C64 BASIC: Game Map Overhead Camera View
Original: C64 Basic: Game Map Overhead “Camera View”
A C64 BASIC tutorial for building an Ultima-style overhead map viewport.
The article explains how to separate world coordinates from a visible viewport in a C64 BASIC game. It starts with a naive 11-by-11 camera slice rendered from a larger map, keeping the player centered where possible. It then improves performance with lookup tables, a flattened map array, loop unrolling, and notes on further retro-system optimizations.
This article is a retro game-development tutorial on how to create an Ultima-like top-down map "camera view" in Commodore 64 BASIC. The core concept is to separate the "complete world map" from the "visible viewport" on the screen: the player has their own world coordinates PX, PY, while the map is stored in a larger array; on each move, the program extracts only a small block centered near the player from the complete map—for example, 11×11 cells—and draws it to a fixed region of the screen. The article first demonstrates an intuitive but very slow version: computing the camera's top-left corner CX, CY, clamping at the map edges to avoid out-of-bounds reads, then using nested loops to POKE each cell into the C64's screen memory, and finally drawing the player at the center or at an edge-corrected position. Next, the author moves into performance optimization, because in C64 BASIC v2, floating-point arithmetic, multiplication, two-dimensional array access, and FOR/NEXT are all very expensive. The first step is to pre-build a screen-row address lookup table to reduce multiplication during each per-cell redraw; the second step is to change the two-dimensional map into a one-dimensional array and then build a map-row offset table, so that the viewport copying process becomes mainly addition; the third step adds an initialization progress indicator to prevent the slow startup from looking like a crash; the fourth step unrolls the fixed 11-row outer loop to lower the cost of the BASIC loop itself. At the end, the article also proposes directions for future improvement, including using PRINT instead of large numbers of POKEs, calling assembly routines from BASIC, using meta tiles to compress the map representation, partially redrawing only newly appearing rows or columns, hardware smooth scrolling, custom character sets, and color-memory updates. The overall focus is not on AI, but on game-map viewports on a retro platform, coordinate-system separation, and low-level performance trade-offs.
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