CHESS ZOOMROTATOR

; ========================================== ; Chess Zoomrotator - inner loop ; Atari 7800 / MADS assembler ; 160B bitmap mode, 16-bit fixed point ; ========================================== ; Zero page variables (16-bit) fx = $40 ; 2 bytes - texture X fy = $42 ; 2 bytes - texture Y fx_ = $44 ; 2 bytes - row start X fy_ = $46 ; 2 bytes - row start Y zx = $48 ; 2 bytes - sin_table val zy = $4A ; 2 bytes - cos_table val tmp = $4C ; work byte pixbuf = $4D ; 4-pixel accumulator col_x = $4E ; pixel counter row_y = $4F ; scanline counter ; ---- Load zx, zy from LUT ---- ldx angle lda sin_table_lo,x sta zx lda sin_table_hi,x sta zx+1 lda cos_table_lo,x sta zy lda cos_table_hi,x sta zy+1 ; ---- Centering: fx_ = $10000-(W/2)*(zx+zy) ; fy_ = $10000-(H/2)*(zx-zy) ; (precalc or unrolled multiply) ---- jsr calc_center ; ---- Outer loop: scanlines ---- lda #0 sta row_y @yloop ; fx = fx_; fy = fy_ lda fx_ sta fx lda fx_+1 sta fx+1 lda fy_ sta fy lda fy_+1 sta fy+1 lda #0 sta col_x ; ---- Inner loop: pixels ---- @xloop ; THE KEY: checker = (fx XOR fy) hi byte lda fx+1 ; high byte of fx eor fy+1 ; XOR with high byte fy and #$10 ; test checker bit beq @color_a ; Color B: pixel value %10 in 160B @color_b lda pixbuf ora mask_b,x ; OR in %10 at position sta pixbuf jmp @advance ; Color A: pixel value %01 in 160B @color_a lda pixbuf ora mask_a,x ; OR in %01 at position sta pixbuf ; ---- Advance fx += zx ---- @advance clc lda fx adc zx sta fx lda fx+1 adc zx+1 sta fx+1 ; ---- Advance fy -= zy ---- sec lda fy sbc zy sta fy lda fy+1 sbc zy+1 sta fy+1 ; ---- Every 4 pixels: write byte ---- inc col_x lda col_x and #$03 bne @xloop lda pixbuf ; write 4 packed pixels sta (fb_ptr),y ; to framebuffer iny lda #0 sta pixbuf ; reset accumulator lda col_x cmp #WIDTH bne @xloop ; ---- Row done: fx_ += zy, fy_ += zx ---- clc lda fx_ adc zy sta fx_ lda fx_+1 adc zy+1 sta fx_+1 clc lda fy_ adc zx sta fy_ lda fy_+1 adc zx+1 sta fy_+1 inc row_y lda row_y cmp #HEIGHT bne @yloop rts ; ---- Pixel bit masks for 160B packing ---- ; 4 pixels per byte: bits 76,54,32,10 ; color A = %01, color B = %10 mask_a .byte %01000000,%00010000,%00000100,%00000001 mask_b .byte %10000000,%00100000,%00001000,%00000010

; Mad Team pseudocode (from madteam.atari8.info) zx = sin_table[angle] zy = cos_table[angle] ; Center on screen (symmetry point) fx_ = 65536 - ((W/2) * (zx+zy)) fy_ = 65536 - ((H/2) * (zx-zy)) for j = 0 to height-1 fx = fx_ fy = fy_ for i = 0 to width-1 if (fx xor fy) >> 8 and $10 = 0 Pixels[i,j] = $ff else Pixels[i,j] = $00 fx = fx + zx fy = fy - zy next i fx_ = fx_ + zy fy_ = fy_ + zx next j

; ========================================== ; LUT Generation (run once at init) ; sin_table / cos_table: rotation + zoom ; 256 entries, 16-bit signed each ; ========================================== ; Generated offline or at startup: angle = (6.284/128)*96 dzo = 0.05 zo = dzo*21 ml = 256 ; fixed-point scale for a = 0 to 255 zo = zo - dzo if (a mod 64) = 0 dzo = -dzo ; bounce zoom sin_table[a] = round((-zo*sin(angle))*ml) cos_table[a] = round((-zo*cos(angle))*ml) angle = angle + 6.284/128 next a ; ---- In ROM as split hi/lo tables: ---- sin_table_lo .byte <val0, <val1, ... <val255 sin_table_hi .byte >val0, >val1, ... >val255 cos_table_lo .byte <val0, <val1, ... <val255 cos_table_hi .byte >val0, >val1, ... >val255 ; 4 × 256 = 1024 bytes of ROM ; Only need 64 unique entries ; (90° then invert palettes)