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diff --git a/doc/devel/planettopo b/doc/devel/planettopo new file mode 100644 index 0000000..7f27a39 --- /dev/null +++ b/doc/devel/planettopo @@ -0,0 +1,92 @@ +Planet Topography Code: sc2code/planets/gentopo.c +---------------------------------------------------- + +This text covers only topography generation code. It does NOT cover the +craters (blemishes) nor the smoothing. Variables names are the same in +"sc2code/planets/gentopo.c". + +A brief overview: + +The algorithm works by applying several elevation/lowering steps on the +same terrain. Each step works by randomly generating two non-intersecting +lines, LineDDA0 and LineDDA1. That delimits two regions. Each region is +then displaced by a certain height delta (depth_delta). That step is +repeated several times. + +There's a pseudocode below, with detailed information about it in the next +section. + +The DeltaTopography() function works as follows: + + Function parameters: + + COUNT num_iterations // Number of height displacements on the + // planet's surface + PSBYTE DepthArray // Target surface, receives topography data + PRECT pRect // Surface dimensions + SIZE depth_delta // The amount of height units to raise/lower + // on each displacement + + for i from 1 to num_iterations + randomly either negates depth_delta's or not, influencing + the displacement direction + randomly pick two line segments, LineDDA0 and LineDDA1 + increase the height of the region between LineDDA0 and + LineDDA1 by depth_delta + decrease the height of the region between LineDDA1 and + LineDDA0 by depth_delta (by wrapping around the surface) + end for + +Detailed information about the algorithm's steps: + +First, in half the steps (randomly) depth_delta is negated. +That influences on the direction of the displacements. +Note that in each step exactly two height displacements occur: +a lifting and a lowering. + +Next, it sets both w1 and w2 to different random WORD values. Those are +used in the line segment generation process. + +LineDDA0 starts on a random position on the top of the surface, and ends +at another random position on the bottom of the surface. +LineDDA0 never wraps around the surface, since their X coordinates are +generated in the range 0..width-1: + + LineDDA0.x_top = LOBYTE (w1) % width; + LineDDA0.x_bot = HIBYTE (w1) % width; + +LineDDA1 is generated much like the way LineDDA0 is, the difference being +LineDDA1's x coordinates are displaced by LineDDA0 ones: + + LineDDA1.x_top = (LOBYTE (w2) % (width - 1)) + LineDDA0.x_top + 1; + LineDDA1.x_bot = (HIBYTE (w2) % (width - 1)) + LineDDA0.x_bot + 1; + +Y coordinates are 0 for top, and 'height' for bottom in both line +segments. + +This delimits basically two regions. One delimited by LineDDA0 on the left +side, and LineDDA1 on the right right, which we call "0->1". The other is +delimited by LineDDA1 on the left, and LineDDA0 on the right, which we call +"1->0". +Note that, since we're talking about a planet, the rightmost side of the +surface if "connected" to the leftmost side. At least one of those regions +"wrap around" the surface. + +The final step in the iteration is to increase the height of the region +0->1 by depth_delta, and decreasing 1->0 by the same value. Note that since +delta_depth may be negative, raising part of the surface by delta_depth +may result in the surface being lowered. + +This step is repeated num_iteration times. + +About the terrain generation algorithm: This method of displacing +line-delimited subsets of a surface is commonly known as Fault Formation. + +The DDA acronym stands for "Digital Differential Analyzer". Hence the names +LineDDA0 and LineDDA1. + +The first version of this document was written by: +Daniel Ribeiro Maciel <daniel.maciel@gmail.com> +on 2006-11-24 + + |