libskry_r

Lucky imaging library

Copyright (C) 2017 Filip Szczerek (ga.software@yahoo.com)

This project is licensed under the terms of the MIT license (see the LICENSE file for details).

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• 1. Introduction
• 2. Performance comparison with C
  • 2.1. Implementation remarks
• 3. Input/output formats support
• 4. Principles of operation
• 5. Change log

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1. Introduction

libskry_r implements the lucky imaging principle of astronomical imaging: creating a high-quality still image out of a series of many (possibly thousands) low quality ones (blurred, deformed, noisy). The resulting image stack typically requires post-processing, including sharpening (e.g. via deconvolution in ImPPG).

libskry_r is a Rust rewrite of libskry, mostly complete. Not yet ported: multi-threading, demosaicing, and video file support via libav.

For a visualization of the stacking process, see the Stackistry video tutorial.

For sample results, see the gallery.

See doc/example1.rs for a usage example.

See also the Algorithms summary in libskry repository.

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2. Performance comparison with C

The two goals of rewriting libskry were practising Rust and comparing the performance to C99 code.

The following figures were obtained on a Core i5-3570K under Fedora 25. Each test was preceded by a pre-run to make sure the input video is cached in RAM. In case of libskry, the program was doc/example1.c. In case of libskry_r, it was doc/example1.rs. Both use the same processing parameters. The raw video sun01.avi (840x612 8 bpp mono, 634 frames, Sun in Hα) can be downloaded in the “Releases” section. The C program was forced to use 1 thread with OMP_NUM_THREADS=1.

Compiler	Options	Execution time
rustc 1.23.0-nightly	RUSTFLAGS="-C opt-level=3 -C target-cpu=native", cargo with --release	23.4 s
GCC 6.4.1	-O3 -ffast-math -march=native	25.7 s
Clang 3.9.1	-O3 -ffast-math -march=native	23.9 s

2.1 Implementation remarks

The processing is not very complex. The calculations include: iterated box blur, block matching (both on 8-bit mono images), bilinear interpolation (on 32-bit floating-point pixel values) and Delaunay triangulation (only once, typically for a few hundred to a few thousand points, with negligible time impact). The only collection type used is vector.

The most time-consuming operation is block matching. Replacing the inner loop’s body in blk_match::calc_sum_of_squared_diffs:

result += sqr!(img_pix[img_offs + x as usize] as i32 -
               rblk_pix[blk_offs + x as usize] as i32) as u64;

with an unsafe block:

unsafe {
    result += sqr!(*img_pix.get_unchecked(img_offs + x as usize) as i32 -
                   *rblk_pix.get_unchecked(blk_offs + x as usize) as i32) as u64;
}

enabled the compiler to vectorize it (using AVX) and reduced the reference point alignment phase’s execution time of example1.rs from 26.7 s to 12.6 s.

On the other hand, doing the same in filters::box_blur_pass and filters::estimate_quality had negligible effect on the quality estimation speed.

Other uses of unsafe are not performance-critical: pixel format conversions, reading structures from a file, creating an uninitialized vector.

In libskry, the user is asked to create the processing phase objects in appropriate order and not to modify the underlying image sequence while processing is in progress. In libskry_r, correct usage in enforced by Rust’s borrow checker; it also necessitated modification of the API. Each processing phase is now represented by a __Proc struct, which holds a mutable reference to the input image sequence, and a __Data struct with this phase’s results (which does not hold any references). Each __Proc is created in a sub-scope; each resulting __Data is then fed to the next phase’s __Proc. See doc/example1.rs for details.

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3. Input/output formats support

Supported input formats:

• AVI: uncompressed DIB (mono or RGB), Y8/Y800
• SER: mono, RGB
• BMP: 8-, 24- and 32-bit uncompressed
• TIFF: 8- and 16-bit per channel mono or RGB uncompressed

Supported output formats:

• BMP: 8- and 24-bit uncompressed
• TIFF: 8- and 16-bit per channel mono or RGB uncompressed

At the moment there is only limited AVI support (no extended or ODML AVI headers).

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4. Principles of operation

Processing of a raw input image sequence consists of the following steps:

1. Image alignment (video stabilization)
2. Quality estimation
3. Reference point alignment
4. Image stacking

Image alignment compensates any global image drift; the result is a stabilized video of size usually smaller than any of the input images. The (rectangular) region visible in all input images is referred to as images’ intersection throughout the source code.

Quality estimation concerns the changes of local image quality. This information is later used to reject (via an user-specified criterion) poor-quality image fragments during reference point alignment and image stacking.

Reference point alignment traces the geometric distortion of images (by using local block matching) which is later compensated for during image stacking.

Image stacking performs shift-and-add summation of image fragments using information from previous steps. This improves signal-to-noise ratio. Note that stacking too many images may decrease quality – adding lower-quality fragments causes more blurring in the output stack.

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5. Change log

• 0.3.0 (2017-12-05)
  • Initial rewrite (from C) of libskry 0.3.0 (a805d0c4a).