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The ATOM provides nine different graphics modes, up to a resolution of 256x192 in black and white, and 128x192 in four selectable colours. The graphics modes use the BASIC statements PLOT, DRAW, and MOVE in an identical way. All the black-and-white graphics commands are present in the unexpanded ATOM, although extra memory will be required for the higher-resolution graphics modes. Colour plotting requires the addition of an assembler routine, or the COLOUR statement provided in the extension ROM.
The ATOM provides nine different graphics modes, up to a resolution of 256x192 in black and white, and 128x192 in four selectable colours. The graphics modes use the BASIC statements PLOT, DRAW, and MOVE in an identical way. All the black-and-white graphics commands are present in the unexpanded ATOM, although extra memory will be required for the higher-resolution graphics modes. Colour plotting requires the addition of an assembler routine, or the COLOUR statement provided in the extension ROM.
The nine graphics modes are listed below:
Mode: Resolution: Memory:
X: Y:
0 64 48 0.5 K
1a 64 64 1 K
1 128 64 1 K
2a 128 64 2 K
2 128 96 1.5 K
3a 128 96 3 K
3 128 192 3 K
4a 128 192 6 K
4 256 192 6 K
This statement clears the screen and puts it into graphics mode. It is followed by a number, or expression in brackets, to specify the mode. The graphics screen is labelled as follows:
| ^ | Y | |
|
| 0,0 | -- X --> |
The smallest square which can be plotted on the display is referred to as a 'pixel' (or 'picture element').
The graphics statements include a versatile 'PLOT K,X,Y' statement, the value of K determining whether to draw or move, plot lines or points, whether to set, clear, or invert, and whether to take the parameters X and Y as the absolute screen position, or as a displacement from the last point. The values K, X, and Y can be arbitrarily-complicated expressions.
K: Function:
0 Move relative to last position
1 Draw line in white relative to last position
2 Invert line relative to last position
3 Draw line in black relative to last position
4 Move to absolute position
5 Draw line in white to absolute position
6 Invert line to absolute position
7 Draw line in black to absolute position
8 Move relative to last position
9 Plot point in white relative to last position
10 Invert point relative to last position
11 Plot point in black relative to last position
12 Move to absolute position
13 Plot point in white at absolute position
14 Invert point at absolute position
15 Plot point in black at absolute position
In addition DRAW and MOVE statements are provided as convenient aliases for drawing a line and moving to an absolute X,Y position. MOVE X,Y is equivalent to PLOT 12, X, Y. DRAW X,Y is equivalent to PLOT 5, X, Y.
The following program illustrates the use of relative plotting using the PLOT statement, and draws random rectangles on the display. The program will work in any of the graphics modes.
10 REM Random Rectangles 13 S=20 16 Z=1;B=0 17 W=64;H=48 18 E=W-S;F=H-S 20 CLEARB 30 FORQ=0TO7 32 MOVE(ABSRND%E),(ABSRND%F) 35 C=ABSRND%S+1;D=ABSRND%S+1;GOSUBs 37 NEXTQ;FOR Q=0TO20000;NEXTQ 38 GOTO20 100sPLOTZ,C,0 110 PLOTZ,0,D 120 PLOTZ,-C,0 130 PLOTZ,0,-D 140 RETURN
Description of Program:
13-18 Set up constants
20 Initialise graphics
30 Draw 41 rectangles
32 Move to random point, leaving margin for size of largest
rectangle.
35 Choose random rectangle
37 Wait; then repeat.
100 140 s: Draw rectangle.
Variables: C,D - Dimensions of rectangle E,F - Dimensions of safe part of screen to start drawing rectangle. H - Screen height Q - Counter S - Size of squares W - Screen width Z - Plot mode; draw relative. Program size: 278 bytes
The following examples are designed for use with the higher-resolution graphics modes, and illustrate some of the applications that are possible using the ATOM's graphics facilities.
This curve is of interest to mathematicians because it has the property that it encloses every interior point of a square, and yet it is a closed curve whose area is less than half that of the square. This program draws successive generations to illustrate how the Sierpinski curve, which is the limit of these polygonal drawings, is constructed.
