媒介
初学canvas,做了一个画板运用,地点点这里 。本篇为canvas的一些基本头脑和注重事项,不是基本api。主假如在于touch事宜上的实践经验
retina屏兼容
retina屏会运用多个物理像素衬着一个自力像素,致使一倍图在retina屏幕上隐约,canvas也是如许,所以我们应当把canvas画布的大小设为canvas元素大小的2或3倍。元素大小在css中设置
const canvas = selector('#canvas')
const ctx = canvas.getContext('2d')
const RATIO = 3
const canvasOffset = canvas.getBoundingClientRect()
canvas.width = canvasOffset.width * RATIO
canvas.height = canvasOffset.height * RATIO
坐标系转化
把相对于浏览器窗口的坐标转化为canvas坐标,须要注重的是,假如兼容了retina,须要乘上devicePixelRatio。背面一切涌现的坐标,都要经由过程这个函数转化
function windowToCanvas (x, y) {
return {
x: (x - canvasOffset.left) * RATIO,
y: (y - canvasOffset.top) * RATIO
}
}
不得不提的是,《HTML5 Canvas核心技术》有一个雷同的函数,然则书上谁人是错的(也有能够我看的那本是假书)
猎取touch点的坐标
function getTouchPosition (e) {
let touch = e.changedTouches[0]
return windowToCanvas(touch.clientX, touch.clientY)
}
画布状况的贮存和恢复
举行画图操纵时,我们会频仍设置canvas画图环境的属性(线宽,色彩等),大多数情况下我们只是暂时设置,比方画蓝色的线段,又要画一个赤色的正方形,为了不影响两个画图操纵,我们须要在每次绘制时,先保留环境属性(save),画图终了后恢复(restore)
ctx.save()
ctx.fillStyle = "#333"
ctx.strokeStyle = "#666"
ctx.restore()
绘制外表的贮存与恢复
重要用于暂时性的画图操纵,比方用手指拖出一个方形时,起首要在touchstart事宜里贮存拖动最先时的绘制外表(getImageData),touchmove的事宜函数中,起首要先恢复touch最先时的画图外表(putImageData),再依据当前的坐标值画出一个方形,继承拖动时,适才画出的方形会被事宜函数的恢复画图外表覆蓋掉,在从新绘制一个方形,所以不管怎样拖动,我们看到的只是画了一个方形,下面是画板demo中方形东西的类
// 东西基本 宽度,色彩,是不是在绘画中,是不是被选中
class Basic {
constructor (width = RATIO, color = '#000') {
this.width = width
this.color = color
this.drawing = false
this.isSelect = false
}
}
class Rect extends Basic {
constructor (width = RATIO, color = '#000') {
super(width, color)
this.startPosition = {
x: 0,
y: 0
}
this.firstDot = ctx.getImageData(0, 0, canvasWidth, canvasHeight)
}
begin (loc) {
this.firstDot = ctx.getImageData(0, 0, canvasWidth, canvasHeight) //在这里贮存画图外表
saveImageData(this.firstDot)
Object.assign(this.startPosition, loc)
ctx.save() // 贮存画布状况
ctx.lineWidth = this.width
ctx.strokeStyle = this.color
}
draw (loc) {
ctx.putImageData(this.firstDot, 0, 0) //恢复画图外表,并最先绘制方形
const rect = {
x: this.startPosition.x <= loc.x ? this.startPosition.x : loc.x,
y: this.startPosition.y <= loc.y ? this.startPosition.y : loc.y,
width: Math.abs(this.startPosition.x - loc.x),
height: Math.abs(this.startPosition.y - loc.y)
}
ctx.beginPath()
ctx.rect(rect.x, rect.y, rect.width, rect.height)
ctx.stroke()
}
end (loc) {
ctx.putImageData(this.firstDot, 0, 0)
const rect = {
x: this.startPosition.x <= loc.x ? this.startPosition.x : loc.x,
y: this.startPosition.y <= loc.y ? this.startPosition.y : loc.y,
width: Math.abs(this.startPosition.x - loc.x),
height: Math.abs(this.startPosition.y - loc.y)
}
ctx.beginPath()
ctx.rect(rect.x, rect.y, rect.width, rect.height)
ctx.stroke()
ctx.restore() //恢复画布状况
}
bindEvent () {
canvas.addEventListener('touchstart', (e) => {
e.preventDefault()
if (!this.isSelect) {
return false
}
this.drawing = true
