eclipse-hdr

Takes a pile of DNG photos of totality during a solar eclipse, and exposure stack them to get a pretty picture of the corona.

It works like this ...

1. Image alignment

If you stuck your camera on a tripod and took a bunch of exposure-bracketed photos during totality, then the sun & moon will have moved a little between each frame.

The alignment stage compensates for this, trying various translations and rotations to find where the images agree the most. It performs sub-pixel alignment, using Catmull Rom interpolation as needed.

2. Image fusion

Once the input images have been aligned, we fuse them into a single HDR (high dynamic range) image. This normalizes across the differing exposure values used for the images.

It generates a .hdr image file, which can be used with other HDR software such as Adobe PhotoShop, or the command line suite pfstmo:

pfsin fused.hdr | pfstmo_fattal02 --white-point 0.00001  | pfsout fattal.png

3. Tone mapping

HDR files can't really be viewed directly - they need to be converted into LDR (low dynamic range) files. This conversion is called tone mapping.

We bundle a number of tone-mapping operators: drago03, durand, fattal02, icam06, reinhard05, and a linear operator. You can see some output in samples/. (Huge thanks to github.com/mdouchement/hdr for most of these !)

How to use it

The biggest prep step is building the DNG SDK on your system - Adobe's library is too hairy to be fully buildable as a Golang package. Luckily that's only a few commands, details here: https://github.com/abworrall/go-dng#using-the-sdk-from-go

Prep steps:

# Install DNG SDK: https://github.com/abworrall/go-dng#using-the-sdk-from-go

# Now, you can build eclipse-hdr
sudo apt install build-essential libfftw3-dev
go install github.com/abworrall/eclipse-hdr/cmd/eclipse-hdr@latest
~/go/bin/eclipse-hdr -h

Usage:

eclipse-hdr images/                   # load everything in the dir
eclipse-hdr images/1234.DNG ...       # load specific file(s)
eclipse-hdr -finetunealign images/    # generate fine-tuned alignment (takes ages)
eclipse-hdr images/ ./conf.yaml       # also load a config file

eclipse-hdr -developer=layer images/  # see which layers get used
eclipse-hdr -width=1.2 images/        # generate images not much wider than the sun

Supported photo files

This tool expects to see DNG files (Adobe Digital Negative). As well as pixel data, and exposure metadata, DNGs provide matrices for color correction and white balance. Plus we can access them as simple linearized sensor data (DNG's "stage 3"), which is what we need for image fusion.

Using TIFFs instead

If for some reason you can't use DNGs, then you can make do with TIFF files and some color configuration. The TIFF files should contain linear (unadjusted) image data, and the easiest way to get those is using Adobe's dng_validate.exe tool to output stage 3 data.

You then need to use exiftool.exe to copy over the EXIF metadata. For more detail, go here.

Take note of your AsShotNeutral and ForwardMatrix info that gets printed by dng_validate.exe -v; you'll need it for conf.yaml.

Note you can build dng_validate.exe for linux; details at https://github.com/abworrall/go-dng#building-the-sdk-on-linux

Alignment fine-tuning

By default, the alignment is pretty coarse - it just lines up the dark moon in each photo. The -alignfinetune argument does much more work, trying hundreds of possible alignments and scoring how well the images agree. This can take many hours, so you only want to do it once, and do it overnight.

When it finishes, it will print out some configuration. You should save this for your conf.yaml (see below).

If you run in verbose mode (-v=2), it will write hundreds of images to disc, each one a luminance diff of a proposed alignment.

conf.yaml

Mostly you should put your alignment info in here, as it takes so long to compute.

If you're using TIFF files, you'll also need your color correction info - the manual overrides for AsShotNeutral and ForwardMatrix that you've figured out some other way.

# Reuse expensive-to-compute fine alignments
alignments:
  5671-5667:
    name: 5671-5667
    translatebyx: 11.75
    translatebyy: -16.5
    rotationcenterx: 3269
    rotationcentery: 1344
    rotatebydeg: 3.191891195797325e-16
    errormetric: 21455.073216304932
  5671-5668:
    name: 5671-5668
    translatebyx: 8.5
    translatebyy: -6.25
    rotationcenterx: 3269
    rotationcentery: 1344
    rotatebydeg: 3.191891195797325e-16
    errormetric: 24212.739338470437

# Needed for TIFF files
manualoverrideasshotneutral:
- 0.501
- 1
- 0.7014
manualoverrideforwardmatrix:
- 0.6227
- 0.3389
- 0.0026
- 0.2548
- 0.9378
- -0.1926
- 0.0156
- -0.133
- 0.9425

Note the forwardmatrix is a row at a time; the one above corresponds to this output from dng_validate.exe:

ForwardMatrix2:
                                         0.6227   0.3389   0.0026
                                         0.2548   0.9378  -0.1926
                                         0.0156  -0.1330   0.9425

You only want one config file to be loaded, the last one overwrites.

Output files

The outputs are all centered on the eclipse itself, are square, and are sized in terms of lunar diameters via -width.

Fused HDR image, suitable for PhotoShop, PFSTMO, etc

The main output is fused.hdr, a high-dynamic range file combining all the exposures. You can process this further in standard software.

Tonemapped LDR images

It will also generate a PNG file for each supported tonemapping operator, e.g. tmo-fattal02.png.

• fattal02 seems the most reliable, though it amplifies noise a bit
• icam06 seems to use a different white reference, so is pinkish and warm
• linear always looks dim, that's why we need fancy tonemappers
• reinhard05 looks great with width<=3, but goes wrong when there is too much dark sky