1 REM Sierpinski Curve 10 INPUT"MODE"0 15 INPUT"SIZE"K 20 CLEAR0 30 S=5 40 J=1 50 FOR I=1 TO 5 60 J=J*2;D=K/J/4 70 X=K-5*D; Y=K-2*D 80 T=1; MOVE X,Y 90 X=X+D; A=J; B=J; GOTO s 100aIF A=J AND B=J GOTO z 110sP=J; Q=a; R=B 120vIF P<2 GOTO z 130 IF P=2 GOSUB o; GOTO a 140 P=P/2 150 IF Q<P OR P+1<Q GOTO n 170 IF R<P OR P+1<R GOTO n 190 GOSUB c; GOTO a 200nIF Q>=p THEN Q=Q-P 210 IF R>=P THEN R=R-P 220 GOTO v 230zREM end of loop 240 FOR N=1 TO 1000;NEXT 250 CLEARO 260 NEXT I 270 END
1000cGOTO(1000+100*T) 1100 X=X+D 1105 PLOTS,X,Y 1110 X=X+D;Y=Y+D;PLOTS,X,Y 1120 Y=Y+D;B=B+1;T=4;RETURN 1200 Y=Y-D 1205 PLOTS,X,Y 1210 X=X+D;Y=Y-D;PLOTS,X,Y 1220 X=X+D;A=A+1;T=1;RETURN 1300 X=X-D 1305 PLOTS,X,Y 1310 X=X-D;Y=Y-D;PLOTS,X,Y 1320 Y=Y-D;B=B-1;T=2;RETURN 1400 Y=Y+D 1405 PLOTS,X,Y 1410 X=X-D;Y=Y+D;PLOTS,X,Y 1420 X=X-D;A=A-1;T=3;RETURN 2000oGOTO(2000+100*T)
2100 X=X+D;PLOTS,X,Y 2110 X=X+D;Y=Y+D;PLOTS,X,Y 2120 X=X+D;Y=Y-D;GOTO 1305 2200 Y=Y-D;PLOTS,X,Y 2210 X=X+D;Y=Y-D;PLOTS,X,Y 2220 X=X-D;Y=Y-D;GOTO 1405 2300 X=X-D;PLOTS,X,Y 2310 X=X-D;Y=Y-9;PLOTS,X,Y 2320 X=X-D;Y=Y+D;GOTO 1105 2400 Y=Y+D;PLOTS,X,Y 2410 X=X-D;Y=Y+D;PLOTS,X,Y 2420 X=X+D;Y=Y+D;GOTO 1205 Description of Program: 50 Plot five generations l000-1420 Plot centre square 2000-2420 Not a centre square Variables: A,B - Coordinates of current square D - Number of cells in a quarter of a square J - Number of squares in picture K - Resolution of screen O - Graphics mode S - Argument for PLOT statement T - Angle in units of 90 degrees. X,Y - Current drawing position Program size: 1047 bytes
Sample plot:
The following program will plot a perspective view of a three-dimensional object or curve as viewed from any specified point in space. The program is simply provided with a subroutine giving the coordinates of the object to be drawn, or the equation of the curve. The program below plots a perspective view of the curve 1/(1+x^2+y^2) for a range of values of x and y. The function has been scaled up by a factor of 300 to bring the interesting part of the curve into the correct range. The program is provided with an equation of the curve, specifying z (the vertical axis) in terms of x, and y (the two horizontal axes), and the view position. It projects every point on the surface onto a plane perpendicular to the line joining the view position to the origin. The example given here draws line of equal y, and the surface is drawn as if viewed from the point x=30, y=40, z=8; i.e. slightly above the surface.
