More Graphics - Tutorial 015

Tutorial 015 More Graphics Programs

***In the following, those marked with an asterisk* have been developed from ideas in Alan Thomas' excellent book Further Programming for the BBC Micro which I bought in 1983 - I saw a second hand copy at the Book Barn, White Cross, Somerset, BS39 6EX, UK, Telephone 01761-451777 the other day for £3-50 (its long been out of print).

Lines

      REM Lines by R.G.Weston
      MODE8
      REPEAT
        INPUTTAB(15,12)"How many lines would you like to draw : "no_lines
        CLS : OFF
        FOR n=1 TO no_lines
          PRINTTAB(0,0);n
          GCOL0, RND(15)
          x1=RND(1279) : y1=RND(1024)
          x2=RND(1279) : y2=RND(1024)
          LINE x1,y1,x2,y2
          delay=INKEY(20)
        NEXT n
        delay=INKEY(300)
        CLS:COLOUR RND(7)
      UNTIL FALSE

Exercises : Speed up the display.
                Make all lines start at the centre of the screen and radiate out from there.

Linescope*

Just keep watching - it goes on for ever....

      MODE8:OFF
      COLOUR 0, 2
      COLOUR 8, 1
      mx=640:my=512
      ORIGIN 640,512
      st=RND(100)
      x=RND(mx) : y=RND(my)
      sw=0
      n=0
      REPEAT
        n=n+1
        GCOL0,n MOD 15
        delay=INKEY(1)
        x1=x: y1=y
        IF sw=1 THEN x1=(mx/st)*RND(st)
        IF sw=0 THEN y1=(my/st)*RND(st)
        MOVE x,y : DRAW x1,y1
        MOVE -x,y : DRAW -x1,y1
        MOVE -x,-y DRAW -x1,-y1
        MOVE x,-y : DRAW x1,-y1
        x=x1 : y=y1
        sw=(sw+1) MOD 2
        G$=INKEY$(10)
      UNTIL n=40
      delay=INKEY(200)
      RUN

Flower*

Play about with some of the parameters to see how this works

      MODE8 : OFF
      PRINTTAB(30)"Press SPACE"
      COLOUR 0, 1
      COLOUR 8,5
      n=40
      dt=PI/n
      a=512
      k=8
      ORIGIN 640,512
      MOVE 0,0
      IF k MOD 2 =0 THEN dt=2*dt
      e=0
      FOR i=1 TO k
        GCOL0,RND(15)
        t=PI/(2*k)
        FOR j=1 TO n
          PRINTTAB(1,3)"j = ";j
          delay=INKEY(10)
          x=a*COS(k*t)*COS(t+e)
          y=a*COS(k*t)*SIN(t+e)
          DRAW x,y
          t=t+dt
          PRINTTAB(1,1)"i = ";i
          G=GET

        NEXT j
        e= e + 0.1
      NEXT i

Epicycles*

Run his program lots of times to see the range of possibilities. (Remember Spirograph?)

A wheel of radius b rolls round the inside of a circle radius a. The program plots the path of an ant sitting on the inner wheel at a distance r from the centre of the smaller, inner wheel.

The outer red circle contains a smaller red circle, which follows the path of the centre of the inner wheel. The green circles show the limits of travel of the ant! If the inner wheel is almost as big as the outer circle the results are quite surprising.

      REM Epicycles
      REM by RG Weston
      REM 22 Feb 2003
      REM After Alan Thomas
      REM Wheels within Wheels 1983
      MODE8:OFF
      PRINTTAB(25)"Press SPACE to go again"
      a=500
      b=20+RND(460)
      r=RND(b)
      d=a-b : e=d/b
      t=0
      dt=0.1
      PRINT" a = ";a
      PRINT" b = ";b
      PRINT" r = ";r
      ORIGIN 640,512
      GCOL0,1
      PLOT69,0,0
      CIRCLE 0,0,a
      CIRCLE 0,0,d
      GCOL0,2
      CIRCLE 0,0,d-r
      CIRCLE 0,0,d+r
      MOVE d+r, 0
      GCOL0,15
      REPEAT
        x=d*COS(t) + r*COS(e*t)
        y=d*SIN(t) - r*SIN(e*t)
        DRAW x,y
        t=t+dt
      UNTIL INKEY$(1)=" "
      RUN

Chessboard*

The following program produces a chessboard too quickly, on most machines, for one to see how it is done.

      REM: Chessboard fast draw
      REM: Richard Weston 24th March 2003
      REM: after Alan Thomas
      MODE8:OFF
      a=100
      x=250:y=100
      s=8*a
      RECTANGLE x,y,s,s
      FOR i=1 TO 8
        FOR j=1 TO 8
          IF (i+j) MOD 2 = 0 THEN
            p=x+a*(i-1)
            q=y+a*(j-1)
            RECTANGLE FILL p,q,a,a
          ENDIF
        NEXT j
      NEXT i

Here I have added a few more lines to show how it works and to slow things up :

      REM: Chessboard Demo
      REM: Richard Weston 24th March 2003
      REM: after Alan Thomas
      MODE8:OFF
      a=100
      x=250:y=100
      s=8*a
      RECTANGLE x,y,s,s
      FOR i=1 TO 8
        FOR j=1 TO 8
          PRINTTAB(35,30)(" i = ");i
          PRINTTAB(7,15)(" j = ");j
          IF (i+j) MOD 2 = 0 THEN
            p=x+a*(i-1)
            q=y+a*(j-1)
            RECTANGLE FILL p,q,a,a
            delay=INKEY(40)
          ENDIF
        NEXT
      NEXT
      PRINTTAB(32,2)"Anyone for Chess?"

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