Canvas drawImage缩放

Question

我正在尝试根据画布按比例缩放图像。我可以用固定的宽度和高度缩放它，如下所示：

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

但我只想按比例调整宽度和高度。类似于以下内容：

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

我已经找遍了我能想到的所有地方，但还没有看到这是否可能。

Answer 1

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

Answer 2

@TechMaze的解决方案非常好。

下面是在对image.onload事件进行一些更正和介绍之后的代码。为了避免任何形式的扭曲，image.onload是非常重要的。

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);
}
}

Answer 3

如果你想根据屏幕尺寸适当地缩放图片，下面是你可以做的数学运算:如果你没有使用JQUERY，用适当的等价选项替换$(window).width。

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);