1 REM Three-Dimensional Plotting 50 L=30;M=40;N=8 110 Z=0;CLEAR4 120 A=#8000;B=#9800 130 FORJ=A TO B STEP4;!J=-1;N. 150 S=L*L+M*M;GOS.s;R=Q 160 S=S+N*N;GOS.s;S=L*L+M*M 170 T=L*L+M*M+N*N 200 F.U=-20TO20 210 V=-20;GOS.c;GOS.b 220 F.V=-19TO20;GOS.c;GOS.a;N.;N. 230 END 400sQ=S/2 410 DOQ=(Q+S/Q)/2 415 U.(Q-1)*(Q-1)<S AND(Q+1)*(Q+1)>S 420 R. 500 REM DRAWTO(U,V,W) 510aZ=3 520bO=T-U*L-V*M-W*N 530 C=T*(V*L-U*M)*4/(R*O)+128 540 D=96+3*Q*(W*S-N*(U*L+V*M))/(R*O) 560 PLOT(Z+4),C,D;Z=0;R. 600cW=300/(10+U*U+V*V)-10;R.
Description of Program: 50 Set up view position 110 Set move mode, and clear screen 120-130 Invert screen 150-170 Calculate constants for linear projection 200-230 Scan X,Y plane evaluating function and plotting projected lines. 400-420 s: Square root routine (see Section 5.2.2). 500-560 a: Calculate projected position of next point and move to it (Z=0) or draw to it (Z=3) 600 c: Function for evaluation
Variables: A - Display area start B - Display area end C,D - Coordinates of projected point J - Display location to be cleared Q,R,S,T - Constants for projection U,V - Scan variables W - Function value Program size: 491 bytes.
Sample Plot:
In the higher graphics modes, modes 1 to 4, characters cannot be plotted on the screen directly but it is fairly simple to draw characters using the graphics statements. The following simple routines will draw the hex characters 0 to F, with any desired scaling, and with an optional slope. The routines are useful for labelling graphs drawn in the higher-resolution graphics modes. Routine p plots a single hex character; routine q plots two hex characters. The routine is demonstrated by drawing random hex characters in a circle.
1 REM Plotting Hex Characters 10 N=TOP; !N=06E3E4477; N!4=#467B6B4D 12 N!8=#795F4F7F; N!12=#1B3B7C33 20 V=2; H=2; S=0 25 CLEAR 0 30 X=30; Y=0 40 MOVE (32+X),(24+Y) 50 X=X+Y/6;Y=Y-X/6 60 A=ABSRND&#F 70 GOSUBp 90 GOTO 40 1000qREM plot B as 2 hex digits 1010 A=B/16; GOSUB p 1020 A=B&#F 2000pREM Plot A in hex 2001 REM uses:A,H,J,K,L,N,Q,V 2010 Q=N?A 2020 FOR J=1 TO 7 2030 K=(2-J%6)%2;L=(2-(J'-1)%4)%2 2040 PLOT(Q&1),(L*H+K*S),(K*V) 2050 Q=Q/2; NEXT J 2060 PLOTO,((H+2)/2),0; RETURN
Description of Program: 10-12 Set up plotting statements for the 16 characters. 20 Scales for letters 30-50 Move X,Y around a circle 60-70 Plot random character 1000-1020 q: Plot low-order byte of B as two hex digits 2000-2060 p: Plot low-order hex digit of A in hex
Variables: A - Hex digit to be plotted B - Byte to be plotted H - Horizontal scaling N - Vector containing character plotting statements Q - Next plot statement; low-order bit determines whether to draw or move. S - Slope factor V - Vertical scaling X,Y - Coordinates of point on circle. Program size: 457 bytes Vector: 16 bytes
The graphics statements are optimised for speed. For example, to draw a diagonal across the screen using: MOVE 0,0 ; DRAW 255,191 takes under 40 msec. The following program uses animated graphics to display a clock whose hands move to show the correct time. The hands are drawn using the statement PLOT 6, and the same statement is repeated to remove each hand’s old position before drawing its new position. The clock keeps accurate time by executing the WAIT statement:
