meep-eep
bd3758d9ff
git-svn-id: svn://svn.code.sf.net/p/sc2/code/trunk@2533 8092fc87-c524-0410-9efc-e669fe64eaf9
93 lines
3.6 KiB
Plaintext
93 lines
3.6 KiB
Plaintext
Planet Topography Code: sc2code/planets/gentopo.c
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----------------------------------------------------
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This text covers only topography generation code. It does NOT cover the
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craters (blemishes) nor the smoothing. Variables names are the same in
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"sc2code/planets/gentopo.c".
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A brief overview:
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The algorithm works by applying several elevation/lowering steps on the
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same terrain. Each step works by randomly generating two non-intersecting
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lines, LineDDA0 and LineDDA1. That delimits two regions. Each region is
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then displaced by a certain height delta (depth_delta). That step is
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repeated several times.
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There's a pseudocode below, with detailed information about it in the next
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section.
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The DeltaTopography() function works as follows:
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Function parameters:
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COUNT num_iterations // Number of height displacements on the
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// planet's surface
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PSBYTE DepthArray // Target surface, receives topography data
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PRECT pRect // Surface dimensions
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SIZE depth_delta // The amount of height units to raise/lower
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// on each displacement
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for i from 1 to num_iterations
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randomly either negates depth_delta's or not, influencing
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the displacement direction
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randomly pick two line segments, LineDDA0 and LineDDA1
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increase the height of the region between LineDDA0 and
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LineDDA1 by depth_delta
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decrease the height of the region between LineDDA1 and
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LineDDA0 by depth_delta (by wrapping around the surface)
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end for
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Detailed information about the algorithm's steps:
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First, in half the steps (randomly) depth_delta is negated.
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That influences on the direction of the displacements.
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Note that in each step exactly two height displacements occur:
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a lifting and a lowering.
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Next, it sets both w1 and w2 to different random WORD values. Those are
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used in the line segment generation process.
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LineDDA0 starts on a random position on the top of the surface, and ends
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at another random position on the bottom of the surface.
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LineDDA0 never wraps around the surface, since their X coordinates are
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generated in the range 0..width-1:
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LineDDA0.x_top = LOBYTE (w1) % width;
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LineDDA0.x_bot = HIBYTE (w1) % width;
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LineDDA1 is generated much like the way LineDDA0 is, the difference being
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LineDDA1's x coordinates are displaced by LineDDA0 ones:
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LineDDA1.x_top = (LOBYTE (w2) % (width - 1)) + LineDDA0.x_top + 1;
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LineDDA1.x_bot = (HIBYTE (w2) % (width - 1)) + LineDDA0.x_bot + 1;
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Y coordinates are 0 for top, and 'height' for bottom in both line
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segments.
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This delimits basically two regions. One delimited by LineDDA0 on the left
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side, and LineDDA1 on the right right, which we call "0->1". The other is
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delimited by LineDDA1 on the left, and LineDDA0 on the right, which we call
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"1->0".
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Note that, since we're talking about a planet, the rightmost side of the
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surface if "connected" to the leftmost side. At least one of those regions
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"wrap around" the surface.
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The final step in the iteration is to increase the height of the region
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0->1 by depth_delta, and decreasing 1->0 by the same value. Note that since
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delta_depth may be negative, raising part of the surface by delta_depth
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may result in the surface being lowered.
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This step is repeated num_iteration times.
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About the terrain generation algorithm: This method of displacing
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line-delimited subsets of a surface is commonly known as Fault Formation.
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The DDA acronym stands for "Digital Differential Analyzer". Hence the names
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LineDDA0 and LineDDA1.
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The first version of this document was written by:
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Daniel Ribeiro Maciel <daniel.maciel@gmail.com>
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on 2006-11-24
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