mickaelb91 - 1 year ago 101

Python Question

I'm trying to create a lookAt function in 2 dimensions using Python, so here's my code right now.

`from math import *`

def lookAt(segment, originPoint):

segmentCenterPoint = getSegmentCenter(segment)

for i in range(2):

vtx = getVertex(segment, i)

x, y = getVertexCoord(vtx)

# Calculate the rotation angle already applied on the polygon

offsetAngle = atan2(y - segmentCenterPoint.y, x - segmentCenterPoint.x)

# Calculate the rotation angle to orient the polygon to an origin point

orientAngle = atan2(segmentCenterPoint.y - originPoint.y, segmentCenterPoint.x - originPoint.x)

# Get the final angle

finalAngle = orientAngle - (pi / 2)

if offsetAngle >= pi:

offsetAngle -= pi

elif offsetAngle < 0:

offsetAngle += pi

finalAngle += offsetAngle

# Temporary move the point to have its rotation pivot to (0,0)

tempX = x - segmentCenterPoint.x

tempY = y - segmentCenterPoint.y

# Calculate coords of the point with the rotation applied

s = sin(finalAngle)

c = cos(finalAngle)

newX = tempX * c - tempY * s

newY = tempX * s + tempY * c

# Move the point to the initial pivot

x = newX + segmentCenterPoint.x

y = newY + segmentCenterPoint.y

# Apply new coords to the vertex

setVertexCoord(vtx, x, y)

I tried some examples manually and worked well, but when I tried to apply the function on thousands of segments, it seems some segment are not well oriented.

I probably missed something but i don't know what it is. Also, maybe there's a faster way to calculate it ?

Thank you for your help.

Here is a visualization to understand better the goal of the lookAt.

The goal is to find A' and B' coordinates, assuming we already know O, A and B ones. ([AB] is the segment we need to orient perpendicularly to the point O)

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Answer Source

To find positions of A' and B', you don't need rotate points (and dealt with angles at all).

```
Find vector OC = segmentCenterPoint - originPoint
Make normalized (unit) vector oc = OC / Length(OC)
Make perpendicular vector P = (-oc.Y, oc.X)
Find CB length lCB
Find A' = C + lCB * P
B' = C - lCB * P
```

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