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RegiStar 1.0.10 Support for Raw Images

For an overview of the new features in RegiStar 1.0.10, see revision history. 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 LibRaw or DCRaw, as explained here. 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.

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