT
Effective use of the 1,612 images contained on the IHW/Large-Scale
Phenomena (L-SP) compressed image CD-ROMs requires that the user of the discs
be able to "browse through the data" quickly to find those images and
intervals which are of high scientific interest. Because of the long
decompression and transfer times of the full-resolution images with current
image display hardware, the goal of efficient browsing of the data can be met
only if the images are placed on the discs at least a second time, in either
subsampled or filtered form, and uncompressed.
The browse images are actually stored in three places within the total
set of IHW CD-ROMs. In addition to the subset of images stored in the BROWSE
directory of each IHW/L-SP compressed image CD-ROM (HAL_0001 - HAL_0018), the
entire set of 1,612 digital images exists on the last of the IHW/L-SP
dedicated discs (HAL_0018) in the "volume subdirectories" of the
SUMMARY\BROWSE directory (sample path is SUMMARY\BROWSE\HAL_0006). The browse
images are also interleaved with data from the other IHW disciplines in the
daily data subdirectories on these "mixed data" CD-ROMs (HAL_0019 - HAL_0023).
A "browsed image" is one that has been generated from the original
uncompressed image. It has been subsampled and is no larger than 256 pixels in
either dimension. In addition, the digital data have been scaled into a
numerical range of 0 to 255 (one byte per pixel; the precision for most of the
original images is 10 bits, requiring two bytes per pixel). The BROWSE
directory of each L-SP compressed image CD-ROM contain datafiles, FITS
headers, and PDS labels for the compressed images on that CD-ROM; this
includes both images of Comet Halley and of calibration objects. Note that
the 1,612 total L-SP digital images (1,439 of the comet, 173 of calibration
objects) are deposited on multiple CD-ROMs "dedicated" to the L-SP imagery.
The browse data were obtained by taking the "n"th row and column for the
original image starting at row "n/2" and column "n/2". The value for "n" was
determined from the larger of the two axes such that the quantity (original
length / n) was less than or equal to 256. For the images which were
digitized at GSFC, the original densitometer values ranged between 0 and 1023.
The density values in the images were divided by 4 in order to convert the
density to a single byte. For those images digitized elsewhere, the density
scaling factor was chosen so that the density in the browse image was less
than or equal to 255.
The FITS header records for the browse images have had their astrometric
information adjusted to reflect the change both in pixel spacing and image
origin. Thus, should the user wish, (crude) astrometry can be performed with
the browse images. In addition, HISTORY keywords have been inserted to
document the linear scale and density scale changes. The creation of the
browse images was accomplished using the program MIDGET, which can be found as
MIDGET.FOR in the SOFTWARE\BROWSE\FORTRAN directory of the compressed image
CD-ROMs.
The filename extension for the files of the browse data (.IBG = image,
.HDR = header, .LBL = PDS label) follow the IHW filename conventions. To
reconstruct the original FITS byte stream, the .HDR and .IBG files for the
appropriate observation should be concatenated.