在画布上绘制多个矩形 [英] Draw multiple rectangle on canvas
本文介绍了在画布上绘制多个矩形的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我尝试在鼠标移动时在画布上添加多个矩形.但是，当我在图像上绘制矩形时，画布上的背面图像也会被清除.我不想删除它.我想要画布上有多个矩形而不清除画布图像.请检查下面的JavaScript代码.
I am trying to add multiple rectangle on canvas on mouse move. But when I am drawing rectangle on image, back image on canvas is also cleared. I don't want to remove it. I want multiple rectangle on canvas without clearing canvas image. Please check below JavaScript code.
var canvas = document.getElementById('canvasarea'),
context = canvas.getContext('2d');
base_image = new Image();
base_image.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg'; //Load Image ;
base_image.onload = function(){
context.drawImage(base_image,0,0);
}
var strokeWidth = 4;
drawCount = 1;
canvas.addEventListener("mousemove", function (e) {
drawRectangleOnCanvas.handleMouseMove(e);
}, false);
canvas.addEventListener("mousedown", function (e) {
drawRectangleOnCanvas.handleMouseDown(e);
}, false);
canvas.addEventListener("mouseup", function (e) {
drawRectangleOnCanvas.handleMouseUp(e);
}, false);
canvas.addEventListener("mouseout", function (e) {
drawRectangleOnCanvas.handleMouseOut(e);
}, false);
function reOffset(){
var BB=canvas.getBoundingClientRect();
recOffsetX=BB.left;
recOffsetY=BB.top;
}
var recOffsetX,recOffsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }
var isRecDown=false;
var startX,startY;
var rects=[];
var newRect;
var drawRectangleOnCanvas={
handleMouseDown:function(e){
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
startX=parseInt(e.clientX-recOffsetX);
startY=parseInt(e.clientY-recOffsetY);
// Put your mousedown stuff here
isRecDown=true;
},
handleMouseUp:function(e){
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-recOffsetX);
mouseY=parseInt(e.clientY-recOffsetY);
// Put your mouseup stuff here
isRecDown=false;
//if(!willOverlap(newRect)){
rects.push(newRect);
//}
drawRectangleOnCanvas.drawAll();
},
drawAll:function(){
context.clearRect(0, 0, canvas.width, canvas.height);
context.lineWidth=strokeWidth;
//context.strokeStyle=$('div.dropdown-menu').find('.selected').attr('data-color');
for(var i=0;i<rects.length;i++){
var r=rects[i];
context.strokeStyle = r.color;
context.globalAlpha=1;
context.strokeRect(r.left,r.top,r.right-r.left,r.bottom-r.top);
context.beginPath();
context.arc(r.left, r.top, 15, 0, Math.PI*2, true);
context.closePath();
context.fillStyle = r.color;
context.fill();
var text = drawCount;
context.fillStyle = "#fff";
var font = "bold " + 2 +"px serif";
context.font = font;
var width = context.measureText(text).width;
var height = context.measureText("w").width; // this is a GUESS of height
context.fillText(text, r.left - (width/2) ,r.top + (height/2));
}
},
handleMouseOut:function(e){
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-recOffsetX);
mouseY=parseInt(e.clientY-recOffsetY);
// Put your mouseOut stuff here
isRecDown=false;
},
handleMouseMove:function(e){
if(!isRecDown){return;}
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mouseX=parseInt(e.clientX-recOffsetX);
mouseY=parseInt(e.clientY-recOffsetY);
newRect={
left:Math.min(startX,mouseX),
right:Math.max(startX,mouseX),
top:Math.min(startY,mouseY),
bottom:Math.max(startY,mouseY),
color:"#000000"
}
drawRectangleOnCanvas.drawAll();
context.strokeStyle = "#000000";
context.lineWidth = strokeWidth;
context.globalAlpha=1;
context.strokeRect(startX,startY,mouseX-startX,mouseY-startY);
}
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<canvas id="canvasarea"></canvas>
推荐答案
