# Intersection between line and cylinder

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I am tracing a ray in Matlab and I have an expression for a "line- to sphere intersection" that works:
$a = 1 + Ax^2 + Ay^2$

$b = 2\cdot(-z_s + A_x\cdot(B_x-x_s) + A_y\cdot(B_y-y_s))$

$c = z_s^2 + (B_x-x_s)^2 + (B_y - y_s)^2 - R^2$

This is part of a code in Matlab, and works fine. It is derived from substituting

$(x=A_x\cdot z+B_x, y=A_y\cdot z+B_y)$ (This is line in parametric, I believe.)

into:

$((x-x_s)^2+(y-y_s)^2+(z-z_s)^2 = R^2)$  (This is the equation for a sphere, modified to find distance)

I am now trying to do the same for a cylinder, by substituting

$(x=A_x\cdot z+B_x, y=A_y\cdot z+B_y)$ (line) into: $((x-x_s)^2+(y-y_s)^2 = R^2)$ (cylinder)

Putting into quadratic formula and ending up with:

$a = A_x^2+A_y^2$;

$b = 2\cdot A_x\cdot B_x - 2\cdot A_x\cdot x_s + 2\cdot A_y\cdot B_y - 2\cdot A_y\cdot y_s;$

$c = -2\cdot B_x\cdot x_s - 2\cdot B_y\cdot y_s + B_x^2 + B_y^2 + x_s^2+y_s^2 - R^2;$

This doesn't seem to work. Can anyone tell me if something seems off with the math or logic?
Any comments at all would be greatly appreciated.

Is there a way to describe a line- cylinder intersection, using a,b,c, like I have for a line- sphere intersection(below)?

$a = 1 + Ax^2 + Ay^2$

$b = 2\cdot(-z_s + A_x\cdot(B_x-x_s) + A_y\cdot(B_y-y_s))$

$c = z_s^2 + (B_x-x_s)^2 + (B_y - y_s)^2 - R^2$

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