Drawing pixels: PIXEL

       Name: PIXEL                                                   [Show more]
       Type: Subroutine
   Category: Drawing pixels
    Summary: Draw a one-pixel dot, two-pixel dash or four-pixel square
  Deep dive: Drawing pixels in the Commodore 64 version
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DOEXP calls PIXEL
             * PTCLS2 calls PIXEL
             * TT22 calls PIXEL
             * PIXEL2 calls via PX4


 Draw a point at screen coordinate (X, A) with the point size determined by the
 distance in ZZ. This applies to the top part of the screen.


 Arguments:

   X                    The screen x-coordinate of the point to draw

   A                    The screen y-coordinate of the point to draw

   ZZ                   The distance of the point, with bigger distances drawing
                        smaller points:

                          * ZZ < 80           Double-height four-pixel square

                          * 80 <= ZZ <= 143   Single-height two-pixel dash

                          * ZZ > 143          Single-height one-pixel dot


 Returns:

   Y                    Y is preserved


 Other entry points:

   PX4                  Contains an RTS


.PIXEL

 STY T1                 ; Store Y in T1 so we can restore it at the end of the
                        ; subroutine

 TAY                    ; Copy the screen y-coordinate from A into Y

 TXA                    ; Each character block contains 8 pixel rows, so to get
 AND #%11111000         ; the address of the first byte in the character block
                        ; that we need to draw into, as an offset from the start
                        ; of the row, we clear bits 0-2

 CLC                    ; The ylookup table lets us look up the 16-bit address
 ADC ylookupl,Y         ; of the start of a character row containing a specific
 STA SC                 ; pixel, so this fetches the address for the start of
 LDA ylookuph,Y         ; the character row containing the y-coordinate in Y,
 ADC #0                 ; and adds it to the row offset we just calculated in A
 STA SC+1

                        ; So SC(1 0) now contains the address of the first pixel
                        ; in the character block containing the (x, y)

 TYA                    ; Set Y = Y mod 8, which is the pixel row within the
 AND #7                 ; character block at which we want to draw our pixel
 TAY                    ; (as each character block has 8 rows)

 TXA                    ; Set X = X mod 8, which is the horizontal pixel number
 AND #7                 ; within the character block where the pixel lies (as
 TAX                    ; each pixel line in the character block is 8 pixels
                        ; wide)

 LDA ZZ                 ; If distance in ZZ >= 144, then this point is a very
 CMP #144               ; long way away, so jump to PX3 to fetch a one-pixel
 BCS PX3                ; point from TWOS and EOR it into SC+Y

 LDA TWOS2,X            ; Otherwise fetch a two-pixel dash from TWOS2 and EOR it
 EOR (SC),Y             ; into SC+Y
 STA (SC),Y

 LDA ZZ                 ; If distance in ZZ >= 80, then this point is a medium
 CMP #80                ; distance away, so jump to PX13 to stop drawing, as a
 BCS PX13               ; two-pixel dash is enough

                        ; Otherwise we keep going to draw another 2 pixel point
                        ; either above or below the one we just drew, to make a
                        ; four-pixel square

 DEY                    ; Reduce Y by 1 to point to the pixel row above the one
 BPL PX3                ; we just plotted, and if it is still positive, jump to
                        ; PX3 to draw our second two-pixel dash

 LDY #1                 ; Reducing Y by 1 made it negative, which means Y was
                        ; 0 before we did the DEY above, so set Y to 1 to point
                        ; to the pixel row after the one we just plotted

.PX3

 LDA TWOS2,X            ; Fetch a two-pixel dash from TWOS2 and EOR it into this
 EOR (SC),Y             ; second row to make a four-pixel square
 STA (SC),Y

.PX13

 LDY T1                 ; Restore Y from T1, so Y is preserved by the routine

.PX4

 RTS                    ; Return from the subroutine