지금까지 우리는 Canvas를 가지고 스스로 도형을 만들고 그 도형에 스타일 적용 해 보았습니다. 이미지를 사용 하는 기능은 {{HTMLElement("canvas")}}의 가장 흥미로운 기능중 하나입니다. 이 기능은 게임의 그래픽 배경이나 혹은 다이나믹한 사진 도음들을 위해 사용 될 수 있습니다. PNG,GIF, JPEG등 브라우저에서 지원되는 포맷형태라면 어떠한 외부 이미지라도 사용 될 수 있습니다. 같은 페이지 소스에서 다른 Canvas요소로 만들어진 이미지 또한 사용할수 있습니다!
이미지를 캔버스로 불러오는것은 기본적으로 두 단계를 필요로 합니다:
drawImage() function을 사용하여 캔버스에 나타난 이미지 위에 그림을 그립니다.이 과정이 어떻게 되는지 봅시다.
canvas API는 아래의 데이터 타입을 이미지 소스로 사용 할 수 있습니다:
Image() constructor를 통해 만들어진 이미지입니다.이렇게 얻은 소스들은 {{domxref("CanvasImageSource")}}.를 사용하여 불러 올 수 있습니다.
다음은 캔버스에 놓인 이미지를 사용하는 여러가지 방법입니다.
우리는 다음을 사용하여 같은 페이지에 있는 캔버스나 이미지를 참고 할 수 있습니다.
Using the {{htmlattrxref("crossorigin", "img")}} attribute of an {{HTMLElement("img")}} element (reflected by the {{domxref("HTMLImageElement.crossOrigin")}} property), you can request permission to load an image from another domain for use in your call to drawImage(). If the hosting domain permits cross-domain access to the image, the image can be used in your canvas without tainting it; otherwise using the image will taint the canvas.
Just as with normal images, we access other canvas elements using either the {{domxref("document.getElementsByTagName()")}} or {{domxref("document.getElementById()")}} method. Be sure you've drawn something to the source canvas before using it in your target canvas.
One of the more practical uses of this would be to use a second canvas element as a thumbnail view of the other larger canvas.
Another option is to create new {{domxref("HTMLImageElement")}} objects in our script. To do this, you can use the convenient Image() constructor:
var img = new Image(); // Create new img element img.src = 'myImage.png'; // Set source path
When this script gets executed, the image starts loading.
If you try to call drawImage() before the image has finished loading, it won't do anything (or, in older browsers, may even throw an exception). So you need to be sure to use the load event so you don't try this before the image has loaded:
var img = new Image(); // Create new img element
img.addEventListener('load', function() {
// execute drawImage statements here
}, false);
img.src = 'myImage.png'; // Set source path
If you're only using one external image this can be a good approach, but once you need to track more than one we need to resort to something more clever. It's beyond the scope of this tutorial to look at image pre-loading tactics, but you should keep that in mind.
Another possible way to include images is via the data: url. Data URLs allow you to completely define an image as a Base64 encoded string of characters directly in your code.
var img = new Image(); // Create new img element img.src = 'data:image/gif;base64,R0lGODlhCwALAIAAAAAA3pn/ZiH5BAEAAAEALAAAAAALAAsAAAIUhA+hkcuO4lmNVindo7qyrIXiGBYAOw==';
One advantage of data URLs is that the resulting image is available immediately without another round trip to the server. Another potential advantage is that it is also possible to encapsulate in one file all of your CSS, JavaScript, HTML, and images, making it more portable to other locations.
Some disadvantages of this method are that your image is not cached, and for larger images the encoded url can become quite long.
You can also use frames from a video being presented by a {{HTMLElement("video")}} element (even if the video is not visible). For example, if you have a {{HTMLElement("video")}} element with the ID "myvideo", you can do this:
function getMyVideo() {
var canvas = document.getElementById('canvas');
if (canvas.getContext) {
var ctx = canvas.getContext('2d');
return document.getElementById('myvideo');
}
}
This returns the {{domxref("HTMLVideoElement")}} object for the video, which, as covered earlier, is one of the objects that can be used as a CanvasImageSource.
