[英]Canvas drawing off center, Erase doesn't work correctly
我写了一个JavaScript,允许用户用鼠标在HTML5画布(类似于MS Paint)上绘制。
现在,我有两个问题:
仅当HTML5 canvas元素位于网页的左上角(0,0)时,绘图功能才起作用,否则它根本不起作用或绘图偏离中心。
我无法删除图形。 当我擦除图形时,它会在再次开始绘制时立即擦除BUT,然后又恢复。
我的代码如下:
HTML画布
<canvas id="can1" width="500" height="500"></canvas>1
画布绘图JavaScript
// Variables
var x1;
var y1;
var isPressed = false;
var myCanvas;
var myContext;
function startCanvas() {
// Canvas stuff
myCanvas = document.getElementById("can1");
myContext = myCanvas.getContext("2d");
// Specify a black background, and white lines that are 3 pixels thick.
myContext.fillStyle = '#fff';
myContext.strokeStyle = '#fff';
myContext.fillRect(0, 0, 500, 500);
myContext.lineWidth = 3;
myContext.fill();
}
function functionMouseDown(e) {
// Get coordinates
x1 = e.clientX
y1 = e.clientY;
isPressed = true;
}
function functionMouseMove(e) {
// If mouse is down and moved start drawing line
if (isPressed == true) {
drawLine(e);
}
}
function functionMouseUp() {
// Stop drawing line
isPressed = false;
//myContext.closePath();
//myContext.stroke();
}
function drawLine(e) {
// Draw line
var x = e.clientX;
var y = e.clientY;
myContext.strokeStyle = '#cc0000';
myContext.lineWidth = 1;
myContext.moveTo(x1, y1);
myContext.lineTo(x, y);
myContext.stroke();
// Set start coordinates to current coordinates
x1 = x;
y1 = y;
}
我用来擦除画布的JavaScript:
myContext.clearRect(0, 0, 500, 500);
我使用以下功能来完成此操作
function relMouseCoords(event){/*needs fixing for general case*/
var totalOffsetX = 0
var totalOffsetY = 0
var canvasX = 0
var canvasY = 0
var currentElement = this
do{
totalOffsetX += currentElement.offsetLeft
totalOffsetY += currentElement.offsetTop
}
while(currentElement = currentElement.offsetParent)
canvasX = event.pageX - totalOffsetX
canvasY = event.pageY - totalOffsetY
return {x:canvasX, y:canvasY}
}
HTMLCanvasElement.prototype.relMouseCoords = relMouseCoords;
然后
var cord = e.target.relMouseCoords(e);
x1 = cord.x;
y1 = cord.y;
...
var cord = e.target.relMouseCoords(e);
var x = cord.x;
var y =cord.y;
最简单的解决方案是使用myCanvas.offsetLeft和myCanvas.offsetTop设置画布的偏移量。
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