PointF 90degrees from X

问题

Yesterday I asked a question - Draw arrow according to path

In that question I explained that I want to draw a arrow head in the direction of my onTouchEvent. I got a answer in the comments saying I should rotate the canvas.

Last night I got a new idea and this is where this question comes in.

I get the x position/coordinate in MotionEvent.ACTION_DOWN by calling event.getX(); inside my onTouchEvent.

I would like to know if it is possible to get a point/coordinate 90 degrees from x?

Here is a demonstration to give move clarity:

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point I want to get --> x1------------x

As shown above, x1 is what I want to get.

Also worth mentioning, the following will not work because the x axis might be at a angle:

x1 = x - value

EDIT:

I think I made the question unclear, sorry about that.

I'm going to create a scenario to explain myself better

In my onTouchEvent inside MotionEvent.ACTION_DOWN I set the starting x and y, lets call it xdown and ydown, by calling event.getX() and event.getY().

Then inside MotionEvent.ACTION_UP I get the x and y, same as above, lets call it xup and yup.

I then get the center point, lets call it centerx and centery.

It will now look like this:

               xdown, ydown
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              |centerx, centery
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               xup, yup

Now I want to get the x and y 90 degrees from centerx, centery, lets say at a distance of 40.

Another demonstaraiton:

                              xdown, ydown
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  nintyX,nintyY <------------|centerx, centery
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                              xup, yup

回答1:

Seems you have some direction and want to get perpendicular direction.

Components of direction vectors are (dx, dy)

dx = centerx - xup
dy = centery - yup

then components of left (note - "leftness" depends on coordinates system handness!) perpendicular vector are

 px =  - dy
 py =   dx

(px=dy and py=-dx if direction is wrong for you)

To get a point ninty at distance D in needed direction, normalize vector and multiply its components by D

lenp = sqrt(px*px + py*py)  
upx = px / lenp
upy = py / lenp
nintyX = centerx + upx * D 
nintyY = centery + upy * D

来源：https://stackoverflow.com/questions/53513066/pointf-90degrees-from-x

标签

java

android

android-canvas