Routines
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6AA3: Render one pixel line of terrain
Renders a single pixel line of the current terrain fragment. Called 16 times per fragment (lines 0-15). Draws left edge sprite, fills left terrain, draws right edge sprite, fills right terrain. Handles special fragments (bridges/roads) separately.
Left edge X = profile_byte + row_offset - 16. The profile_byte comes from data_terrain_profiles[profile_number][line]. The -16 adjusts for the edge sprite width. Right edge depends on state_terrain_extras mode.
render_terrain_fragment 6AA3 LD A,($5F77) Set up profile lookup: HL = data_terrain_profiles, DE = TERRAIN_PROFILE_SIZE.
6AA6 LD HL,$8063
6AA9 LD DE,$0010
6AAC OR A
6AAD SBC HL,DE
locate_terrain_profile 6AAF ADD HL,DE Locate profile: data_terrain_profiles[state_terrain_profile_number].
6AB0 DEC A
6AB1 JR NZ,locate_terrain_profile
6AB3 LD A,($5F7D) Increment state_terrain_position (line 0-15).
6AB6 INC A
6AB7 LD ($5F7D),A
6ABA CP $10 If line reached TERRAIN_PROFILE_SIZE, jump to render_terrain_row to load next fragment.
6ABC JP Z,render_terrain_row
6ABF AND $0F Index into profile: HL = profile_base + (line mod TERRAIN_PROFILE_SIZE).
6AC1 LD D,$00
6AC3 LD E,A
6AC4 ADD HL,DE
6AC5 LD BC,($5F78) Load row offset (B) from state_terrain_element_23 and profile byte (A).
6AC9 LD A,(HL)
6ACA BIT 7,A If bit 7 of profile byte set, jump to handle_special_terrain_fragment (special fragment).
6ACC JP NZ,handle_special_terrain_fragment
6ACF ADD A,B Left edge X = profile_byte + row_offset - 16. Save original X, work with adjusted X.
6AD0 PUSH AF
6AD1 LD B,$00
6AD3 SUB $10
6AD5 LD D,A Increment terrain_edge_counter. Set up sprite lookup from terrain_edge_left.
6AD6 LD HL,$693B
6AD9 INC (HL)
6ADA LD HL,$89F2 Sprite index = (X AND $06). Look up 2-byte edge sprite from terrain_edge_left.
6ADD LD A,D
6ADE AND $07
6AE0 SRL A
6AE2 LD C,A
6AE3 SLA C
6AE5 LD A,D Screen address = screen_ptr + (X >> 3). Copy 2-byte sprite via LDIR.
6AE6 ADD HL,BC
6AE7 EX DE,HL
6AE8 LD C,A
6AE9 LD HL,($5F7B)
6AEC LD B,$00
6AEE SRL C
6AF0 SRL C
6AF2 SRL C
6AF4 ADD HL,BC Fill count = X >> 3. Fill leftward with $FF (solid terrain).
6AF5 EX DE,HL
6AF6 LD BC,$0002
6AF9 LDIR
6AFB DEC DE Restore original X. Dispatch based on state_terrain_extras: 1=calc_terrain_right_mirrored, 2=calc_terrain_right_offset.
6AFC DEC DE
6AFD LD B,A
6AFE SRL B
6B00 SRL B
6B02 SRL B
6B04 LD A,$FF
fill_terrain_left_loop 6B06 DEC DE
6B07 LD (DE),A
6B08 DJNZ fill_terrain_left_loop
6B0A POP AF
6B0B LD D,A
6B0C LD BC,($5F78)
6B10 LD A,($5F7A) Right sprite index = (right_X AND $06). Look up from terrain_edge_right.
6B13 CP $01
6B15 JP Z,calc_terrain_right_mirrored
6B18 LD A,($5F7A)
6B1B CP $02
6B1D JP Z,calc_terrain_right_offset
This entry point is used by the routines at calc_terrain_right_mirrored and calc_terrain_right_offset.
draw_terrain_right_edge 6B20 LD D,A }
6B21 LD B,$00 Screen address = screen_ptr + (right_X >> 3). Copy 2-byte sprite via LDIR.
6B23 LD HL,$89FA
6B26 LD A,D
6B27 AND $06
6B29 LD C,A
6B2A LD A,D
6B2B ADD HL,BC
6B2C EX DE,HL
6B2D LD C,A
6B2E LD HL,($5F7B)
6B31 SRL C Fill count = 30 - (right_X >> 3). Fill rightward with $FF.
6B33 SRL C
6B35 SRL C
6B37 ADD HL,BC
6B38 EX DE,HL
6B39 LD BC,$0002
6B3C LDIR
6B3E LD B,A
6B3F SRL B
6B41 SRL B
6B43 SRL B
6B45 LD A,$1E If island active (state_island_line_idx != 16), call render_island_line.
6B47 SUB B
6B48 LD B,A
6B49 LD A,$FF
fill_terrain_right_loop 6B4B LD (DE),A
6B4C INC DE
6B4D DJNZ fill_terrain_right_loop
6B4F LD A,($5EFD)
6B52 CP $10
6B54 CALL NZ,render_island_line
6B57 RET
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