Nigel Dore's machine-code

             program transforms the otherwise

               laborious routine of filling

             in irregular shapes into a quick

                  and simple operation.



                       Paint Routine



Filling in irregular shapes on the Spectrum can be a long

and laborious process, which can be avoided by using this

fill routine. The program, which is entirely in machine

code, will fill any enclosed area simply given one point

inside of it. The machine code is approximately 500 bytes

long but requires about 6K for storage of a second screen.

This use of memory is no problem on a 48K Spectrum but

leaves the 16K with about 2.

  The fill routine is entered into memory by using the

program in figure 1. [This program is on Fill.tzx under the

name of "codereader".] The length of the machine code means

that mistakes are easily made and for that reason the pro-

gram is extensively check-summed. So much so that the com-

puter will tell you in which lines you have made mistakes.

  Having entered the program, save it before you run it,

then run it. If there are no detected mistakes the Basic

program will now self-destruct. [The version on the TZX

will not.] If mistakes are detected then correct them and

rerun the program, repeating if necessary. Once the machine

code has been entered it can be Saved with the command:

    SAVE "fill"CODE 32000,500

and loaded with the command

    CLEAR 25800: LOAD "fill"CODE

[This first version is called "fill 16" on the TZX, for a

reason which will become clear later, if it isn't already.]

Now we can check the program, first we must reserve space

for the second screen, thus type:

    CLEAR 25800

Now we must draw an enclosed shape, for instance:

    CIRCLE 100,100,50

Next we give the computer one point inside our shape with

the plot command, thus enter:

    PLOT 100,100

Then we instruct the computer to fill the area in with a

USR call:

    LET a=USR 32000

The circle should now be filled and as an added bonus A now

contains the number of points plotted which is proportional

to the area of the shape.

  It should be noted that the program will fill any enclo-

sed shape and not just circles. Great care must be taken

that a gap is not left in the shape because a gap of just

one pixel will allow the fill to spread to the whole

screen. The normal ink, bright and flash commands are

recognised only if there is no return to command mode

between them and the USR call.

  The program you now have in memory will work on either a

16K or 48K Spectrum. If you have a 48K Spectrum you will

want to move it to a higher address. To do this, enter and

run the relocator program in figure 2 [on the TZX as, no

surpise, "relocator"]. The clear address now becomes 58600

and the USR address 64800. The code can be saved with:

    SAVE "fill"CODE 64800,500

[And this is on the TZX under the name "fill 48".]

  The algorithm used by the program is fairly simple to

understand. Imagine two pieces of paper, the first repre-

sents the screen with the shape to be filled drawn on it.

The second represents a corresponding blank screen stored

in the computer's memory. The one point given to the com-

puter is plotted onto the second sheet of paper.

  The computer then starts at the top left of the second

sheet and scans from left to right and down looking for a

point. If it finds one, it then in turn checks each of the

four points surrounding that point and plots each of them

on both sheets of paper only if the point is not already

set on the first sheet.

  Once one scan is complete it is repeated but in the

reverse direction and this continues until no more points

can be plotted. Thus the overall effect is that the spread-

ing of points is halted by a line on the first sheet.

  Figure 3 is a set of routines for drawing various geo-

metrical shapes which can then be filled. The routine which

draws an ellipse is particularly interesting, the ellipse

is in fact made up of 30 straight lines. This makes the

program much faster than the more usual method of calcu-

lating each point individually from sines and cosines.



[This article was illustrated with some samples of filled

shapes: one ring, one each of the shapes from Figure 3,

and one very complicated shape filled twice, once from a

point inside the shape and once from a point outside. There

are two demonstration programs on fill.tzx, one for the 16K

Spectrum and one for the 48K. "Demo 16" draws the ring and

the Figure 3 shapes, and fills in each of them. "Demo 48"

also has memory enough for the complicated shape, filled

and counter-filled. In both of these, lines 9000 and up are

the listing from Figure 9.]