Canvas drawImage scaling

Question

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.

score 116 · Accepted Answer · answered May 31 '12 at 22:54

116

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

answered May 31 '12 at 22:54

Gaurav

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For best performance, make sure your image is saved with the correct width and height and preloaded. If that is not possible, you can fall back to the other answers below. – Gaurav Jul 14 '20 at 14:28

score 8 · Answer 2 · edited Mar 02 '22 at 04:48

@TechMaze's solution is quite good.

Here is the code after some corrections and the introduction of the image.onload event. image.onload is essential in order to avoid 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);
  }
}

score 3 · Answer 3 · edited Mar 02 '22 at 04:50

And if you want to properly scale the picture as per the screen size, here is the code you can use:

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);

If you are not using jQuery, replace $(window).width with the appropriate equivalent option.

Canvas drawImage scaling

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