canvas——橡皮筋式線條繪圖應用

什麼叫橡皮筋式

指畫圖時橡皮筋一樣伸縮自如。。
例子如下👇

思路

思路很簡單，只有橡皮筋式繪製功能要注意，以下總結mousedown，mousemove，mouseup三個階段的思路
mousedown：記錄start位置，drag（記錄是否處於拖動狀態）設置爲true，getImageData(橡皮筋效果關鍵1)
mousemove：獲取拖動時的位置pos，putImageData(對應getImageData，橡皮筋效果關鍵2)，根據pos與start畫直線
mouseup：drag恢復爲false
關鍵就在於putImageData()與getImageData()這兩個canvas的方法，putImageData()記錄了鼠標點下時的圖像，getImageData()對應還原。如果沒有執行這兩個方法就會出現以下的效果

putImageData()相當於把“掃描”出來的線都擦掉

代碼

    <canvas id="canvas" width="600" height="400" style="border: 1px solid black;"> </canvas>
    <script type="text/javascript">
        let canvas = document.getElementById('canvas'),
            ctx = canvas.getContext('2d'),
            canvasLeft = canvas.getBoundingClientRect().left, //getBoundingClientRect()獲取元素位置
            canvasTop = canvas.getBoundingClientRect().top;
        let imageData; //記錄圖像數據
        let start = new Map([['x',null],['y',null]]);
        let drag = false;//記錄是否處於拖動狀態
        canvas.onmousedown = function (e) {
            let pos = positionInCanvas(e, canvasLeft, canvasTop);
            start.set('x', pos.x);
            start.set('y', pos.y);
            drag = true;
            //記錄imageData
            imageData = ctx.getImageData(0,0,canvas.width,canvas.height);
        }
        canvas.onmousemove = function (e) {
            if(drag === true){
               let pos = positionInCanvas(e, canvasLeft, canvasTop);
               //相當於把掃描出來的線都擦掉，重新畫
               ctx.putImageData(imageData, 0, 0);
               ctx.beginPath();
               ctx.moveTo(start.get('x'), start.get('y'));
               ctx.lineTo(pos.x, pos.y);
               ctx.stroke();
            }

        }
        canvas.onmouseup = function  (e) {
            drag = false;
        }
        function positionInCanvas (e, canvasLeft, canvasTop) {//獲取canvas中鼠標點擊位置
         return {
                  x:e.clientX - canvasLeft,
                  y:e.clientY - canvasTop
          }       
        }    

    </script>