Bresenham Line Drawing Lines

Question

My homework on computer graphics is to implement OpenGL algorithms using only the ability to draw dots.

So obviously, I need to get drawLine() to work before I can draw anything else. drawLine() should only be executed using integers. No floating point.

This is what I was taught. In principle, the lines can be divided into 4 different categories, positive steep, positive small, negative steep and negative small. This is the picture I have to draw:

and this is the image that my program draws:

Colors are made for us. We are given the vertices, and we need to use the Bresenham Line algorithm to draw lines based on the start and end points.

This is what I still have:

 int dx = end.x - start.x; int dy = end.y - start.y; //initialize varibales int d; int dL; int dU; if (dy > 0){ if (dy > dx){ //+steep d = dy - 2*dx; dL = -2*dx; dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; y <= end.y; y++){ Vertex v(x,y); drawPoint(v); if (d >= 1){ d += dL; }else{ x++; d += dU; } } } else { //+shallow d = 2*dy - dx; dL = 2*dy; dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; x <= end.x; x++) { Vertex v(x,y); drawPoint(v); // if choosing L, next y will stay the same, we only need // to update d by dL if (d <= 0) { d += dL; // otherwise choose U, y moves up 1 } else { y++; d += dU; } } } } else { if (-dy > dx){ cout << "-steep\n"; //-steep d = dy - 2*dx; //south dL = 2*dx; //southeast dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; y >= end.y; --y){ Vertex v(x,y); drawPoint(v); //if choosing L, next x will stay the same, we only need //to update d if (d >= 1){ d -= dL; } else { x++; d -= dU; } } } else { cout << "-shallow\n"; //-shallow d = 2*dy - dx; dL = 2*dy; dU = 2*dy - 2*dx; for (int x = start.x, y = start.y; x <= end.x; x++){ Vertex v(x,y); drawPoint(v); if (d >= 0){ d += dL; } else { --y; d -= dU; } } } } 

I know that my mistake will be something stupid, but I honestly cannot understand what I'm doing wrong. Why are some of the lines not drawn correctly as shown above?

Answer 1

If someone wondered what the problem was, I still don't know what it was. What I ended up was overrated by my code, so -shallow and -steep used the same algorithm as + small and + cool, respectively. After adjusting the x, y coordinates (negating the x or y coordinate), when I went to their graph, I denied my original negation so that it displayed in the right place.

Answer 2

You can find the complete C ++ code for drawing a string using the Breshenem algorithm at http://www.etechplanet.com/codesnippets/computer-graphics-draw-a-line-using-bresenham-algorithm.aspx :

 /*BRESENHAAM ALGORITHM FOR LINE DRAWING*/ #include<iostream.h> #include<graphics.h> #include<stdio.h> #include<conio.h> #include<stdlib.h> #include<math.h> #include<dos.h> void bhm_line(int,int,int,int,int); void main() { int ghdriver=DETECT,ghmode,errorcode,x1,x2,y1,y2; initgraph(&ghdriver,&ghmode,"..\\bgi"); errorcode = graphresult(); if(errorcode !=grOk) { cout<<"Graphics error:%s\n"<<grapherrormsg(errorcode); cout<<"Press any key to halt:"; getch(); exit(1); } clrscr(); cout<<"Enter the coordinates (x1,y1): "; cin>>x1>>y1; cout<<"Enter the coordinates (x2,y2): "; cin>>x2>>y2; bhm_line(x1,y1,x2,y2,1); getch(); } void bhm_line(int x1,int y1,int x2,int y2,int c) { int x,y,dx,dy,dx1,dy1,px,py,xe,ye,i; dx=x2-x1; dy=y2-y1; dx1=fabs(dx); dy1=fabs(dy); px=2*dy1-dx1; py=2*dx1-dy1; if(dy1<=dx1) { if(dx>=0) { x=x1; y=y1; xe=x2; } else { x=x2; y=y2; xe=x1; } putpixel(x,y,c); for(i=0;x<xe;i++) { x=x+1; if(px<0) { px=px+2*dy1; } else { if((dx<0 && dy<0) || (dx>0 && dy>0)) { y=y+1; } else { y=y-1; } px=px+2*(dy1-dx1); } delay(0); putpixel(x,y,c); } } else { if(dy>=0) { x=x1; y=y1; ye=y2; } else { x=x2; y=y2; ye=y1; } putpixel(x,y,c); for(i=0;y<ye;i++) { y=y+1; if(py<=0) { py=py+2*dx1; } else { if((dx<0 && dy<0) || (dx>0 && dy>0)) { x=x+1; } else { x=x-1; } py=py+2*(dx1-dy1); } delay(0); putpixel(x,y,c); } } }