Once we have a reference to our source image object we can use the drawImage() method to render it to the canvas. As we will see later the drawImage() method is overloaded and has several variants. In its most basic form it looks like this:
CanvasImageSource specified by the image parameter at the coordinates (x, y).SVG images must specify a width and height in the root <svg> element.
In the following example, we will use an external image as the backdrop for a small line graph. Using backdrops can make your script considerably smaller because we can avoid the need for code to generate the background. In this example, we're only using one image, so I use the image object's load event handler to execute the drawing statements. The drawImage() method places the backdrop at the coordinate (0, 0), which is the top-left corner of the canvas.
<html> <body onload="draw();"> <canvas id="canvas" width="180" height="150"></canvas> </body> </html>
function draw() {
var ctx = document.getElementById('canvas').getContext('2d');
var img = new Image();
img.onload = function() {
ctx.drawImage(img, 0, 0);
ctx.beginPath();
ctx.moveTo(30, 96);
ctx.lineTo(70, 66);
ctx.lineTo(103, 76);
ctx.lineTo(170, 15);
ctx.stroke();
};
img.src = 'https://mdn.mozillademos.org/files/5395/backdrop.png';
}
The resulting graph looks like this:
{{EmbedLiveSample("Example_A_simple_line_graph", 220, 160, "https://mdn.mozillademos.org/files/206/Canvas_backdrop.png")}}
The second variant of the drawImage() method adds two new parameters and lets us place scaled images on the canvas.
width and height parameters, which indicate the size to which to scale the image when drawing it onto the canvas.In this example, we'll use an image as a wallpaper and repeat it several times on the canvas. This is done simply by looping and placing the scaled images at different positions. In the code below, the first for loop iterates over the rows. The second for loop iterates over the columns. The image is scaled to one third of its original size, which is 50x38 pixels.
Note: Images can become blurry when scaling up or grainy if they're scaled down too much. Scaling is probably best not done if you've got some text in it which needs to remain legible.
<html> <body onload="draw();"> <canvas id="canvas" width="150" height="150"></canvas> </body> </html>
function draw() {
var ctx = document.getElementById('canvas').getContext('2d');
var img = new Image();
img.onload = function() {
for (var i = 0; i < 4; i++) {
for (var j = 0; j < 3; j++) {
ctx.drawImage(img, j * 50, i * 38, 50, 38);
}
}
};
img.src = 'https://mdn.mozillademos.org/files/5397/rhino.jpg';
}
The resulting canvas looks like this:
{{EmbedLiveSample("Example_Tiling_an_image", 160, 160, "https://mdn.mozillademos.org/files/251/Canvas_scale_image.png")}}
The third and last variant of the drawImage() method has eight parameters in addition to the image source. It lets us cut out a section of the source image, then scale and draw it on our canvas.
image, this function takes the area of the source image specified by the rectangle whose top-left corner is (sx, sy) and whose width and height are sWidth and sHeight and draws it into the canvas, placing it on the canvas at (dx, dy) and scaling it to the size specified by dWidth and dHeight.
To really understand what this does, it may help to look at the image to the right. The first four parameters define the location and size of the slice on the source image. The last four parameters define the rectangle into which to draw the image on the destination canvas.
Slicing can be a useful tool when you want to make compositions. You could have all elements in a single image file and use this method to composite a complete drawing. For instance, if you want to make a chart you could have a PNG image containing all the necessary text in a single file and depending on your data could change the scale of your chart fairly easily. Another advantage is that you don't need to load every image individually, which can improve load performance.
In this example, we'll use the same rhino as in the previous example, but we'll slice out its head and composite it into a picture frame. The picture frame image is a 24-bit PNG which includes a drop shadow. Because 24-bit PNG images include a full 8-bit alpha channel, unlike GIF and 8-bit PNG images, it can be placed onto any background without worrying about a matte color.