1 REM Clock 10 CLEAR4;E=128;F=96 15 J=71;K=678;Q=100;R=#B001 20 X=0;Y=8000;G=90 30 MOVE(X/Q+E),(Y/Q+F) 40 FORL=0 TO 59 45 IF L%5<>0 GOTO c 50 DRAW(X/G+E),(Y/G+F) 55 MOVE(X/Q+E),(Y/Q+F) 60cGOSUBi;GOSUBp 68 NEXTL 70 X=0;Y=5000;S=0 72 DO A=0;B=6600 80 FOR H=0 TO 4 82 GOSUBh;C=X;D=Y;X=A;Y=B 84 FOR M=0 TO 11 85 GOSUBh;A=X;B=Y 87 X=0;Y=7000 88 IF ?R<>#FF GOTO b 90 FOR L=0 TO 59 110 GOSUB s 120 FOR N=S TO 55;WAIT;NEXT N 130 S=0 140 GOSUBs;GOSUBi 150 NEXT L 155bX=A;Y=B 160 GOSUBh;GOSUBi 170 NEXT M 175 A=X;B=Y;X=C;Y=D 180 GOSUBh;GOSUBi 200 NEXT H; UNTIL 0 399 REM 400hMOVE E,F 410 V=X/2/Q;U=Y/2/Q;W=V/5;T=U/5 415 WAIT 420 PLOT6,(V-T+E),(U+W+F) 430 PLOT6,(X/Q+E),(Y/Q+F) 440 PLOT6,(V+T+E),(U-W+F) 450 PLOT6,E,F;S=S+5;RETURN 500iWAIT;X=X+J*Y/K 510 Y=Y-J*X/K;S=S+1;RETURN 600sMOVE E,F 620pWAIT;PLOT6,(X/Q+E),(Y/Q+F) 630 S=S+1;RETURN
Description of Program: 40-68 Draw clock face 80-84 Do hours and minutes 88 If shift key down miss out seconds 90-150 Do seconds 120 Use up remainder of each second 400-450 h: Draw hour/minute hand from centre of screen to X,Y 500-510 i: Increment X,Y one sixtieth of way around circle. 600 s: Draw second hand 620-630 p: Plot to point X,Y Variables: A,B - Coordinates of tip of minute hand C,D - Coordinates of tip of hour hand E,F - Coordinates of centre of screen H - Twelves of minutes counter J,K - Incremental variables; J/K = 2*PI/60 approx. L - Seconds counter M - Minutes counter N - Counter Q - Scaling factor R - Address of shift key S - Sixtieths of a second used out of current second X,Y - Coordinates on screen scaled by Q Program size: 806 bytes
Sample Plot:
To set the correct time hold the shift key down after typing RUN, and release it when the hour and minute hands are in the correct positions.
To illustrate how the plotting statements work, the following BASIC programs will plot points on the screen in the different graphics modes without using PLOT, DRAW, or MOVE.
The following BASIC program will plot a point in the graphics mode 0; the main program sets up a vector V which contains bytes with a single bit set to denote the bit to be plotted. Subroutine p plots a point at the coordinates X and Y.
1 REM Plot in Mode 0 10 DIM V(5) 20 !V=#04081020; V!4=#102 100 REM Plot point at X,Y 110 REM Changes: P; Uses V,X,Y 120pP=X/2+(47-Y)/3*32+#8000 130 ?P=?P\V?(X&1+(47-Y)%3*2);RETURN
Using this method it is possible to determine the state of any point on the screen, as well as actually plotting points. For example, changing line 130 to:
130 Q=(?p&(v?(x&1+(47-y)%3*2))<>0)
uses Q as a logical variable whose value is set to ’true' if the point X,Y is set, and to 'false' if the point is clear. Note that the screen should be cleared by writing #40 in every location (or with the statement CLEAR 0) before plotting in graphics mode zero with this routine.
