Change Image() object dimensions (width and height)

问题

I am attempting to resize an image's dimensions, whilst retaining it's aspect ratio. This seems like a very simple task, but I cannot find a rellevent answer anywhere.. (relating to JavaScript's Image() object). I'm probably missing something very obvious. Below is what I am trying to achieve:

var img = new window.Image();
img.src = imageDataUrl;
img.onload = function(){
    if (img.width > 200){
        img.width = 200;
        img.height = (img.height*img.width)/img.width;
    }
    if (img.height > 200){
        img.height = 200;
        img.width = (img.width*img.height)/img.height;
    }
};

This is to proportionally resize an image before being drawn onto a canvas like so: context.drawImage(img,0,0,canvas.width,canvas.height);.However it would appear I cannot directly change Image()dimensions, so how is it done? Thanks.

Edit: I haven't correctly solved the image proportions using cross-multiplication. @markE provides a neat method of obtaining the correct ratio. Below is my new (working) implementation:

var scale = Math.min((200/img.width),(200/img.height));
img.width = img.width*scale;
img.height = img.height*scale;
clearCanvas(); //clear canvas
context.drawImage(img,0,0,img.width,img.height);

回答1:

Here's how to scale your image proportionally:

function scalePreserveAspectRatio(imgW,imgH,maxW,maxH){
    return(Math.min((maxW/imgW),(maxH/imgH)));
}

Usage:

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");

var img=new Image();
img.onload=start;
img.src="https://dl.dropboxusercontent.com/u/139992952/stackoverflow/balloon.png";
function start(){

  canvas.width=100;
  canvas.height=100;

  var w=img.width;
  var h=img.height;

  // resize img to fit in the canvas 
  // You can alternately request img to fit into any specified width/height
  var sizer=scalePreserveAspectRatio(w,h,canvas.width,canvas.height);

  ctx.drawImage(img,0,0,w,h,0,0,w*sizer,h*sizer);

}

function scalePreserveAspectRatio(imgW,imgH,maxW,maxH){
  return(Math.min((maxW/imgW),(maxH/imgH)));
}

body{ background-color: ivory; }
canvas{border:1px solid red;}

<h4>Original Balloon image resized to fit in 100x100 canvas</h4>
<canvas id="canvas" width=100 height=100></canvas>

回答2:

The image dimensions are set in the constructor

new Image(width, height)
// for proportionally consistent resizing use new Image(width, "auto")

the context.drawImage()

the arguments are as follows:

context.drawImage(image source, x-coordinate of upper left portion of image,
y-coordinate of upper left portion of image,image width,image height);

simply position the the image with the first two numeric coordinates and then manually adjust the size with the last two (width,height)

//ex.
var x = 0;
var y = 0;
var img = new Image (200, "auto");
    img.src = "xxx.png";
context.drawImage(img,x,y,img.width,img.height);

来源：https://stackoverflow.com/questions/35613799/change-image-object-dimensions-width-and-height

标签

javascript

canvas

aspect-ratio