Converting lines from normal to slope-intercept form

Several algorithms return lines in the normal form (like The Hough transform) while other algorithms may require these lines in another format (like drawing a line on screen). The slop-intercept form of lines is widely known, and you can easily convert a line in slope-intercept form to other forms. So knowing how to efficiently do the conversion of parameters is a must! Here I’ll show you one way to do this.

But just in case, here’s a brief review of the normal and slope-intercept forms.

The normal form

The normal form of a line uses two parameters: p and θ to describe the line. Here, p is the length of the perpendicular from the origin to the line. And θ is the angle between this perpendicular and the x-axis.

The line equation with these parameters is:

p = x*cosθ + y*sinθ

The slope-intercept form

This form of the line uses m and c as the parameters. m is the slope of the line (the tan of the angle between the line and the x-axis). And c is the position on the y-axis where the line intersects it.

The line equation with these parameters is:

y = m*x + c

The conversion

We’ll start with the line equation in normal form:

p = x*cosθ + y*sinθ

Rearranging the equation, we get this:

-x*cosθ + p =  y*sinθ

and dividing by sinθ we get:

-x*cotθ + p*cosecθ =  y

or,

y = -x*cotθ + p*cosecθ

and there you have the line in slope intercept form! Let me make it more explicit:

y = (-cotθ)*x + (p*cosecθ)

-cotθ is the slope of the line, and p*cosecθ is the intercept.

Thus, we can summarize the conversion as:

m = -cotθ
c = p*cosecθ

Doing these calculations, you’ll have your line in slope-intercept form instead of the normal form! Great!

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