how to alternate colors in a circle, so that circle looks like rotating?

Question

The expected output should be like this with the colors changing their position as well:

Expected output-:

the colors should change their positions in a circle so that it looks like they are moving without changing the position of circle.

though my code is written in codeblocks in c/c++, i will be happy to get answers in any other programming languages.

my present code

#include<graphics.h>
#include<stdlib.h>
#include<stdio.h>
#include<conio.h>
#include<math.h>
#include<string.h>
#include<iostream>

using namespace std;
void vvcircle(float xk,float yk,float radius);


int i=0;

int main()
{
float xk,yk,radius;

    int gdriver=DETECT,gmode,errorcode;
    initgraph(&gdriver,&gmode,"C:\\TURBOC3\\BGI");
   // cout<<"enter the value of x, y and radius of circle"<<endl;
    //cin>>xk>>yk>>radius;


    vvcircle(200,200,100);

    getch();
    closegraph();
    return 0;
}

void vvcircle(float xk,float yk,float radius)
{
    int color[60]={0,1,2,3,4,5,6,7,8,9};
    while(radius>0)
    {

    float xo,yo;
    float P;
    xo=0.0;
    yo=radius;
P=1-radius;


///   vvcircle(200,200,100);

for(;xo<=yo;)
{



     putpixel(xo+xk,yo+yk,1);

     putpixel(yo+xk,xo+yk,1);


     putpixel(-yo+xk,xo+yk,2);


     putpixel(xo+xk,-yo+yk,2);


     putpixel(-yo+xk,-xo+yk,4);

          putpixel(-xo+xk,-yo+yk,4);

     putpixel(yo+xk,-xo+yk,4);

     putpixel(-xo+xk,+yo+yk,4);



    if(P<0)
    {

        xo=xo+1;
        yo=yo;
        P=P+2*xo+1;

    }
    else
    {

        xo=xo+1;
        yo=yo-1;
        P=P+(2*xo)-(2*yo)+1;
       //  putpixel(xo,yo,WHITE);

    }


}
radius=radius-1;

    }
}

Present output-:

i get many concentric circles with colors. but i want to move the colors so that it looks like the circle is moving and it is not achieved.

Spektre · Answer 1 · 2020-02-07T09:50:26.060

How about something like this:

#include <math.h>
void my_circle(int xc,int yc,int r,float a) // center(x,y), radius, animation angle [rad]
    {
    const int n=4;          // segments count
    int x,sx,xx,x0,x1,rr=r*r,
        y,sy,yy,y0,y1,i,
        dx[n+1],dy[n+1],    // segments edges direction vectors
        c[n]={5,1,2,3};     // segments colors
    float da=2.0*M_PI/float(n);
    // BBOX
    x0=xc-r; x1=xc+r;
    y0=yc-r; y1=yc+r;
    // compute segments
    for (i=0;i<=n;i++,a+=da)
        {
        dx[i]=100.0*cos(a);
        dy[i]=100.0*sin(a);
        }
    // all pixels in BBOX
    for (sx=x0,x=sx-xc;sx<=x1;sx++,x++){ xx=x*x;
     for (sy=y0,y=sy-yc;sy<=y1;sy++,y++){ yy=y*y;
        // outside circle?
        if (xx+yy>rr) continue;
        // compute segment
        for (i=0;i<n;i++)
         if ((x*dy[i  ])-(y*dx[i  ])>=0)
          if ((x*dy[i+1])-(y*dx[i+1])<=0)
           break;
        // render
        putpixel(sx,sy,c[i]);
        }}
    }

It simply loop through all pixels of outscribed square to your circle, determines if pixel is inside and then detect which segment it is in and color it with segments color.

The segments are described by direction vectors from circle center towards the segments edges. So if pixel is inside it mean its CW to one edge and CCW to the other so in 2D inspecting z coordinate of the cross product between vector to pixel and vectors to edges will tell if the pixel is in or not ...

As you can see I did not use floating point math in the rendering it self, its needed only to compute the segments edge vectors prior rendering...

I used standard 256 color VGA palette (not sure what BGI uses I expect 16 col) so the colors might be different on your platform here preview:

The noise is caused by my GIF capturing tool dithering the render itself is clean ...

Do not forget to call the my_circle repeatedly with changing angle ...

PS. I encoded this in BDS2006 without BGI so in different compiler there might be some minor syntax problem related to used language quirks...

