[Gambas-user] algorithm for scaling 2D coordinates centered on an arbitrary point

[Kevin Fishburne]

On 08/17/2011 12:48 AM, Kevin Fishburne wrote:
> I'm sure this is ridiculously simple, but my simple brain can't seem 
> to figure it out.
>
> I have a set of screen coordinates, xPos and yPos. I need to adjust 
> their positions based on a zoom level, CameraZoom. The zoom level is 
> relative to the center of the screen (swidth / 2, sheight / 2), and 
> the zoom level range is from 1 to 2. If the zoom is at 1, the x and y 
> coordinates wouldn't change. If the zoom is at 2, they would be half 
> their previous distance from the center of the screen. This simulates 
> the positions of objects changing as the camera zooms in and out.
>
> There has to be some simple equation to produce this. I've attached 
> the procedure that needs this just for reference. Any clues 
> appreciated, as always. 

Maybe I'm not quite as dumb as I thought. ;) I got it.

I needed to do this:

   ' Assign initial values to variables
   CameraZoom = 1 / Zoom.Current

   ' Modify screen coordinates for zoom level.
   xPos = (swidth / 2) + ((xPos - (swidth / 2)) / CameraZoom)
   yPos = (sheight / 2) + ((yPos - (sheight / 2)) / CameraZoom)

   ' Modify dimensions for zoom level.
   Width = Width / CameraZoom
   Height = Height / CameraZoom

I could probably just multiply by Zoom.Current instead of dividing by 
CameraZoom, but whatever. Hopefully someone will find the algorithm 
useful someday.

-- 
Kevin Fishburne
Eight Virtues
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