Re: A82: Drawing a box

To: assembly-82@lists.ticalc.org
Subject: Re: A82: Drawing a box
From: ADAMMAN106 <ADAMMAN106@aol.com>
Date: Thu, 16 Apr 1998 22:47:15 EDT
Delivered-To: assembly-82-outgoing@towerguard.unix.edu.sollentuna.se
Reply-To: assembly-82@lists.ticalc.org


In a message dated 98-04-16 22:21:16 EDT, you write:

> Can someone help me write a simple routine that draws a "white" box on
>  the screen? The box can't just be an outline 'cuz it'll be drawn on a
>  black background. The inputs should be the upper left-hand corner and the
>  bottom right-hand corner (which ever's easier). Feel free to use any
>  registers you like. Thanks a bundle

let's say that the input is (d,e) - top left and (h,l) - bottom right.  All
you do is make a routine that calculates the distance from d to h (a simple
subtraction will do) and loop from d that many times (store it in b, and you
can use djnz).  then, for each loop, erase a line of pixels from e down to l,
effectively creating a loop nested within a loop.  Use a routine like the one
below:

ErasePixel:
  call FIND_PIXEL
  ld de, GRAPH_MEM
  add hl, de
  cpl
  and (hl)
  ld (hl), a
  ret

If you want the coordinates to start at the top left instead of the bottom
left, include this:

  ld a, 63
  sub c
  ld c, a

Note: when you call it you must take your coords and put them into b and c.
This obviously works with the graph mem, so if you want to use the LCD you
will have to use a different routine to erase the pixel.
Please tell me if I did anything wrong!

~Adamman