Planet surface rendering info

git-svn-id: svn://svn.code.sf.net/p/sc2/code/trunk@1365 8092fc87-c524-0410-9efc-e669fe64eaf9

sc2/doc/devel/planetrender

@@ -0,0 +1,42 @@
+Planet surface rendering code
+--------------------------------------------------------------
+
+The way the planets appear on the screen (based on planet type
+definition in plandata.c) is determined by several factors:
+
+1. Topography algorithm (PlanetFrame.Type)
+2. Color map (PlanetFrame.CMapInstance)
+3. Elevation -> colormap index xlat table (PlanetFrame.XLatTabInstance)
+4. Base elevation (PlanetFrame.base_elevation)
+5. Several others not covered here: faults, craters (blemishes)
+
+First, the planet topography is generated according to the algorithm
+selected for the planet: TOPO_ALGO, CRATERED_ALGO, GAS_GIANT_ALGO.
+TOPO_ALGO and CRATERED_ALGO use a fractal algorithm to pseudo-randomly
+generate a surface, then 'craters' are added and, at the end, the map is
+dithered for CRATERED_ALGO. GAS_GIANT_ALGO is of course different
+because it needs to produce a series of ring-like structures with
+'storms' added at the end.
+
+The topographical map is a rectangle with each pixel representing an
+elevation level relative to 0, so the topography is expressed in terms
+of elevation relative to base elevation. Base elevation is applied to
+the generated map.
+
+At this point, the elevation levels are translated to the actual colors
+using the xlat table and color map. First, the elevation is translated
+to an index into the colormap using the xlat table (.xlt files), and
+after that, the actual RGB color is drawn from the colormap (.ct files)
+at this index.
+
+There are 3 colormaps per instance specified in PlanetFrame. The active
+colormap is selected from these 3 based on the surface temperature of
+the planet: cold, normal and hot. The framework is also laid down for
+3 xlat tables (one per surface temperature), but, AFAIK, distinct xlat
+tables are never actually used. Currently, there is only 1 xlat table
+per instance in each file.
+
+The colormap files (.ct) seem to be in original DOS format. Probably
+because they do not translate to the CLUT system very well. The colormaps
+are carefully crafted and synchronized across all planet types to make
+the entire system function properly.

sc2/doc/devel/strtab

@@ -38,6 +38,26 @@ The full palette contains 256 values, which are the original 32 values
     multiplied by 1 to 8.
 
 
+For .ct files (planet surface version; in ipanims) an item is as follows:
+           1       Index of first color in this colortab.
+           1       Index of last color in this colortab.
+  Then for each color (6 bits per channel):
+           1       bits 0-5: red value
+           1       bits 0-5: green value
+           1       bits 0-5: blue value
+The planet surface palette files were probably copied verbatim from DOS version
+and left alone because they do no exactly fit into the whole CLUT system.
+
+For .xlt files (planet surface; in ipanims) an item is as follows:
+           2*3     3 elevation levels (not used, probably informational)
+           1*256   256 colormap indices each corresponding to a particular
+                   elevation level 0..255; only colormap indices 128..255
+                   can be used currently because colormaps only define
+                   colors 128..255
+The .xlt files combined with planet surface .ct files are used to select colors
+for planet surface rendering.
+
+
 For .snd files an item is as follows:
            N-2     Signed, 8-bit, 1 channel PCM samples of the sound
                    effect, where N = (length of string)