In this example I'm going to use the same rhino as we've seen above, but now I'm going to slice its head out and composite it into a picture frame. The image of the picture frame includes a dropshadow which has been saved as a 24-bit PNG image. Because 24-bit PNG images include a full 8-bit alpha channel, unlike GIF and 8-bit PNG images, I can place it onto any background and don't have to worry about a matte color.

在这个例子里面我用到上面已经用过的犀牛图像，不过这次我要给犀牛头做个切片特写，然后合成到一个相框里面去。相框带有阴影效果，是一个以 24-bit PNG 格式保存的图像。因为 24-bit PNG 图像带有一个完整的 8-bit alpha 通道，与 GIF 和 8-bit PNG 不同，我可以将它放成背景而不必担心底色的问题。

I took a different approach to the loading of the images than the example above. I just placed the images directly in my HTML document and used a CSS rule to hide them from view (display:none). I assigned both images an id attribute to make them easier to select. The script itself is very simple. I first draw the sliced and scaled image on the canvas (first drawImage statement), and then place the frame on top (second drawImage statement).

我用一个与上面用到的不同的方法来装载图像，直接将图像插入到 HTML 里面，然后通过 CSS 隐藏(display:none)它。两个图像我都赋了 id ，方便后面使用。看下面的脚本，相当简单，首先对犀牛头做好切片(第一个 drawImage )放在 canvas 上，然后再上面套个相框(第二个 drawImage )。

function draw() { var canvas = document.getElementById('canvas'); var ctx = canvas.getContext('2d'); // Draw slice ctx.drawImage(document.getElementById('source'), 33,71,104,124,21,20,87,104); // Draw frame ctx.drawImage(document.getElementById('frame'),0,0); } Art gallery example

In the final example of this chapter I've made a little art gallery. The gallery consists of a table containing several images. When the page is loaded, for each image in the page a canvas element is inserted and a frame is drawn arround it.

In my case, all images have a fixed width and height, and so does the frame that's drawn around it. You could enhance the script so that it uses the image's width and height to make the frame fit perfectly around it.

The code below should be self-explanatory. We loop through the images array and add new canvas elements accordingly. Probably the only thing to note, for those not so familar with the DOM, is the use of the insertBefore method. insertBefore is a method of the parent node (a table cell) of the element (the image) before which we want to insert our new node (the canvas element).