<html>
<body onload="draw();">
<canvas id="canvas" width="150" height="150"></canvas>
<div style="display:none;">
<img id="source" src="https://mdn.mozillademos.org/files/5397/rhino.jpg" width="300" height="227">
<img id="frame" src="https://mdn.mozillademos.org/files/242/Canvas_picture_frame.png" width="132" height="150">
</div>
</body>
</html>
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);
}
We took a different approach to loading the images this time. Instead of loading them by creating new {{domxref("HTMLImageElement")}} objects, we included them as {{HTMLElement("img")}} tags directly in our HTML source and retrieved the images from those. The images are hidden from output by setting the CSS property {{cssxref("display")}} to none for those images.
{{EmbedLiveSample("Example_Framing_an_image", 160, 160, "https://mdn.mozillademos.org/files/226/Canvas_drawimage2.jpg")}}
The script itself is very simple. Each {{HTMLElement("img")}} is assigned an ID attribute, which makes them easy to select using {{domxref("document.getElementById()")}}. We then simply use drawImage() to slice the rhino out of the first image and scale him onto the canvas, then draw the frame on top using a second drawImage() call.
In the final example of this chapter, we'll build a little art gallery. The gallery consists of a table containing several images. When the page is loaded, a {{HTMLElement("canvas")}} element is inserted for each image and a frame is drawn around it.
In this case, every image has a fixed width and height, as does the frame that's drawn around them. 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 {{domxref("document.images")}} container and add new canvas elements accordingly. Probably the only thing to note, for those not so familiar with the DOM, is the use of the {{domxref("Node.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).
<html>
<body onload="draw();">
<table>
<tr>
<td><img src="https://mdn.mozillademos.org/files/5399/gallery_1.jpg"></td>
<td><img src="https://mdn.mozillademos.org/files/5401/gallery_2.jpg"></td>
<td><img src="https://mdn.mozillademos.org/files/5403/gallery_3.jpg"></td>
<td><img src="https://mdn.mozillademos.org/files/5405/gallery_4.jpg"></td>
</tr>
<tr>
<td><img src="https://mdn.mozillademos.org/files/5407/gallery_5.jpg"></td>
<td><img src="https://mdn.mozillademos.org/files/5409/gallery_6.jpg"></td>
<td><img src="https://mdn.mozillademos.org/files/5411/gallery_7.jpg"></td>
<td><img src="https://mdn.mozillademos.org/files/5413/gallery_8.jpg"></td>
</tr>
</table>
<img id="frame" src="https://mdn.mozillademos.org/files/242/Canvas_picture_frame.png" width="132" height="150">
</body>
</html>
And here's some CSS to make things look nice:
body {
background: 0 -100px repeat-x url(https://mdn.mozillademos.org/files/5415/bg_gallery.png) #4F191A;
margin: 10px;
}
img {
display: none;
}
table {
margin: 0 auto;
}
td {
padding: 15px;
}
Tying it all together is the JavaScript to draw our framed images:
function draw() {
// Loop through all images
for (var i = 0; i < document.images.length; i++) {
// Don't add a canvas for the frame image
if (document.images[i].getAttribute('id') != 'frame') {
// Create canvas element
canvas = document.createElement('canvas');
canvas.setAttribute('width', 132);
canvas.setAttribute('height', 150);
// Insert before the image
document.images[i].parentNode.insertBefore(canvas,document.images[i]);
ctx = canvas.getContext('2d');
// Draw image to canvas
ctx.drawImage(document.images[i], 15, 20);
// Add frame
ctx.drawImage(document.getElementById('frame'), 0, 0);
}
}
}
{{EmbedLiveSample("Art_gallery_example", 725, 400)}}
As mentioned previously, scaling images can result in fuzzy or blocky artifacts due to the scaling process. You can use the drawing context's {{domxref("CanvasRenderingContext2D.imageSmoothingEnabled", "imageSmoothingEnabled")}} property to control the use of image smoothing algorithms when scaling images within your context. By default, this is true, meaning images will be smoothed when scaled. You can disable this feature like this:
ctx.mozImageSmoothingEnabled = false; ctx.webkitImageSmoothingEnabled = false; ctx.msImageSmoothingEnabled = false; ctx.imageSmoothingEnabled = false;
{{PreviousNext("Web/API/Canvas_API/Tutorial/Drawing_text", "Web/API/Canvas_API/Tutorial/Transformations")}}