To set the ATOM to a higher graphics mode the following character should be stored in location #B000:
| Mode: | Value: | GM2 | GM1 | GM0 | A/G | ||
| 0 | #00 | 0 | 0 | 0 | 0 | ||
| 1a | #10 | 0 | 0 | 0 | 1 | ||
| 1 | #30 | 0 | 0 | 1 | 1 | ||
| 2a | #50 | 0 | 1 | 0 | 1 | ||
| 2 | #70 | 0 | 1 | 1 | 1 | ||
| 3a | #90 | 1 | 0 | 0 | 1 | ||
| 3 | #B0 | 1 | 0 | 1 | 1 | ||
| 4a | #D0 | 1 | 1 | 0 | 1 | ||
| 4 | #F0 | 1 | 1 | 1 | 1 |
This operation is performed automatically for modes 0, 1, 2, 3, and 4 by the CLEAR statement. Modes 1a, 2a, 3a, and 4a are colour graphics modes; see section 11.9 below. To illustrate plotting in the higher modes the following BASIC program will plot a point on the screen at the coordinates X,Y in the highest-resolution graphics mode:
10 DIM V(7) 20 !V=#10204080; V!4=#1020408 30 ?#B000=#F0 100 REM Plot point at X,Y 110 REM Changes: P; Uses: V,X,Y 100pP=X/8+(191-Y)*32+#8000 102 ?P=?\V?(X&7);RETURN
Again the program can be modified to test the state of points of the screen.
The ATOM provides three additional graphics modes which provide graphics in four selectable colours up to a maximum definition of 128x192. These modes are known as 1a, 2a, 3a, and 4a. The BASIC's PLOT, DRAW, and MOVE statements can be used in the 4-colour modes provided that a point-plotting routine, written in assembler, is provided to replace the black-and-white point plotting routines. Alternatively the COLOUR statement, provided in the extension ROM, can be used; see Section 22.2. The address of the point-plotting routine used by PLOT, MOVE, and DRAW is stored in RAM at #3FE and #3FF. The following information is passed down to the point-plotting routine in zero page:
| Location: | Function: |
| 5A | X coordinate - low byte |
| 5B | " " high byte |
| 5C | Y coordinate - low byte |
| 5D | " " high byte |
| 5E | 1: set bit, 2: invert bit, else, clear bit. |
| 5F 60 |
Free for workspace " " |
The following BASIC program demonstrates how an assembler point-plotting routine can be provided to give four-colour plotting in graphics mode 4a, the highest-resolution colour graphics mode:
10 REM 4-Colour Plot 12 GOSUB 400 16 CLEAR4;?#B000=#D0 18 ?#3FE=Q;#03FF=Q&#FFFF/256 30 FOR J=0 TO 64 STEP 2 40 ?C=J%3*4;MOVE J,0 50 DRAW 127,J;DRAW(127-J),191 60 DRAW 0,(191-J);DRAW J,0 70 NEXT J 80 END 400 DIM V(11),C(0),P(-1),Q(-1) 420 !V=#01041040;V!4=#02082080;V!8=#030C30C0 430 P.$21 508[ 510 LDA@0;STA #5F 520 LDA#5C;LSR A;ROR #5F 530 LSRA;ROR#5F;LSRA;ROR#5F 540 STA#60;LDA#5A;LSRA;LSRA 550 CLC;ADC#5F;STA#5F 560 LDA#60;ADC@#80;STA#60 570\#5F AND #60 CONTAIN ADDRESS 580 LDA#5A;AND@3;CLC;ADCC;TAY 590 LDX@0;LDAV,Y;ORA(#5F,X) 600 STA(#5F,X);RTS 610] 620 P.$6 630 RETURN
Description of Program: 12 Assemble point plotting routine 16 Clear display; set mode 3a 18 Change point plotter vector 30-70 Demonstration program; curve stitching in 4 colours 400 Set up variable space 420 Vectors for three colours 430 Disable assembler listing 508-610 Assembler point-plotter program 620 Turn screen back on
Variables: C - Colour: 0, 4, or 8. P - Location counter Q - Address of point-plotting routine V - Vectors for setting bits Program size: 558 bytes Vectors: 13 bytes
Note that the routine only sets bits, and plots in three colours - the fourth colour being the background colour. It would be a simple matter to modify the routine so that it was able to set or unset bits; i.e. plot in the background colour.