I faked the putpixel with this:

void putpixel(int x,int y,BYTE c)
    {
    static const DWORD pal[256]=
        {
        0x00000000,0x000000A8,0x0000A800,0x0000A8A8,0x00A80000,0x00A800A8,0x00A85400,0x00A8A8A8,
        0x00545454,0x005454FC,0x0054FC54,0x0054FCFC,0x00FC5454,0x00FC54FC,0x00FCFC54,0x00FCFCFC,
        0x00000000,0x00101010,0x00202020,0x00343434,0x00444444,0x00545454,0x00646464,0x00747474,
        0x00888888,0x00989898,0x00A8A8A8,0x00B8B8B8,0x00C8C8C8,0x00DCDCDC,0x00ECECEC,0x00FCFCFC,
        0x000000FC,0x004000FC,0x008000FC,0x00BC00FC,0x00FC00FC,0x00FC00BC,0x00FC0080,0x00FC0040,
        0x00FC0000,0x00FC4000,0x00FC8000,0x00FCBC00,0x00FCFC00,0x00BCFC00,0x0080FC00,0x0040FC00,
        0x0000FC00,0x0000FC40,0x0000FC80,0x0000FCBC,0x0000FCFC,0x0000BCFC,0x000080FC,0x000040FC,
        0x008080FC,0x009C80FC,0x00BC80FC,0x00DC80FC,0x00FC80FC,0x00FC80DC,0x00FC80BC,0x00FC809C,
        0x00FC8080,0x00FC9C80,0x00FCBC80,0x00FCDC80,0x00FCFC80,0x00DCFC80,0x00BCFC80,0x009CFC80,
        0x0080FC80,0x0080FC9C,0x0080FCBC,0x0080FCDC,0x0080FCFC,0x0080DCFC,0x0080BCFC,0x00809CFC,
        0x00B8B8FC,0x00C8B8FC,0x00DCB8FC,0x00ECB8FC,0x00FCB8FC,0x00FCB8EC,0x00FCB8DC,0x00FCB8C8,
        0x00FCB8B8,0x00FCC8B8,0x00FCDCB8,0x00FCECB8,0x00FCFCB8,0x00ECFCB8,0x00DCFCB8,0x00C8FCB8,
        0x00B8FCB8,0x00B8FCC8,0x00B8FCDC,0x00B8FCEC,0x00B8FCFC,0x00B8ECFC,0x00B8DCFC,0x00B8C8FC,
        0x00000070,0x001C0070,0x00380070,0x00540070,0x00700070,0x00700054,0x00700038,0x0070001C,
        0x00700000,0x00701C00,0x00703800,0x00705400,0x00707000,0x00547000,0x00387000,0x001C7000,
        0x00007000,0x0000701C,0x00007038,0x00007054,0x00007070,0x00005470,0x00003870,0x00001C70,
        0x00383870,0x00443870,0x00543870,0x00603870,0x00703870,0x00703860,0x00703854,0x00703844,
        0x00703838,0x00704438,0x00705438,0x00706038,0x00707038,0x00607038,0x00547038,0x00447038,
        0x00387038,0x00387044,0x00387054,0x00387060,0x00387070,0x00386070,0x00385470,0x00384470,
        0x00505070,0x00585070,0x00605070,0x00685070,0x00705070,0x00705068,0x00705060,0x00705058,
        0x00705050,0x00705850,0x00706050,0x00706850,0x00707050,0x00687050,0x00607050,0x00587050,
        0x00507050,0x00507058,0x00507060,0x00507068,0x00507070,0x00506870,0x00506070,0x00505870,
        0x00000040,0x00100040,0x00200040,0x00300040,0x00400040,0x00400030,0x00400020,0x00400010,
        0x00400000,0x00401000,0x00402000,0x00403000,0x00404000,0x00304000,0x00204000,0x00104000,
        0x00004000,0x00004010,0x00004020,0x00004030,0x00004040,0x00003040,0x00002040,0x00001040,
        0x00202040,0x00282040,0x00302040,0x00382040,0x00402040,0x00402038,0x00402030,0x00402028,
        0x00402020,0x00402820,0x00403020,0x00403820,0x00404020,0x00384020,0x00304020,0x00284020,
        0x00204020,0x00204028,0x00204030,0x00204038,0x00204040,0x00203840,0x00203040,0x00202840,
        0x002C2C40,0x00302C40,0x00342C40,0x003C2C40,0x00402C40,0x00402C3C,0x00402C34,0x00402C30,
        0x00402C2C,0x0040302C,0x0040342C,0x00403C2C,0x0040402C,0x003C402C,0x0034402C,0x0030402C,
        0x002C402C,0x002C4030,0x002C4034,0x002C403C,0x002C4040,0x002C3C40,0x002C3440,0x002C3040,
        0x00000000,0x00000000,0x00000000,0x00000000,0x00000000,0x00000000,0x00000000,0x00000000,
        };
    if ((x<0)||(x>=Main->xs)) return;
    if ((y<0)||(y>=Main->ys)) return;
    Main->pyx[y][x]=pal[c];
    }

Where Main->xs, Main->ys is my window resolution and Main->pyx is direct pixel acces to its canvas for more info see:

Graphics rendering: (#4 GDI Bitmap)

how to alternate colors in a circle, so that circle looks like rotating?

1 Answers1