清除画布后，您还需要在drawAll()方法中绘制图像. context.drawImage(base_image, 0, 0);
After clearing the canvas you need to draw the image as well in your drawAll() method. context.drawImage(base_image, 0, 0);
var canvas = document.getElementById('canvasarea'),
context = canvas.getContext('2d');
var base_image = new Image();
base_image.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg'; //Load Image ;
base_image.onload = function() {
context.drawImage(base_image, 0, 0);
}
function drawImage() {
}
var strokeWidth = 4;
drawCount = 1;
canvas.addEventListener("mousemove", function(e) {
drawRectangleOnCanvas.handleMouseMove(e);
}, false);
canvas.addEventListener("mousedown", function(e) {
drawRectangleOnCanvas.handleMouseDown(e);
}, false);
canvas.addEventListener("mouseup", function(e) {
drawRectangleOnCanvas.handleMouseUp(e);
}, false);
canvas.addEventListener("mouseout", function(e) {
drawRectangleOnCanvas.handleMouseOut(e);
}, false);
function reOffset() {
var BB = canvas.getBoundingClientRect();
recOffsetX = BB.left;
recOffsetY = BB.top;
}
var recOffsetX, recOffsetY;
reOffset();
window.onscroll = function(e) {
reOffset();
}
window.onresize = function(e) {
reOffset();
}
var isRecDown = false;
var startX, startY;
var rects = [];
var newRect;
var drawRectangleOnCanvas = {
handleMouseDown: function(e) {
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
startX = parseInt(e.clientX - recOffsetX);
startY = parseInt(e.clientY - recOffsetY);
// Put your mousedown stuff here
isRecDown = true;
},
handleMouseUp: function(e) {
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mouseX = parseInt(e.clientX - recOffsetX);
mouseY = parseInt(e.clientY - recOffsetY);
// Put your mouseup stuff here
isRecDown = false;
//if(!willOverlap(newRect)){
rects.push(newRect);
//}
drawRectangleOnCanvas.drawAll();
},
drawAll: function() {
context.clearRect(0, 0, canvas.width, canvas.height);
context.drawImage(base_image, 0, 0);
context.lineWidth = strokeWidth;
for (var i = 0; i < rects.length; i++) {
var r = rects[i];
context.strokeStyle = r.color;
context.globalAlpha = 1;
context.strokeRect(r.left, r.top, r.right - r.left, r.bottom - r.top);
context.beginPath();
context.arc(r.left, r.top, 15, 0, Math.PI * 2, true);
context.closePath();
context.fillStyle = r.color;
context.fill();
var text = drawCount;
context.fillStyle = "#fff";
var font = "bold " + 2 + "px serif";
context.font = font;
var width = context.measureText(text).width;
var height = context.measureText("w").width; // this is a GUESS of height
context.fillText(text, r.left - (width / 2), r.top + (height / 2));
}
},
handleMouseOut: function(e) {
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mouseX = parseInt(e.clientX - recOffsetX);
mouseY = parseInt(e.clientY - recOffsetY);
// Put your mouseOut stuff here
isRecDown = false;
},
handleMouseMove: function(e) {
if (!isRecDown) {
return;
}
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mouseX = parseInt(e.clientX - recOffsetX);
mouseY = parseInt(e.clientY - recOffsetY);
newRect = {
left: Math.min(startX, mouseX),
right: Math.max(startX, mouseX),
top: Math.min(startY, mouseY),
bottom: Math.max(startY, mouseY),
color: "#000000"
}
drawRectangleOnCanvas.drawAll();
context.strokeStyle = "#000000";
context.lineWidth = strokeWidth;
context.globalAlpha = 1;
context.strokeRect(startX, startY, mouseX - startX, mouseY - startY);
}
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<canvas id="canvasarea"></canvas>
这篇关于在画布上绘制多个矩形的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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