Scan-line fill OpenGL/GLUT algorithm in C++ - c++

Question

I am trying to learn the scan-line fill algorithm implemented in OpenGL/GLUT. I cannot wrap my mind around the concept. Could anybody explain to me the algorithm in a reasonably simple fashion? The algo is below:
#include<GL/glut.h>
float x1,x2,x3,x4,y1,y2,y3,y4;
void draw_pixel(int x,int y)
{
glColor3f(0.0,1.0,1.0);
glPointSize(1.0);
glBegin(GL_POINTS);
glVertex2i(x,y);
glEnd();
}
void edgedetect(float x1,float y1,float x2,float y2,int *le,int *re)
{
float temp,x,mx;
int i;
if(y1>y2)
{
temp=x1,x1=x2,x2=temp;
temp=y1,y1=y2,y2=temp;
}
if(y1==y2)
mx=x2-x1;
else
mx=(x2-x1)/(y2-y1);
x=x1;
for(i=int(y1);i<=(int)y2;i++)
{
if(x<(float)le[i]) le[i]=(int)x;
if(x>(float)re[i]) re[i]=(int)x;
x+=mx;
}
}
void scanfill(float x1,float y1,float x2,float y2,float x3,float y3,float x4,float y4)
{
int le[500],re[500],i,j;
for(i=0;i<500;i++)
le[i]=500,re[i]=0;
edgedetect(x1,y1,x2,y2,le,re);
edgedetect(x2,y2,x3,y3,le,re);
edgedetect(x3,y3,x4,y4,le,re);
edgedetect(x4,y4,x1,y1,le,re);
for(j=0;j<500;j++)
{
if(le[j]<=re[j])
for(i=le[j];i<re[j];i++)
draw_pixel(i,j);
}
}
void display()
{
x1=250.0;y1=200.0;x2=150.0;y2=300.0;x3=250.0;
y3=400.0;x4=350.0;y4=300.0;
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0,0.0,1.0);
glBegin(GL_LINE_LOOP);
glVertex2f(x1,y1);
glVertex2f(x2,y2);
glVertex2f(x3,y3);
glVertex2f(x4,y4);
glEnd();
scanfill(x1,y1,x2,y2,x3,y3,x4,y4);
glFlush();
}
void init()
{
glClearColor(1.0,1.0,1.0,1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0,499.0,0.0,499.0);
}
void main(int argc,char **argv)
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500,500);
glutCreateWindow("scanline");
glutDisplayFunc(display);
init();
glutMainLoop();
}

Answer 1

The scanfill function is filling the the pixels coverted by the 4-sided polygon described by the four x-y coordinates. To do this, it uses the edgedetect function, which simulates drawing the edges of the polygon and remembers the minimum and maximum x coordinate for each y coordinate. The scanfill function then goes through each y coordinate and draws a horizontal line of pixels across the remembered range.