c++ - Reposition rectangle after zooming -

i have rectangle defined r1:x1,y1-x2,y2 , after applying zoom, rectangle r2:x1,y1-x2,y2.

+--------------+---+ |              |   | |     r1       |   | |              |   | +--------------+   | |              r2  | +------------------+ 

as can see, r2 expanded based on r1's origin. not desired effect.

what want accomplish re-calculate origin based on mouse pointer when zooming operation performed.

for example:

   +-----------------------+     | +-----------------+   |    | |  o              |   |    | |       r1        |   |    | |                 |   |    | +-----------------+   |    |            r2         |    +-----------------------+ 

here, mouse pointer set on point "o", zoomed, resulting on rectangle r2. please note r2 not centered on r1, little displaced right , bottom.

how can re-position origin after zooming?.

this isn't programming specific question, math problem.

if mouse in center of screen, each side expands equally. if mouse way 1 side, rectangle expands in direction. need determine size increase ratio.

so, need setup few variables here: width_delta, height_delta, mouse_x_relative, mouse_y_relative, rect_width, rect_height.

• width delta new width minus old width.
• height delta new height minus old height.
• mouse x relative x-coordinate relative rect's left side
• mouse y relative y-coordinate relative rect's top side

with each delta, when mouse centered, can calculate change in rectangles sides delta - delta / 2, , delta / 2. results in half of delta going 1 side, other half other. instead of dividing 2, need find out how relates mouse position , size of rect.

easy enough: rect_width / mouse_x_relative. let's rect width 10, mouse in center @ 5, 10 / 5 2, causing delta distributed equally both sides of rect. need divide delta rect_width / mouse_x_relative.

left_delta = width_delta / rect_width / mouse_x_relative right_delta = width_delta - left_delta  

but can clean be:

left_delta = width_delta * mouse_x_relative / rect_width right_delta = width_delta - left_delta  

i believe that should work expected behavior, unlike last answer. when zoom in (shrink) rect closes in on mouse (centering), when zoom out moves away mouse (un-centering, if will), inverse of way moved in.