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If you changed the blend to pure altitude, the snow line would be
governed entirely by the curve editor, and our current curve (discussed
in just a minute) would give a very definite flat boundary right around
45,000 meters.
Selecting the "Altitude and Slope" shows a slope weight of 15,000.
This is used to "adjust" the color boundary for more level terrain.
For a slope-altitude combo, the slope value (between 0, for a vertical,
and 1, for a horizontal) is multiplied by the weight and added to the
actual terrain altitude to get an "effective" altitude. This tends
to drop the snow line when the terrain is more level. A flat plateau
15,000 meters below the "official" snow line would be white.
As a side note, slope values are not linear; they are computed as
the cosine of the slope angle, which gives 1 for 0 degrees and 0 for
90 degrees.
The real boundary determination is done in the Output Control curve
editor. This gives a sharp change toward the right-hand side of the
graph.
To figure this out:
- The horizontal (X) axis is the input, which is modified terrain height.
The range of input values or altitudes covered by this graph is 0 to
50,000 as given by the curve Min and Max Input fields.
- Click on the low point at the bottom of the sharp rise. The input
value at this point, displayed at the bottom of the curve editor, is
just about 44,333. This is where color change will begin.
- Click on the high point at the top of the rise. The input value here
is 45,833. After this, the color will be completely switched.
- Values in between these two altitudes will be a mixture of green and
white. Add a point in the middle of the slope, half way between the
top and the bottom, which would indicate a 50-50 mixture of green and
white colors. The X value (altitude) for this point should be pretty
close to 45,000.
In fact, when the Y axis is at 0.5 (the output value), the input is
45,333.
Now, which color is which? The "first" color, which is the material
above the blend, is the green, and is associated with the vertical
(Y) axis at the bottom. The second color, below the blend node, is
the white snow color, and is assigned to the higher end of the Y axis.
This means a modified altitude of 44,333 is about where the green will
begin to change to the white. Since the curve is extremely sharp and
rises almost vertically, the change will be quite abrupt. At 45,833
the curve maxes out, and height values larger than this will be
colored snow white.
The variation in snowline is provided by the slope of the mountain
terrain. Flatter slopes below this line will have snow. In fact,
a plateau at 30,000 meters would be adjusted by the slope weight to
something closer to 45,000 meters. This is going to be partially
snow-covered according to the blend curve.
Part One sections:
Part Two sections:
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