什么叫橡皮筋式
指畫(huà)圖時(shí)橡皮筋一樣伸縮自如。。
例子如下:point_down:
思路
思路很簡(jiǎn)單，只有橡皮筋式繪制功能要注意，以下總結(jié)mousedown，mousemove，mouseup三個(gè)階段的思路
mousedown：記錄start位置，drag（記錄是否處于拖動(dòng)狀態(tài)）設(shè)置為true，getImageData( 橡皮筋效果關(guān)鍵1 )
mousemove：獲取拖動(dòng)時(shí)的位置pos，putImageData( 對(duì)應(yīng)getImageData，橡皮筋效果關(guān)鍵2 )，根據(jù)pos與start畫(huà)直線
mouseup：drag恢復(fù)為false
關(guān)鍵 就在于putImageData()與getImageData()這兩個(gè)canvas的方法，putImageData()記錄了鼠標(biāo)點(diǎn)下時(shí)的圖像，getImageData()對(duì)應(yīng)還原。如果沒(méi)有執(zhí)行這兩個(gè)方法就會(huì)出現(xiàn)以下的效果
putImageData()相當(dāng)于把“掃描”出來(lái)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(shù)據(jù)
let start = new Map([['x',null],['y',null]]);
let drag = false;//記錄是否處于拖動(dòng)狀態(tài)
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(dāng)于把掃描出來(lái)的線都擦掉，重新畫(huà)
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(biāo)點(diǎn)擊位置
return {
x:e.clientX - canvasLeft,
y:e.clientY - canvasTop
}
}
</script>
以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持腳本之家。
