Drawing objects: DrawEdge

       Name: DrawEdge                                                [Show more]
       Type: Subroutine
   Category: Drawing objects
    Summary: Draw an edge, overwriting whatever is already on-screen
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawObjectEdge (Part 3 of 5) calls DrawEdge


 This routine rejoins the DrawObjectEdge routine to move on to the next edge in
 the current object part.


 Arguments:

   (Q P)                The address of the dash data block to draw in

   Y                    The top track line

   blockOffset          The bottom track line

   A                    The pixel byte for the edge


.dred1

 STA (P),Y              \ Draw the pixel byte into the screen buffer by writing
                        \ it to the Y-th byte of the relevant dash data block

 DEY                    \ Decrement the track line counter in Y to move down to
                        \ the next pixel line on-screen

.DrawEdge

                        \ This is the entry point for the routine

 CPY blockOffset        \ If Y <> blockOffset then loop back to draw the next
 BNE dred1              \ byte, as we haven't reached the bottom track line

 JMP draw29             \ Jump to draw29 to fill the inside of the object part
                        \ from the previous block to the current one