HTML5 canvas 设置背景图

页面代码如下

canvas2image.js代码如下

/**
 * covert canvas to image
 * and save the image file
 */

var Canvas2Image = function () {

	// check if support sth.
	var $support = function () {
		var canvas = document.createElement('canvas'),
			ctx = canvas.getContext('2d');

		return {
			canvas: !!ctx,
			imageData: !!ctx.getImageData,
			dataURL: !!canvas.toDataURL,
			btoa: !!window.btoa
		};
	}();

	var downloadMime = 'image/octet-stream';

	function scaleCanvas (canvas, width, height) {
		var w = canvas.width,
			h = canvas.height;
		if (width == undefined) {
			width = w;
		}
		if (height == undefined) {
			height = h;
		}

		var retCanvas = document.createElement('canvas');
		var retCtx = retCanvas.getContext('2d');
		retCanvas.width = width;
		retCanvas.height = height;
		retCtx.drawImage(canvas, 0, 0, w, h, 0, 0, width, height);
		return retCanvas;
	}

	function getDataURL (canvas, type, width, height) {
		canvas = scaleCanvas(canvas, width, height);
		return canvas.toDataURL(type);
	}

	function saveFile (strData) {
		document.location.href = strData;
	}

	function genImage(strData) {
		var img = document.createElement('img');
		img.src = strData;
		return img;
	}
	function fixType (type) {
		type = type.toLowerCase().replace(/jpg/i, 'jpeg');
		var r = type.match(/png|jpeg|bmp|gif/)[0];
		return 'image/' + r;
	}
	function encodeData (data) {
		if (!window.btoa) { throw 'btoa undefined' }
		var str = '';
		if (typeof data == 'string') {
			str = data;
		} else {
			for (var i = 0; i < data.length; i ++) {
				str += String.fromCharCode(data[i]);
			}
		}

		return btoa(str);
	}
	function getImageData (canvas) {
		var w = canvas.width,
			h = canvas.height;
		return canvas.getContext('2d').getImageData(0, 0, w, h);
	}
	function makeURI (strData, type) {
		return 'data:' + type + ';base64,' + strData;
	}


	/**
	 * create bitmap image
	 * 按照规则生成图片响应头和响应体
	 */
	var genBitmapImage = function (oData) {

		//
		// BITMAPFILEHEADER: http://msdn.microsoft.com/en-us/library/windows/desktop/dd183374(v=vs.85).aspx
		// BITMAPINFOHEADER: http://msdn.microsoft.com/en-us/library/dd183376.aspx
		//

		var biWidth  = oData.width;
		var biHeight	= oData.height;
		var biSizeImage = biWidth * biHeight * 3;
		var bfSize  = biSizeImage + 54; // total header size = 54 bytes

		//
		//  typedef struct tagBITMAPFILEHEADER {
		//  	WORD bfType;
		//  	DWORD bfSize;
		//  	WORD bfReserved1;
		//  	WORD bfReserved2;
		//  	DWORD bfOffBits;
		//  } BITMAPFILEHEADER;
		//
		var BITMAPFILEHEADER = [
			// WORD bfType -- The file type signature; must be "BM"
			0x42, 0x4D,
			// DWORD bfSize -- The size, in bytes, of the bitmap file
			bfSize & 0xff, bfSize >> 8 & 0xff, bfSize >> 16 & 0xff, bfSize >> 24 & 0xff,
			// WORD bfReserved1 -- Reserved; must be zero
			0, 0,
			// WORD bfReserved2 -- Reserved; must be zero
			0, 0,
			// DWORD bfOffBits -- The offset, in bytes, from the beginning of the BITMAPFILEHEADER structure to the bitmap bits.
			54, 0, 0, 0
		];

