網頁上也能做刮刮樂!運用 HTML5 canvas 合成功能

我就先不賣關子,底下這個 fiddle 就是這篇文章的刮刮樂範例完整版。
可以玩玩看,試著瞭解裡面的程式腳本,或是 fork 來改改看。
有興趣但看不太懂 code 的話, 就跟著本篇文章的介紹帶你入門吧!

如果大家有看過九月份的「Firefox OS 讓你儘情享受每一刻」活動網頁(註 1),

應該很好奇一開始的刮刮樂是如何做到的吧?
在研究該如何實作這個功能的時候,因為刮刮樂和繪圖的概念很類似,
所以第一直覺就是想到要用 HTML5 Canvas。

但 Canvas 如此博大精深,究竟要從何做起呢?讓我們繼續看下去……

瞭解 Canvas 的合成參數設定

CanvasglobalCompositeOperation 屬性設定可以打破原本圖形只能依繪製順序往上覆蓋的限制,它可以將圖形畫在另一圖形之下,也可用來遮蔽、清除圖形區域。讓圖形的繪製組合更有彈性。在 MDN 的 Canvas 合成效果教學文件(註 2)裡可以看到各種不同合成參數的執行效果:

canvas 合成參數圖例

(注意其中 darker 已從 canvas 標準規範中移除並不再支援)

圖例中藍色是設定合成參數之前繪製上去的,也就是 destination;
紅色則是設定合成參數之後繪製上去的,也就是 source。

哇靠 12 種也太多了吧!簡直是大海撈針!
但冷靜下來仔細想想,其實刮刮樂不就和小畫家的橡皮擦功能差不多嗎?
如果藍色方塊是我想要擦掉的圖形,
而紅色的圓圈是橡皮擦工具,刮完後剩下的不就是一個缺圓角的方塊了嗎?
因此答案很明顯了:登登登- destiniation-out 就是你啦!

用滑鼠刮掉圖案

要開始寫一個功能時,第一件要做的事絕對不是建立一個空白的 javascript 檔案,重新發明輪子,
網路上充滿了熱心的神人,你想得到的功能幾乎早就有人做出來並公開原始碼。
我們只需要想出對的關鍵字,去把它找出來就行了。
於是我在谷歌輸入「javascript scratcher」,運氣很不錯,
第一個搜尋結果(註 3)就是我需要的範本。

但仔細研究以後發現三個問題:

  1. 我想讓刮刮樂好刮一點,讓滑鼠經過之後就開始刮,而不需要點下滑鼠左鍵。
  2. 我的刮刮樂圖形是圓的,且有透明背景,套用在上面時透明部分會刮出不必要的顏色來。
  3. 刮完後我想用 CSS 在底圖上加上簡單動畫特效,底圖若載入 canvas 內就無法做到。

於是我做了一些修改:

首先我把程式裡處理 mouseup/mousedown 的部分刪去,只留下 mousemove 的部分,
用來判斷是否按下滑鼠的 flag 則一律設定成 true,如此一來就可以直接刮不需要按滑鼠左鍵。

接下來我把原本初始化時,傳進的前後景兩張圖的部分,改成只剩下刮刮樂的前景,
canvas 只用在處理刮除的前景部分,而背景則放在 canvas 外的另一個元素中。

最後因為原腳本中兩張圖時用的合成模式是 source-atop
這部分改成前面提到的 destination-out。並把原本處理兩張圖合成的部分刪去。
如此一來就可以做出刮刮樂(橡皮擦)的效果。

有了這段刮刮樂腳本後,只要像 fiddle 中那樣把 HTML 元素和其 CSS 寫好,
再加入這段程式腳本,你就有一個基本的網頁刮刮樂了!

上面的腳本中 "fx-scratcher"canvas 元素的 id 。你也可以換成自訂的 id
scratcherImage 是刮刮樂圖片(灰色圓形圖)的檔案路徑。
在前面的 fiddle 中因為 canvas 載入的外部圖片會因為安全性限制,
而無法進行我們下面要接著介紹的「取得刮除進度」。
所以我改成用 data url 的方式嵌入圖片,若在本機用自己的圖測試則用檔案路徑即可。

以上只點出修改的重點,完整的變動差異比較可以在這個 Gist Revisions 裡看到。

取得刮除進度

這個函式是原本的範本程式裡就寫好的,用途是取得目前刮剩下的部分比例。
我只把原本前面的 canvas.draw 改成 canvas.main
因為前面修改過後我們只需要一個 canvas 即可。

這段程式腳本的重點是透過 ctx.getImageData 將圖片轉為陣列資料,
再用迴圈去檢查陣列內還留有顏色的比例。
stride 參數是用來設定檢查的頻率,數字愈大則檢查的頻率愈低,
避免每次都要掃過所有資料影響執行效能。
pdata[i] = 0 代表已被刮除。

腳本中的函式最後回傳的是一個介於 1 ~ 0 之間的小數,
代表的意義是「未刮除部分所佔比例」。

瞭解 fullAmount 的運作方式和回傳值意義之後,
接著只要在 scratch 事件發生時去檢查 fullAmount 的回傳值,
即可依據目前的刮除進度做出對應的效果。

上面這段腳本是判斷剩餘未刮除的部分小於 3% 時就當作刮除完成,
並在刮刮樂元素上加上 "complete"class
接下來的「開獎」效果只要透過 CSS 來處理即可。

One more thing – 做個假掰的開獎效果

我要做的開獎效果分成兩個部分,先是讓剩下沒刮完的部分淡出消失,
再來就是讓 Firefox 的圖示放大淡出,以做出開啟 App 的效果。

要用 CSS 達成這些效果就要靠 CSS 的轉場功能 - 也就是 transition 啦!
這裡腳本中使用的 transition 屬性是四個轉場屬性的簡易表示式,

這行裡指定的屬性依序為轉場套用屬性 (transition-property)、轉場持續時間 (transition-duration)、
轉場速率變化曲線函式 (transition-timing-function)、轉場延遲時間 (transition-delay)。
要注意的是 transition-property 若在不需要太多屬性同時進行轉場的狀況下,
建議明確列出所有需要轉場的屬性(而不要使用 all),以防執行效能被拖慢。
詳細的說明可參考 MDN 的 CSS 轉場文件(註 4)。

寫好轉場效果的設定後,接下來就是在 .scratcher.complete CSS 選擇器底下,
指定各別元素的樣式變化。只需要簡單幾行 CSS 就能達成。
完成之後,只要當刮除進度超過 97 %,並執行到這行腳本時:

CSS 就會偵測到 class 的改變,並開始執行漸變轉場效果。
是不是很有趣呢?你也來試著動手做做看吧!

 

參考連結:

 

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