Game math (distance increments)

I have 2 points (start, end) represented by POINTS (ptStart, ptEnd). The 2 points represent the barrel of the players gun and the target, respectively (it's a 2d iso game).

I know that to get the distance between the points I would use:

distance = sqrt(((ptEnd.x-ptStart.x)*(ptEnd.x-ptStart.x))+((ptEnd.y-ptStart.y)*(ptEnd.y-ptStart.y)))

What I don't know is how to figure the incremental amounts added/subtracted to the original point to move the object to the end point. Unless my end point is and equal distance from my start x AND y, I can't just increment through the distance.

What I'm looking for is a smooth transition from point a to point b. I just don't know what to add/subtract in my loop to get it.

Thanks in advance for any help you can give.

Lets see if I can explain this..I'll give it a try ask me again if I don't get it clear

Y = KX + M (Standard linear equation)

deltaY = EndY - StartY
deltaX = EndX - StartX

K = deltaY / deltaX

You know know the K value for your line. We now need to know M

M = Y - KX (Standard linear equation)
M = StartY - K*StartX

You can now increment your X value from start to end and get the Y value for your linear curve by using Y =KX + M

That doesn't work (unless I misunderstood something, which is quite possible).

Here's the code:

Code:

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POINT ptStart, ptEnd, ptMob;

DeltaX = ptEnd.x - ptStart.x
DeltaY = ptEnd.y - ptStart.y
K = DeltaY / DeltaX
M = ptStart.y - K * ptStart.x
Y = K * ptStart.x + M
ptMob.x = ptMob.x + 1
ptMob.y = ptMob.y + K

---

With the following initial settings:

ptStart.x = 5
ptStart.y = 200
ptEnd.x = 200
ptEnd.y = 20
ptMob.x = ptStart.x
ptMob.y = ptStart.y

It gives me this:
DeltaX = 195
DeltaY = -180
K = -1
M = 205
Y = 200

-180/195 is not -1. You can't use int. cast to double. The method I oulined will also give you problem with perfectly vertical and horizontal lines so you have to test for those two cases if(DeltaX == 0 || DeltaY == 0) before you solve the linear equation

If you look at his math for a second you will realize that it is the old y=mx+b from school.


A line is a linear interpolation between y and y2 and a linear interpolation between x or x2 or a bi linear interpolation of x,x2,y,y2.

Is there a better way to do this? I don't like the way that it works with vertical lines. It feals like a kludge.

Quote:

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Originally posted by Barjor
[B]-180/195 is not -1. You can't use int. cast to double.

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I'm using 2d coords (screen x,y). Are you saying to cast to double and then convert and use just the int portion? Would that give accurate enough results?


*kicks self*
Now I wish I would have paid better attention in high school (unless I did and I'm just too old to remember!) :D