html - How to keep viewBox centered when "zooming" in SVGs? -


often use viewbox attribute "zoom" svg element. zoom accomplished of course using lower width , height values viewbox attribute - x , y values tricky figure out keeping scene centered.

below snippet viewbox="0 0 100 100". ( x y width height ) starting point. scene scale , starts @ top left corner.

<head>    <link rel="stylesheet" href="https://codepen.io/basement/pen/oepkxy.css">  </head>    <body>     <iframe src="https://s.codepen.io/basement/debug/zdpvyv/pnavylzmjrqr"            id="grid"    ></iframe>      <div class="wrp">             <!-- svg relevant code below-->      <svg xmlns="http://www.w3.org/2000/svg"           viewbox="0 0 100 100"           width="100" height="100"           class="canvas"      >         <defs>             <style type="text/css">               circle {              fill: #0ff;              fill-opacity: 0.25;              stroke-width: 0.25;              stroke: #0ee;            }            </style>               </defs>          <circle cx="37.5" cy="50" r="25" />        <circle cx="62.5" cy="50" r="25" />        <circle cx="50" cy="71.65" r="25" />          </svg>          </div>  </body>

( viewable in full page. snippets responsive )

zooming in

changing width , height values 50 in viewbox zooms scene. had adjust x , y values both half viewbox dimensions: 25 keep drawing centered.

viewbox="25 25 50 50"

<head>    <link rel="stylesheet" href="https://codepen.io/basement/pen/oepkxy.css">  </head>    <body>     <iframe src="https://s.codepen.io/basement/debug/zdpvyv/pnavylzmjrqr"            id="grid"    ></iframe>      <div class="wrp">             <!-- svg relevant code below-->      <svg xmlns="http://www.w3.org/2000/svg"           viewbox="25 25 50 50"           width="100" height="100"           class="canvas"      >         <defs>             <style type="text/css">               circle {              fill: #0ff;              fill-opacity: 0.25;              stroke-width: 0.25;              stroke: #0ee;            }            </style>               </defs>          <circle cx="37.5" cy="50" r="25" />        <circle cx="62.5" cy="50" r="25" />        <circle cx="50" cy="71.65" r="25" />          </svg>          </div>  </body>

halving width , height isn't consistent

following pattern of halving width , height center scene:

viewbox="12.5 12.5 25 25" should continue zoom , stay centered. doesn't stay centered. question formula or technique svg use can figure out mathematically x , y value need center specific width , height value in viewbox?

note: i'm aware of libraries snap.svg , raphaël. i'm avoiding libraries in effort understand fundamentals.

one more thing: i'm asking if have width , height value in viewbox how find center coordinates of scene. not vice versa.

external stylesheets included visual cognition. relevant code question pertaining svg elements.

your calculations wrong. if width , height of viewbox smaller, x , y values need bigger.

initial viewbox:  0 0 100 100 zoom x2:          25 25 50 50 zoom x4:          37.5 37.5 25 25

to x or y values, need subtract half new width or height halfway point of last (or original) viewbox.

zoom x2:  centre - newwidth/2           = (100/2) - (50/2) = 25 zoom x4:  (100/2) - (25/2) = 37.5, or           (25 + 50/2) - (25/2) = 37.5

another waty calculate subtract current width original width , divide two.

zoom x4:  (100 - 25) / 2 = 37.5 

<head>    <link rel="stylesheet" href="https://codepen.io/basement/pen/oepkxy.css">  </head>    <body>     <iframe src="https://s.codepen.io/basement/debug/zdpvyv/pnavylzmjrqr"            id="grid"    ></iframe>      <div class="wrp">             <!-- svg relevant code below-->      <svg xmlns="http://www.w3.org/2000/svg"           viewbox="37.5 37.5 25 25"           width="100" height="100"           class="canvas"      >         <defs>             <style type="text/css">               circle {              fill: #0ff;              fill-opacity: 0.25;              stroke-width: 0.25;              stroke: #0ee;            }            </style>               </defs>          <circle cx="37.5" cy="50" r="25" />        <circle cx="62.5" cy="50" r="25" />        <circle cx="50" cy="71.65" r="25" />          </svg>          </div>  </body>


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