Lesson 25 · DXYN - wrapping and clipping

DXYN - wrapping and clipping

Real sprites land near the edges, so DXYN needs two edge rules: the starting position wraps around the screen, but the sprite itself clips rather than spilling over. Today you pin both.

The goal

Make DXYN wrap the starting coordinate modulo the screen and clip pixels that fall past an edge.

Start here - the target

TO DO

Scenario: The start wraps but the sprite clips

GivenI points at a byte 0xFF, V1 = 62 (x), V2 = 0 (y), on a blank screen, about to execute 0xD121 (draw 1 row)

WhenStep runs it

Thenonly columns 62 and 63 of row 0 turn on - the remaining six pixels are clipped at the right edge, NOT wrapped to columns 0 and 1

Andwith V1 = 68 the start wraps (68 modulo 64 = 4), so the same draw lights columns 4 through 11

Andclipping applies to the bottom edge too: a 3-row sprite (three 0xFF bytes) drawn with V1 = 0, V2 = 31 lights only row 31 columns 0 through 7 - the 2nd and 3rd rows fall past row 31 and are dropped, not wrapped to the top

Background

Two different edge behaviours meet in DXYN, and mixing them up is a classic bug. First, the starting coordinate wraps: because VX and VY are bytes but the screen is only 64 by 32, the start corner is taken modulo the screen size. A sprite told to start at x = 68 actually starts at column 4 (68 mod 64). This is what lets a program position a sprite anywhere using a byte coordinate.

Second, once drawing begins from that wrapped corner, the individual pixels clip - they do not wrap. A sprite whose start is near the right edge simply loses the pixels that would fall past column 63; they are dropped, not drawn on the opposite side. So a sprite starting at column 62 lights only columns 62 and 63, and the rest vanish. Pinning both in one lesson - one draw that clips at the edge and one whose start wraps into range - keeps the two rules from bleeding into each other, which is exactly the mistake that makes sprites smear across the screen.

Make it work

// wrap only the starting corner into range:
px := int(v.V[x]) % 64
py := int(v.V[y]) % 32
// ...then, per pixel, skip (clip) any column >= 64 or row >= 32
// instead of wrapping the individual pixel

CheckpointDONE

DXYN wraps the sprite's start position and clips pixels past the screen edge. Commit and stop here.