## Introduction

Today's image processing technology helps us in many areas such as determining tumors, text extraction, checking images,... In this algorithm, we can get the angle of a line in an image. Of course, there are several ways to do this with advanced algorithms, but this is a simple algorithm.

## Features

- Works in real images with several colors
- Simple to implement
- Gets the length of the line
- Very light-weight code

## Source Code

I have commented the code clearly. First, we must define a global `CImage`

and then load the image file from the hard disk:

CImage i;

After that, we need to draw an image when cleared with the `OnPaint()`

function:

void CDetectangleoflineDlg::OnPaint()
{
if(!i.IsNull()){ CClientDC dc(this);
i.BitBlt(dc.m_hDC,0,0,SRCCOPY);
}
}

Next, we load the image:

void CDetectangleoflineDlg::OnBnClickedButton1(){
CFileDialog f(1); TCHAR tc[260];
CString s;
GetCurrentDirectory(260,tc);
s=tc;
if(f.DoModal()==IDOK){ if(!i.IsNull()) i.Destroy(); i.Load(f.GetFolderPath()+_T("\\")+f.GetFileName());
//load image to i (CImage)
OnPaint();//call draw image
b.EnableWindow(1);
}
}

Finally, we use the Process algorithm to determine the angle:

void CDetectangleoflineDlg::OnBnClickedButton2(){
CImage i1;
i1=i; CDC cdc; cdc.Attach(i1.GetDC());
CPen p(PS_SOLID,1,RGB(255,255,0)),p1(PS_SOLID,1,RGB(0,0,255));
cdc.SelectObject(&p);
register int x1,x2,y1,y2;
x1=x2=y1=y2=-1;
double pi=3.1415926535;
register long r,g,b,r1,g1,b1;
register COLORREF c,c1;
register int tol=30,d=20;
c=i1.GetPixel(0,0);
r=GetRValue(c);
g=GetGValue(c);
b=GetBValue(c);
bool ar[600][450];
for(register int y=0;y<i.GetHeight();y++)for(register int x=0;x<i.GetWidth();x++)
ar[y][x]=0;
for(register int y=0;y<i.GetHeight();y++)
for(register int x=0;x<i.GetWidth();x++){
c1=i.GetPixel(x,y);r1=GetRValue(c1);g1=GetGValue(c1);b1=GetBValue(c1);
if((abs(r-r1)>tol)&&(abs(g-g1)>tol)&&(abs(b-b1)>tol))
ar[y][x]=1;
else ar[y][x]=0;
}
for(register int y=0;y<i.GetHeight();y++)
for(register int x=0;x<i.GetWidth();x++){
if(ar[y][x]){ if(x1==-1){ x1=x2=x;y1=y2=y;
}else if(sqrt(pow(x-x2,2.0)+pow(y-y2,2.0))<=d){ x2=x;y2=y;
ar[y][x]=1;
}else ar[y][x]=0;
}
}
for(register int y=0;y<i.GetHeight();y++)
for(register int x=0;x<i.GetWidth();x++)
if(ar[y][x])
cdc.SetPixel(x,y,RGB(255,255,0));
register float radius=sqrt(pow(x1-x2,2.0)+pow(y1-y2,2.0));
float t=atan((float)(y2-y1)/(x2-x1))*180/pi;
cdc.SelectObject(&p1); cdc.MoveTo((x1+x2)/2-radius/2,(y1+y2)/2);
cdc.LineTo((x1+x2)/2+radius/2,(y1+y2)/2);
cdc.MoveTo((x1+x2)/2,(y1+y2)/2-radius/2);
cdc.LineTo((x1+x2)/2,(y1+y2)/2+radius/2);
CClientDC dc(this); if(t>=-180&&t<=180){ char a[10]="";
itoa(t,a,10);
CString s;
s=a;
dc.TextOut(0,300,s+_T(" degree"));
}
i1.ReleaseDC();
cdc.Detach();
i1.BitBlt(dc.m_hDC,i.GetWidth()+10,0,SRCCOPY); i1.Destroy();
}

## Algorithm

First, we need a color for the `backcolor`

, '`c`

', and a color to get the points of the image, '`c1`

'. We have to find the first and last points of the line `(x1, y1)`

and `(x2, y2)`

. We set **-1** as the default value of `x1`

. This means we don't find the first point yet. For detecting the first point, we move and look around the image at `(x, y)`

. When `c1!=c && x==-1`

, we find `(x1, y1)`

. We imagine a virtual radius like '`d`

' and with a default value `10`

. After that, we move and look again, and when `c!=c && sqrt((x-x2)^2+(y-y2)^2))<=d`

, this point is related to the previous point and line. We continue this till the end. Finally, we'll find `(x1,y2),(x2,y2)`

and we can get the length of the line and get the angle of the line with `arctan`

.

teta=arttan((y2-y1)/(x2-x1))