RegiStar 1.0.10 Support for Raw Images
For an overview of the new features in RegiStar 1.0.10, see
Detail on the new support for raw images is given below. For further information, see the RegiStar Help system.
Raw images are produced by cameras whose sensors have a color Bayer array.
A layer of microfilters restricts each sensor pixel to capturing red (R), green (G), or blue (B) light,
which means each pixel of the image contains exactly one R, G, or B value instead
of the full complement of R, G, and B in traditional RGB images. RegiStar reads only the native,
original pixel values when it opens a raw image. It does all interpolation required to produce full RGB values
(or “de-Bayer” the image) internally.
Raw-image types: RegiStar reads two types of raw images: camera-raw and FITS-raw.
Camera-raw images are in a proprietary format determined by the camera manufacturer.
For example, Canon DSLRs write .CR2 or .CR3 files, Nikons write .NEF files, Sonys write .ARW files,
and some brands write .DNG files. There are dozens of camera-raw formats, and to avoid clutter RegiStar
is initially set to recognize only these five suffixes. To add additional extensions (e.g. .ORF, .PEF),
or to remove unneeded extensions,
use the File menu or Open dialog. Camera-raw images are decoded using either
DCRaw, as explained
FITS-raw images are produced by image-acquisition software that controls a
DSLR or a cooled “one-shot-color” astro-camera and do not require decoding.
Raw parameters: Because of variations in the relative sensitivity
between the tiny R, G, and B filters in the Bayer array and also any
external filters used, compensation by color-specific multipliers must be
performed so raw images are color balanced. Also, the correct Bayer array must
be assigned. RegiStar allows raw parameters to be set
and modified in a number of ways, and remembers values in effect at the time an
image is closed so that processing can continue in another session.
Raw-to-RGB conversion: By default, RegiStar displays raw
images in color, filling in missing R, G, or B values by interpolating from
nearby pixels and then applying color adjustment. Raw images may
also be viewed in greyscale mode in which the Bayer checkerboard pattern is
visible. Regardless of its display, the image itself has original, raw pixel
values. Only a RegiStar operation actually outputs an RGB image; display
calculations produce transient data only.
Raw-image Display Control: Raw images contain linear data
representing literal counts of photons of the designated color at the given
pixel location. Like all linear images, they display as dark unless a gamma
adjustment is applied for rendering. Also, the entire allowed pixel range may
not be realized. By default, RegiStar applies gamma adjustment and pixel-range
adjustment so raw images render well on screen.
Raw-image operations and interpolation: The first operation
(not including preprocessing) performed on a raw image is typically registration. At
this key stage in the processing sequence, RegiStar produces a registered RGB
version of the original raw image with channel-adjusted, interpolated pixel
values. Conversion to RGB here minimizes the total amount of interpolation
needed in the entire processing sequence and yields the best final image
quality. Strictly channel-specific formulas are used, to preserve color purity. Bicubic
interpolation is used by default and best preserves sharp detail, such as stars,
but bilinear interpolation may be chosen instead for speed.
RegiStar is designed to ease the management of raw images, which can
nevertheless require some preplanning. For additional information and pointers on how to best take
advantage of RegiStar’s raw-image design, see the “Raw images” chapter in the RegiStar Help system.