Canvas drawImage scaling

问题

I'm trying to scale an image proportionately to the canvas. I'm able to scale it with fixed width and height as so:

context.drawImage(imageObj, 0, 0, 100, 100)

But I only want to resize the width and have the height resize proportionately. Something like the following:

context.drawImage(imageObj, 0, 0, 100, auto)

I've looked everywhere I can think of and haven't seen if this is possible.

回答1:

context.drawImage(imageObj, 0, 0, 100, 100 * imageObj.height / imageObj.width)

回答2:

solution of @TechMaze is quite good.

here is the code after some correctness and introduction of image.onload event. image.onload is too much essential in order to abstain from any kind of distortion.

function draw_canvas_image() {

var canvas = document.getElementById("image-holder-canvas");
var context = canvas.getContext("2d");
var imageObj = document.getElementById("myImageToDisplayOnCanvas");

imageObj.onload = function() {
  var imgWidth = imageObj.naturalWidth;
  var screenWidth  = canvas.width;
  var scaleX = 1;
  if (imgWidth > screenWidth)
      scaleX = screenWidth/imgWidth;
  var imgHeight = imageObj.naturalHeight;
  var screenHeight = canvas.height;
  var scaleY = 1;
  if (imgHeight > screenHeight)
      scaleY = screenHeight/imgHeight;
  var scale = scaleY;
  if(scaleX < scaleY)
      scale = scaleX;
  if(scale < 1){
      imgHeight = imgHeight*scale;
      imgWidth = imgWidth*scale;          
  }

  canvas.height = imgHeight;
  canvas.width = imgWidth;

  context.drawImage(imageObj, 0, 0, imageObj.naturalWidth, imageObj.naturalHeight, 0,0, imgWidth, imgHeight);
}
}

回答3:

And if you want to properly scale the picture as per screen size, here is the math you can do: IF YOU ARE NOT USING JQUERY, REPLACE $(window).width with appropriate equivalent option.

var imgWidth = imageObj.naturalWidth;
var screenWidth  = $(window).width() - 20; 
var scaleX = 1;
if (imageWdith > screenWdith)
    scaleX = screenWidth/imgWidth;
var imgHeight = imageObj.naturalHeight;
var screenHeight = $(window).height() - canvas.offsetTop-10;
var scaleY = 1;
if (imgHeight > screenHeight)
    scaleY = screenHeight/imgHeight;
var scale = scaleY;
if(scaleX < scaleY)
    scale = scaleX;
if(scale < 1){
    imgHeight = imgHeight*scale;
    imgWidth = imgWidth*scale;          
}
canvas.height = imgHeight;
canvas.width = imgWidth;
ctx.drawImage(imageObj, 0, 0, imageObj.naturalWidth, imageObj.naturalHeight, 0,0, imgWidth, imgHeight);

来源：https://stackoverflow.com/questions/10841532/canvas-drawimage-scaling