キャンバスの描画は、contain または cover モードで適応され、中央に配置されます。

        /**
         * @param {Number} sx fixes the x coordinate of the box, sy fixes the y left scale of the box
         * @param {Number} box_w the width of the fixed box, box_h the height of the fixed box
         * @param {Number} source_w the width of the original image, source_h the height of the original image
         * @return {Object} {drawImage's parameters, x coordinate, y coordinate, width and height of the scaled image}, corresponding to drawImage(imageResource, dx, dy, dWidth, dHeight)
         */
        function containImg(sx, sy , box_w, box_h, source_w, source_h){
            var dx = sx,
                dy = sy,
                dWidth = box_w,
                dHeight = box_h;
            if(source_w > source_h || (source_w == source_h && box_w < box_h)){
                dHeight = source_h*dWidth/source_w;
            dy = sy + (box_h-dHeight)/2;

            }else if(source_w < source_h || (source_w == source_h && box_w > box_h)){
                dWidth = source_w*dHeight/source_h;
                dx = sx + (box_w-dWidth)/2;
            }
            return{
                dx,
                dy,
                dWidth,
                dHeight
            }
        }


        var c=document.getElementById("myCanvas");
        var ctx = c.getContext("2d");
        ctx.fillStyle = '#e1f0ff';
        // fix the position and size of the box - the image needs to be placed inside this box
        ctx.fillRect(30, 30, 150, 200);

        var img = new Image();
        img.onload = function () {
            console.log(img.width,img.height);
            
            var imgRect = containImg(30,30,150,200,img.width,img.height);
            console.log('imgRect',imgRect);
            ctx.drawImage(img, imgRect.dx, imgRect.dy, imgRect.dWidth, imgRect.dHeight); 
            
        }
        img.src = ". /timg2.jpg";  
        //Note: In img preload mode, onload should be placed above the value assigned to src to avoid the situation where the onload event cannot be triggered if there is already a cache and the event in onload is not executed

        /**
         * @param {Number} box_w width of the fixed box, box_h height of the fixed box
         * @param {Number} source_w the width of the original image, source_h the height of the original image
         * @return {Object} {the intercepted image information}, corresponding to drawImage(imageResource, sx, sy, sWidth, sHeight, dx, dy, dWidth, dHeight) parameter
        */
        function coverImg(box_w, box_h, source_w, source_h){
            var sx = 0,
                sy = 0,
                sWidth = source_w,
                sHeight = source_h;
            if(source_w > source_h || (source_w == source_h && box_w < box_h)){
                sWidth = box_w*sHeight/box_h;
                sx = (source_w-sWidth)/2;
            }else if(source_w < source_h || (source_w == source_h && box_w > box_h)){
                sHeight = box_h*sWidth/box_w;
                sy = (source_h-sHeight)/2;
            }
            return{
                sx,
                sy,
                sWidth,
                sHeight
            }
        }


        var c=document.getElementById("myCanvas");
        var ctx = c.getContext("2d");
        ctx.fillStyle = '#e1f0ff';
        // fix the position and size of the box - the image needs to be placed inside this box
        ctx.fillRect(30, 30, 150, 200);

        var img = new Image();
        img.onload = function () {
            console.log(img.width,img.height);
            
            var imgRect = coverImg(150,200,img.width,img.height);
            console.log('imgRect',imgRect);
            ctx.drawImage(img, imgRect.sx, imgRect.sy, imgRect.sWidth, imgRect.sHeight, 30, 30, 150, 200); 
        }
        img.src = ". /timg2.jpg";  
        //Note: In img preload mode, onload should be placed above the value assigned to src to avoid the situation where the onload event cannot be triggered if there is already a cache, and thus the event in onload does not execute