Image
Processing techniques
Joerg
Gabi & I were all participants on the same commercial eclipse
expedition to the Gobi desert. After the eclipse, Joerg &
I have corresponded about image processing techniques. I recognised
that Joerg's original images were of a very high quality so I
asked Joerg to complete the initial stitch at his end. Joerg
sent me a 16 bit 2400 x 3600 Px image with no manipulation applied
after the stitching.
Russel
Brown's procedure is a very well documented generic procedure.
Unfortunately, no photograph can be generically processed if
it is to be optimally processed. The workflow & methods I
apply are guided by the changing appearance of the image as I
work. Therefore the work flow, that is the selection and order
of application of image processing techniques, is different for
every image I work on. If you don't already know a lot about
Photoshop, the description I provide may not be of much help.
When
I have more time, I might try to develop a set of actions &
make them available for download - a sort of eclipse chasers
toolbox - for working on solar eclipse images .
Broadly
I can describe the processing as follows :
Step
1
I
first applied high pass sharpening rather than the unsharp masking
described in Russel Brown's tutorial. This is a simple technique.
Copy the layer. Apply a high pass filter of radius between 1
and 3 pixels to the image. Set the blending mode to overlay.
Adjust the opacity if you want to tone it down.
Step
2
Dust
in the air left a reddish diffraction ring surrounding the sun
at Rs=2.5. I applied a colour correction layer that corrected
for the gradient colour in the sky and some colour fringing that
is an artefact of the high pass sharpening. Create a new layer;
set blending to colour, fill with a colour sampled from the part
of the sky that is the right shade of blue.
Step
3
I
have also applied variable proportional contrast masking (contrast
increase that is only applied to parts of the image that need
it without killing shadow details) and luminosity adjustments
using photoshop. To do this, copy the layer and set the blending
mode to multiply. Create an alpha mask and invert it. Adjust
the opacity to suit.
Step
4
Adjust
the residual colour cast left by the sky colour correction.
Step
5
The
image was resized down using factorial interpolation (changing
the size by no more than 10% each step) At each displayed image
size 600Px & 1600Px, some sharpening was applied to the final
sized image appropriate to that size.
Joe
Cali
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