how i can find the next point on slope when the point can be changed?

Question

iam try to find the next point on the same slope of an line i I had a problem when I tried to add A small number to find the next point add it the next point The distance has become very large i try on this formula:

          slope = (P2.Y - P1.Y) / (P2.X - P1.X);
          nx2 = P2.X +add;
          ny2 = P2.Y+ slope * add;

The nx2 is the next x , ny2 is the next y and p1,p2 is the point1 and point2; add : is the number i want to add in this case i used 18

so i used this formula in this exmaple this tow point {X=517, Y=336}--{X=505, Y=530.5} the next point is *{x=535,y=44} i just add 18 for * but on othe point the distance is normal but on other point like this distance be very far

thank you

Answer 1

This is doing what it should. Your first two points have an $x$ difference of $12$ and a $y$ difference of about $-200$. The third point is $18$ away in $x$, so the $y$ difference should be about $\frac {18}{12}(-200)=-300$ and it is.

Answer 2

The slope can be calculated by either

$\frac {530.5 - 336}{505-517} = \frac {194.5}{-12}= -16 \frac 5{24}$.

Or by $\frac {336 - 44}{517- 535} = \frac {292}{-18}=-16 \frac 29$.

Or by $\frac {530.5 - 44}{505-535} = \frac {486.5}{-30} = -16 \frac 7{60}$.

These slopes are no exact but they all are aproximately $-16.2$ which means for every one you go over you go down $16.2$.

So to go from $x =517;y = 535$ to $x = 535$ you go over $18$. So you expect to go down $18*16.2 = 291.6$. So you go down from $y = 336$ to $y = 336-291.6 = 44.4$. Which is very close to where we did end up at $y= 44$.

"but on othe point the distance is normal but on other point like this distance be very far"

That because $-16.2$ is a very steep slope. Going over $12$ ($517-505= 12$) means going down nearly $200$ ($336 - 530.5= -194.5$). And going over $18$ $(535-517$) means going down nearly $300$ ($44-336=-292$). This is consistent. $18$ is $1\frac 12$ times $12$ and $-292$ is (about) $1\frac 12$ times $-194.5$.