UVIS PREVIEW IMAGES
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Mark Showalter, PDS Rings Node, April 2012

These preview images are designed to provide a quick view of the contents
of each UVIS data product. The appearance of each image is defined by the
instrument detector with which it was obtained and the form of the data
product as a 3-D cube, 2-D spectrogram, or 1-D time series.

All images have been generated in PNG format and are provided at the following
sizes:
    thumbnail   100 x 100 pixels
    small       256 x 256 pixels
    medium      512 x 512 pixels
    large       1024 x 1024 pixels
Note that these sizes apply regardless of the dimensions of the data products
themselves. Smaller products are enlarged or striped, and larger products are
scaled down to match the specified size and shape.

The frame color of the image defines the detector used, as follows:
    EUV         blue
    FUV         green
    HSP         red
    HDAC        yellow

(1) Spectral image cubes

These images are oriented with the 64-sample UVIS slit vertical (increasing
upward). The sweep of the instrument along the perpendicular axis is plotted
toward the right. The spectral wavelengths of the cube are divided up into
three equal bands, and then the samples at the same spatial location within
each band are coadded, resulting in three 2-D images. The brightest sample
among the three images is defined as unit brightness, and all the other pixels
are scaled accordingly. The three images are then combined into one color
image, using the shortest wavelength band for the blue channel and the longest
wavelength band for the red channel.

The color image is then re-sized to fit within a square. For products that are
especially wide, the image is broken into segments of equal width and these
are stacked to create the final image, with the first segment at the top and
each subsequent segment beneath it. In effect, these horizontally striped
images should be read like the lines of text in a book.

(2) 2-D spectrograms

These images are oriented with the 64-sample UVIS slit vertical and the
wavelength increasing toward the right. The resulting grayscale image is
re-scaled so that the brightest sample is white. The image is then re-shaped
into a square.

In the resulting image, spatial information varies along the vertical axis and
wavelength increases along the horizontal. To make this easier to interpret,
we also add a violet marker indicating the mean value in each column, and a
cyan marker indicating the mean value in each row. The violet points can be
interpreted as the spectrum averaged over the slit, and the cyan points can
be interpreted as the spatial profile averaged over all wavelengths (and then
flipped on its side).

(3) Time series data

These images are created by generating a horizontal strip of measured
intensity vs. time step as a grayscale image, which is then re-scaled so that
the maximum sample appears as white. This strip is then re-shaped into a
square, where especially wide strips are wrapped into up to six segments that
are stacked horizontally, in the same manner as described above for wide
spectral image cubes.

To simplify the interpretation of these plots, the intensity value in each
column is also plotted as a yellow point. These yellow points can therefore be
interpreted as a profile of intensity vs. sample number along the horizontal
axis.