let loc = getTouchPosition(e)
this.begin(loc)
})
canvas.addEventListener('touchmove', (e) => {
e.preventDefault()
if (!this.isSelect) {
return false
}
if (this.drawing) {
let loc = getTouchPosition(e)
this.draw(loc)
}
})
canvas.addEventListener('touchend', (e) => {
e.preventDefault()
if (!this.isSelect) {
return false
}
let loc = getTouchPosition(e)
this.end(loc)
this.drawing = false
})
}
}
椭圆的绘制要领(匀称紧缩法)
道理是在紧缩过的坐标系中绘制一个圆形,那看起来就是一个椭圆了。由于是经由过程拖动绘制椭圆,所以在我们拖动时,必定拖出了一个方形,那实在就是以方形的中间为圆心,较长边的一半为半径画圆,这个圆要画在紧缩过的坐标系中,紧缩比例就是较窄边与较长边的比,圆心的坐标也要依据紧缩比例做坐标变更,圆形东西类代码以下
class Round extends Basic{
constructor (width = RATIO, color = '#000') {
super(width, color)
this.startPosition = {
x: 0,
y: 0
}
this.firstDot = ctx.getImageData(0, 0, canvasWidth, canvasHeight)
}
drawCalculate (loc) {
ctx.save()
ctx.lineWidth = this.width
ctx.strokeStyle = this.color
ctx.putImageData(this.firstDot, 0, 0) //恢复画图外表
const rect = {
width: loc.x - this.startPosition.x,
height: loc.y - this.startPosition.y
} // 盘算方形的宽高(带有正负值)
const rMax = Math.max(Math.abs(rect.width), Math.abs(rect.height)) // 选出较长边
rect.x = this.startPosition.x + rect.width / 2 // 盘算紧缩前的圆心坐标
rect.y = this.startPosition.y + rect.height / 2
rect.scale = {
x: Math.abs(rect.width) / rMax,
y: Math.abs(rect.height) / rMax
} // 盘算紧缩比例
ctx.scale(rect.scale.x, rect.scale.y)
ctx.beginPath()
ctx.arc(rect.x / rect.scale.x, rect.y / rect.scale.y, rMax / 2, 0, Math.PI * 2)
ctx.stroke()
ctx.restore()
}
begin (loc) {
this.firstDot = ctx.getImageData(0, 0, canvasWidth, canvasHeight) //贮存画图外表
saveImageData(this.firstDot)
Object.assign(this.startPosition, loc)
}
draw (loc) {
this.drawCalculate(loc)
}
end (loc) {
this.drawCalculate(loc)
}
bindEvent () {
canvas.addEventListener('touchstart', (e) => {
e.preventDefault()
if (!this.isSelect) {
return false
}
this.drawing = true
let loc = getTouchPosition(e)
this.begin(loc)
})
canvas.addEventListener('touchmove', (e) => {
e.preventDefault()
if (!this.isSelect) {
return false
}
if (this.drawing) {
let loc = getTouchPosition(e)
this.draw(loc)
}
})
canvas.addEventListener('touchend', (e) => {
e.preventDefault()
if (!this.isSelect) {
return false
}
let loc = getTouchPosition(e)
this.end(loc)
this.drawing = false
})
}
}
打消操纵
上述例子中都有个 saveImageData() 函数,这个函数是把当前画图外表贮存在一个数组中,点击打消的时刻用于恢复上一步的画图外表
const lastImageData = []
function saveImageData (data) {
(lastImageData.length == 5) && (lastImageData.shift()) // 上限为贮存5步,太多了怕挂掉
lastImageData.push(data)
}
document.getElementById("cancel").addEventListener('click', () => {
if(lastImageData.length < 1) return false
ctx.putImageData(lastImageData[lastImageData.length - 1], 0, 0)
lastImageData.pop()
})
总结
有一些看上去嵬峨上的东西,了解了今后就会发明很简单,有了基本的模子今后,再去一点一点雄厚功用,所以有些时刻不能老是看看看,一定要着手,yeah
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