Now let's take a look at a bunch of filter
effects we might want to use on this picture.
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into 3D: draw
a shape, and see it
automatically turn it into 3D
Archipelis
Designer
cool
tool for rapid 3D model creation and prototyping from images
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First and foremost, you may want to adjust brightness, contrast, gamma
and similar aspects of the image. Use
menu: Filter
> Adjust > Value...
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One the coolest filters for images like these is Mystic vision, part of
the Blur filters:
menu: Filter > Blur > Mystic vision
It's a bit like the Zoom blur but it is selective. It applies to bright
pixels only. (in the Animation menu, its counter part exsts too:
shadow vision, useful for casting rays of apparent volumetric shadows)
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Here's an idea of what Mystic vision does: those few bright areas in
the dark cloud appear to cast rays of light. In this case the hotspot
is near the center of the dark cloud blob. Light rays appear to emanate
from there in all directions.
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You can change the position of the 'hotspot' for the filter. That can
be used for a more top-down sunrays effect. The hotspot is placed
near the top right border. You can even drag it way higher and away,
above or anywhere outside of the image for very long rays.
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Alpha
Channel for selection
You could also use it to generate a set of rays just from one sub-area
of the image, namely the brightest areas. This could be done by using a
mask in the alpha channel to prevent the filter's effect from applying
in the remote areas or in the wrong side and direction.

You can use the Lasso selection tool (one of the 4 selection tools for
alpha masking in the Tools panel below the first row of tools). You
could for example start by selecting the general area around the cloud.
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Select
menu: Alpha
> Blur alpha...
in order to avoid a sudden transition in the mask from selected
(within) to not selected (outside of selection)
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Set the Blur factor to the max.
Apply it once, two or three times even.
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It's a god idea to store the alpha channel content (the mask as a grey
scale image). You could do this before bluring, then again after
blurring.
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Here's an example of the stored alpha buffer, before blurring. Notice
the very crisp, sudden transition from black to white. Black is outside
of the selection, i.e. non-selected. White indicates a set of pixels
that are selected.
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Here's another saved alpha buffer, after blurring the alpha.
You can see now that the white region gradually transitions through
greys before turning into the black area.
Well, it looks close to white, and black, but with enough blurring it
may actually have faded a little, and be off-white and off-black.
You may thus want to adjust brightness, contrast etc... to enhance the
dynamic range of the alpha buffer.
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You could thus adjust alpha:
menu: Alpha
> Adjust alpha...
This is somewhat equivalent to Filter>Adjust>Value...
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The alpha channel turns into a greyscale image, and shows the familiar
control sliders to adjust value, brightness, contrast and gamma.
When you click OK or Cancel, the modified greyscale disappears, and is
loaded into the new alpha channel.
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