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When you look at the
image below, you'll notice that we didn't apply the light diffusion
filter yet. The window in the house is crisp and bright. However,
you might expect some of the snowflakes and immediate surrounding air
near that lit window to also be brighter.
We'lll explore a few tricks and ideas to make it so.
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Using
the lasso selection tool in the Alpha tools group, like we did in
earlier parts of this tutorial, you can 'draw' a rough quick selection
of the zone that should be affected by the light from the window.
Imagine a cone of light starting by the window and spreading at wide
angle to the left.
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That selection would
be too crisp and precise for a gradual brightening of the air and
snowflakes inside that cone. We need it more subtly changing from
outside and inside the cone.
Use the alpha blur tool:
menu: Alpha >
Blur alpha...
Make it a very high blur value, why not all the way to the max, and
perhaps even apply it a few times for further bluring.
Remember that you can use the ALpha>Store-alpha option to take a
quick peek at the result in alpha space as a greyscale image.
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Adjusting
the alpha for less contrast
The goal of this cone shaped alpha mask is not to totally block the
rendering of snowflakes which are outside of the cone. We'll
still expect the flakes to be visible, just at a diminished intensity
or brightness.
You should thus adjust the alpha, by brightening it to the point that
the area outside of the cone is not totally 'off' but still has perhaps
a 50% selection value.
This will help ensure that the snow flakes brighten up inside the cone,
and darken outside of it but still remain seen.
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Use an increased value for the Brightness slider, and a decreased value
for the Value slider to prevent it from going too bright. Not that it
matters too much.Just a thought. You can also use the Contrast value
slider.
The ideal is that the area inside the light cone is near the maximum
bright white value (i.e. fully selected or enabled in
alpha-lingo), and the area outside would be at mid grey, for partial
selection of those pixels. This mask becomes weighted mask of sorts.
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Render another pass of the snoflakes filter in the Timeline now. Don't
use too many for the snowfall count parameter. Use average to large
size (these are relatively close-up flakes), and bright intensity.
Even in the small preview thumbnail you should notice that more bright
snow is appearing 'in front of the window'.
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Adding a
Lightcone - the Linear Alpha Fader
Now we'll add some
brightening, in another cone coming from the window again, but this
time the brightening should not just apply to the flakes passing in
front. This brightening will apply to the mass of air in front of the
window, to simulate volumetric air moisture or fog scattering the
light. The farther away from the window you look, the dimmer the
scattered light will be, until it fades to normal darkness near the
left edge of the image or wherever you deem appropriate.
Use the lasso selection tool again like before. This time perhaps a
more elongated light cone?
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Select the Linear Alpha fader. It is one of the tools in the Linear
tools collection: right-click on the linear tool  (or line tool in version 3 of PD Pro and earlier), so you
can see the popup menu which includes linear gradient, circular
gradient and other tools like Nova, lens flare, lightning,... which all
have some form of linear or curvi-linear element in their application.
< Select the Linear alpha fader
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The linear alpha fader applies a decreasing gradient of alpha values in
the direction that you draw the line. You start with a click-and-drag
inside the cone for example, such as near the upper edge of it, where
it's still fully selected and should remain so. You keep moving to the
top and outside of the cone selection and release the mouse button aobe
that.
 The
short (or not-so-short) lrubberband line you've drawn defines the
transitional phase of the fader: That's where it makes the alpha values
fade from fully enabled (white) to fully blocked (black)
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Do another from the mid-bttom downwards. Note that you can draw them at
a slight angle too, in order to 'open up' the cone on the left side.
and tighten the alpha grip to the right side.
And you can do this more than once to gradually chip away at the
original, crisp cone.
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The final and perhaps most important use and application of the linear
alpha fader will be the one controlling the fading of the lightcone as
a result of the distance from the window. Start at or near the window,
perhaps even slightly on the right side of it. Drag the linear alpha
rubberband to the far left and left go.
The linear alpha fader is a multiplicative operator: it multiplies the
fading alpha values from 255 to 0 (or 1.0 to 0.0 in a normalized
sense), i.e. white to black multiplied to the current alpha values. It
is thus cumulative if you apply it several times.
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After just one application across the length of the cone it may look
like this in alpha: half of it may appear gone. But don't be alarmed or
misled: the marching ants only show the location of the 50% threshold.
Thus the seemingly sudden dropoff at the leftside vertical line is not
actually an abrupt border of alpha selection.
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You can in fact use 'Store alpha' to verify the result. It should look
like a code that's fading to the left since we draw the linear alpha
fader right-to-left.
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With that gradual, fading alpha mask in place, we can now use it in
several ways. If you're working on just one image, use the erase tool
with the right button and select Clear selected to primary or
secondary, whichever holds the color you want to erase into the mask of
the fading cone.
You can also 'fade last action' (Filter menu) if the erase effect is
too intense just by the window.
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If the color used for clearing was a light yellowish tint, you'd get
something like this now.
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Combine
with Swap across all frames
For the animation, let's instead use the swap buffer to carry the color
(or multiple colors) that we'll combine over or into the images from
the animation's frames in the main buffer.
Temporarily jump to the swap buffer - shortcut 'j' or
menu: Image > Swap buffers
And clear the whole buffer (not just the selection, although that might
work too if starting from white)
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Then use the filter we've used before when we applied the bright color
to the windows in each frame:
Animation>
Timeline > Combine with swap > Combine add
this filter will combine the colors of the pixels from the swap buffer
and the main buffer, frame by frame. The color from the swap buffer
will be added to that of the main buffer. The effect is modulated by
the alpha mask, so the resulting effect fades away to the left.
Plus, if there are bright snow flakes whisking bye they will brighten
up even more while in the fading lightcone.
This looks good but perhaps a bit too intense.
You might want to undo, and change the alpha mask to a slightly dimmed
value. As before, use either linear alpha fader once mode to reduce the
brightness of the selection mask from right to left once mode. Or, use
the
menu: Alpha > Adjust alpha...
and bring the brightness and/or value down to 50% or so.
Below is an image resulting from this new application with a dimmed
alpha mask. The 'volumetric' light cone is more subtle.
I've also added light diffusion, and prior to that also added lights to
some of the other windows.
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click for larger image
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