3D FPS engine for the Atari 2600 ; Originally by Adam Abrams ; License: MIT ; ; ; A 3D portal rendering game engine for the ATARI 2600 (VCS)
; 3D pane generation: ; Most of the screen is taken up by a playfield drawn from a network of zone descriptions. ; Zones are connected via portals (each zone having 0-n portals) ; Zone walls are described via a series of vertices (x,y) and some meta-data forming a convex polygon ; One panel half is drawn while the other is reflected by the VCS. ; The info for display of the pane is pre-computed and put into RAM in compressed format to be extracted ; during kernel display (see below). The format is designed to allow the kernel easy extraction and leave ; enough processing time free for sprites and some logic. ; The pane is 192 lines in height, double lines are used. Reflection mode is used to keep PF0 at the extreme left and right sides. ; ; 3D pipeline consists of the following:
; 1. Backface culling - Going over all vertices in the zone ; 2. Rotation - One vertex at a time ; 3. Translation - One vertex at a time ; 4. 2D Clipping - Based on current and previous vertex (clip against FOV line) ; 5. Projection - Either special-case at FOV edges, or simple 2D world -> 2D screen projection ; ; Zones are kept in ROM unrotated (first & last coordinate always on 0,0 - last is assumed, not in ROM) ; When player is in a zone or zone is viewable via portal its coordinates are rotated, translated, clipped & projected with the results stored in RAM ; Only visible walls are moved into RAM, backface culling done via comparison with NORMAL angles stored in ROM per zone wall ; 2D clipping in world coordinates is done as well before data is stored in RAM ; Rotation is done via sine, cosine tables as well as 8 by 8 bit signed multiply relying on a table ; Each zone has a displacement+rotation based on player or player+portal(s). ; It is assumed all zones are convex and amount of vertices (visible in zone+portal zones) must be kept 21 or less ; 3D pipeline is peformed only upon request of the game code (player turned, moved, etc.) ; ROM Zone Vertex -> Backface culling -> Rotate -> 2D World Clip -> Project
; * Polyfill Kernel ; ; The kernel is split into mini kerels, each specializing in specific task. ; The list of kernels and roles is below ; Once the trapeze is drawn it is reflected by the VCS, looking like a wall with correct perspective ; No overlap/gap is allowed, 3D pipeline must order polygons perfectly for kernels in stack
; RAM structure ; ; 63b - Zone + Portal zones vertices (projection result with metadata) ; 9b - More variables (Pointers, Player info, etc.) ; 24b - Game data ; 32b - Scratch RAM, used differently in different code areas ; --- ; 128b - Total
; ROM structure (top down plan, not exact to the single byte level) ; ; Level data begin ; 128b - 8 Sprites data ; 128b - 8 Textures data ; 256b - 64 portals data ; 1024b - 256 vertices data (walls) ; 512b - Texture Y Scale tables ; Level data end ; 96b - Projection tables (inc. Fast ASLx4) ; 511b - Mutiplication tables (8b by 8b with 8b product, signed) ; 16b - Init ; 384b - 3D engine (backface culling, rotation, translation, clipping, projection) ; 512b - Game code (handle input and game logic) ; 384b - Kernels (world & game displays) ; 48b - ArcTan table ; 16b - Sine table ; 64b - Cosine table (overlap with Sine) ; 13b - Padding ; --- ; 4096b - Total ; ; *** Data Structures *** ; ; Portals List (key: ID, 6 bits) ; ; Bytes: Trans_X, Trans_Y, Meta, NextZone
; Vertices List (key: ID, 8 bits. Array ordered clockwise from 0,0 back to 0,0 - last assumed) ; ; Byte: Meta, Normal angle ; Byte: Vertex X,Y (-96..+95) - Special case is used for vertex [0,0], see below
; Polygon list for kernel draw ; ; Stack based and compressed starting at $FF, 3 bytes per entry (describes a polygon) ; M - D7: 0 Portal Entry / 1 Wall ; D0-D2 - Texture ; D3-5 - Brightness ; Y - Height of polygon on screen (0-15) ; X - Normally describes screen X coordinate -96 to +95 ; - When new portal entered it signifies the texture/light location fraction in 4.4 fixed point unsigned (0 means start of polygon)