Tempest Level Data Structure
----------------------------

This document describes the data structures used by Atari's Tempest to store it's levels.

The information required is contained in the ROM file '136002.118'. TempEd uses the following C structure:

typedef unsigned char uint8;     // Byte type
typedef signed char sint8;

typedef struct
{
	uint8 x[16][16];
	uint8 y[16][16];
	uint8 angle[16][16];

	uint8 remap[16];
	uint8 scale[16];
	uint8 y3d[16];
	uint8 y2d[16];
	sint8 y2db[16];
	uint8 open[16];
	uint8 fscale[16];
	uint8 fscale2[16];
} level_data;

The data is stored at an offset of 380 into the 2K ROM file, i.e. :

...
uint8 data[2048];
level_data *LevelData;
...

	// Read '136002.118' into 'data'
	...

	LevelData = (level_data*)&data[380];
	...



Details
-------

x	: the x-coordinates of the points in the level. x[L][P] is the x-coordinate of point P of level L.
y	: the y-coordinates of the points in the level. y[L][P] is the y-coordinate of point P of level L.

x & y are in the range 0-255.
Note that the arcade game applies a y-scale of 125% to everything drawn.


angle	: the angles of the sectors in the level. angle[L][S] is the angle of sector S of level L.

angle is in the range 0-15 (representing 0-360 degrees).
It proved difficult to calculate the angle in a way that exactly matched the values in Tempest's original levels. Presumably, these values were calculated by hand and possibly tweaked a little. If these values are too far out, problems arise such as flippers rotating constantly, unable to 'connect' with the next sector.


remap	: the order of the levels as encountered in the game.

remap is in the range 0-15 (representing levels 1-16).
Level L is actually stored as level remap[L]. You will note that the original levels are not stored in the order in which they are played.


scale	: the scale of the levels, or more accurately one-over the scale since small values give big levels.

scale is in the range 1-255, although 10-28 is the range of the original levels.


y3d	: the 3D y-offset, or 'camera height' for the levels.

y3d is in the range 0-255.


y2d	: 'Low' byte of 2D y-offset.
y2db	: 'High' byte of 2D y-offset. This is a SIGNED value.

y2d is in the range 0-255. y2db is (usually) in the range -2 to +2.
The 2D y-offset is stored as a 16-bit value, i.e. :
yoffset = LevelData->y2db[L]*256 + LevelData->y2d[L];


open	: open / closed status of the levels.

open is either 255 or 0.
A closed level has 16 sectors and is looped. An open level has 15 sectors and does not loop. If open[L] is 255, then level L is open.


fscale	: 'Low' byte of flipper scale.
fscale2	: 'High' byte of flipper scale.

The 'flipper scale' is the value used to scale the flippers when they are in the act of flipping. When flippers are lying fully in a sector, they take their shape from that sector's co-ordinates. Like the level scale, small values mean big flippers.
This was the most troublesome piece of data to work out. Although it seems to be a 16-bit value, it is not stored in the usual way. Instead, fscale only represents the first 7 bits, and fscale2 the rest. So the full value is recovered as:
fs = LevelData->fscale2[L]*128 + LevelData->fscale[L];
TempEd calculates this value based on the average sector width of the level (actually, one-over the average width).

-------------------------

12/12/99
Simon Mills
http://www.thundercross.freeserve.co.uk