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Elite on the BBC Micro

Drawing lines: LOIN (Part 7 of 7) (Disc version)

Name: LOIN (Part 7 of 7) [View in context] Type: Subroutine [Compare versions] Category: Drawing lines Summary: Draw a steep line going up and right or down and left Deep dive: Bresenham's line algorithm
This routine draws a line from (X1, Y1) to (X2, Y2). It has multiple stages. If we get here, then: * The line is going up and right (no swap) or down and left (swap) * X1 >= X2 and Y1 >= Y2 * Draw from (X1, Y1) at bottom left to (X2, Y2) at top right
.LFT LDA SWAP \ If SWAP = 0 then we didn't swap the coordinates above, BEQ LI18 \ jump down to LI18 to skip plotting the first pixel DEX \ Decrement the counter in X because we're about to plot \ the first pixel .LIL6 LDA R \ Fetch the pixel byte from R EOR (SC),Y \ Store R into screen memory at SC(1 0), using EOR STA (SC),Y \ logic so it merges with whatever is already on-screen .LI18 DEY \ Decrement Y to step up along the y-axis BPL LI19 \ If Y is positive we are still within the same \ character block, so skip to LI19 DEC SCH \ Otherwise we need to move up into the character block LDY #7 \ above, so decrement the high byte of the screen \ address and set the pixel line to the last line in \ that character block .LI19 LDA S \ Set S = S + P to update the slope error ADC P STA S BCC LIC6 \ If the addition didn't overflow, jump to LIC6 ASL R \ Otherwise we just overflowed, so shift the single \ pixel in R to the left, so the next pixel we plot \ will be at the previous x-coordinate BCC LIC6 \ If the pixel didn't fall out of the left end of R \ into the C flag, then jump to LIC6 ROL R \ Otherwise we need to move over to the next character \ block, so first rotate R left so the set C flag goes \ back into the right end, giving %0000001 LDA SC \ Subtract 7 from SC, so SC(1 0) now points to the SBC #7 \ previous character along to the left STA SC CLC \ Clear the C flag so it doesn't affect the additions \ below .LIC6 DEX \ Decrement the counter in X BNE LIL6 \ If we haven't yet reached the left end of the line, \ loop back to LIL6 to plot the next pixel along LDY YSAV \ Restore Y from YSAV, so that it's preserved .HL6 RTS \ Return from the subroutine