画布里如果超出一张图片  clip裁剪图片效果失效   如果只有一张图片  裁剪可以使用  超出一张就GG了

教程也没有说明

canvas: function (e) {

var that = this

var width = 0

var height = 0

var user_img = that.data.user_img

// 用户名字

var user_name = wx.getStorageSync('userInfo').name

console.log(that.data.poster)

// 活动

var activity = that.data.activity

wx.getSystemInfo({

success: res => {

console.log(res)

width = res.windowWidth

height = res.windowHeight

that.setData({

width: res.windowWidth,

height: res.windowHeight - 50

})

}

})

console.log('可使用的宽度为' + width + 'px')

console.log('可使用的高度为' + height + 'px')

// 宽度比例

var ratio = width / 1080

ratio = ratio.toFixed(2)

// 白色背景

var white_back = 200 * ratio

// 背景图片高度

var image_width = width

var image_height = 320 * ratio

// 用户logo

var user_logo = 110 * ratio

var user_height = 325 * ratio

var info = 409 * ratio

// 标签高度

var biaoqian = 720 * ratio

var biaoqian_text = 780 * ratio

// 名字高度

var name_height = 410 * ratio

// 活动信息

var ac_name = 990 * ratio

var ac_address = 1150 * ratio

var ac_time = 1310 * ratio

// 二维码

var code_text = 610 * ratio

var code_height = 600 * ratio

var inte_height = 550 * ratio

// 二维码高度

var poster_width = 300 * ratio

var poster_height = 250 * ratio

// 图片的自身宽高

var acti_logo_width = poster_width*0.45

var acti_logo_hei = poster_height + poster_width * 0.27

// 计算裁剪位置

var cutting_right = code_text + 10 + acti_logo_width

var cutting_height = poster_height

// 图片宽度

var img_width = that.data.img_width

var img_height = that.data.img_height

var proportion = img_height / img_width

if (img_width < width) {

img_width = width

var img_wid = width * proportion

var img_cut = img_height - 300

img_cut = img_cut / 2

var img_left = img_width - width

img_left = img_left / 2

} else {

var img_left = img_width - width

img_left = img_left / 2

if (img_height > 300) {

var img_cut = img_height - 300

img_cut = img_cut / 2

} else {

var img_cut = 0

}

}

var ctx = wx.createCanvasContext('firstCanvas')

ctx.setFillStyle('#fff')

ctx.rect(0, 0, width, h