		//
		//  typedef struct tagBITMAPINFOHEADER {
		//  	DWORD biSize;
		//  	LONG  biWidth;
		//  	LONG  biHeight;
		//  	WORD  biPlanes;
		//  	WORD  biBitCount;
		//  	DWORD biCompression;
		//  	DWORD biSizeImage;
		//  	LONG  biXPelsPerMeter;
		//  	LONG  biYPelsPerMeter;
		//  	DWORD biClrUsed;
		//  	DWORD biClrImportant;
		//  } BITMAPINFOHEADER, *PBITMAPINFOHEADER;
		//
		var BITMAPINFOHEADER = [
			// DWORD biSize -- The number of bytes required by the structure
			40, 0, 0, 0,
			// LONG biWidth -- The width of the bitmap, in pixels
			biWidth & 0xff, biWidth >> 8 & 0xff, biWidth >> 16 & 0xff, biWidth >> 24 & 0xff,
			// LONG biHeight -- The height of the bitmap, in pixels
			biHeight & 0xff, biHeight >> 8  & 0xff, biHeight >> 16 & 0xff, biHeight >> 24 & 0xff,
			// WORD biPlanes -- The number of planes for the target device. This value must be set to 1
			1, 0,
			// WORD biBitCount -- The number of bits-per-pixel, 24 bits-per-pixel -- the bitmap
			// has a maximum of 2^24 colors (16777216, Truecolor)
			24, 0,
			// DWORD biCompression -- The type of compression, BI_RGB (code 0) -- uncompressed
			0, 0, 0, 0,
			// DWORD biSizeImage -- The size, in bytes, of the image. This may be set to zero for BI_RGB bitmaps
			biSizeImage & 0xff, biSizeImage >> 8 & 0xff, biSizeImage >> 16 & 0xff, biSizeImage >> 24 & 0xff,
			// LONG biXPelsPerMeter, unused
			0,0,0,0,
			// LONG biYPelsPerMeter, unused
			0,0,0,0,
			// DWORD biClrUsed, the number of color indexes of palette, unused
			0,0,0,0,
			// DWORD biClrImportant, unused
			0,0,0,0
		];

		var iPadding = (4 - ((biWidth * 3) % 4)) % 4;

		var aImgData = oData.data;

		var strPixelData = '';
		var biWidth4 = biWidth<<2;
		var y = biHeight;
		var fromCharCode = String.fromCharCode;

		do {
			var iOffsetY = biWidth4*(y-1);
			var strPixelRow = '';
			for (var x = 0; x < biWidth; x++) {
				var iOffsetX = x<<2;
				strPixelRow += fromCharCode(aImgData[iOffsetY+iOffsetX+2]) +
							   fromCharCode(aImgData[iOffsetY+iOffsetX+1]) +
							   fromCharCode(aImgData[iOffsetY+iOffsetX]);
			}

			for (var c = 0; c < iPadding; c++) {
				strPixelRow += String.fromCharCode(0);
			}

			strPixelData += strPixelRow;
		} while (--y);

		var strEncoded = encodeData(BITMAPFILEHEADER.concat(BITMAPINFOHEADER)) + encodeData(strPixelData);

		return strEncoded;
	};

	/**
	 * saveAsImage
	 * @param canvasElement
	 * @param {String} image type
	 * @param {Number} [optional] png width
	 * @param {Number} [optional] png height
	 */
	var saveAsImage = function (canvas, width, height, type) {
		if ($support.canvas && $support.dataURL) {
			if (typeof canvas == "string") { canvas = document.getElementById(canvas); }
			if (type == undefined) { type = 'png'; }
			type = fixType(type);
			if (/bmp/.test(type)) {
				var data = getImageData(scaleCanvas(canvas, width, height));
				var strData = genBitmapImage(data);
				saveFile(makeURI(strData, downloadMime));
			} else {
				var strData = getDataURL(canvas, type, width, height);
				saveFile(strData.replace(type, downloadMime));
			}
		}
	};

	var convertToImage = function (canvas, width, height, type) {
		if ($support.canvas && $support.dataURL) {
			if (typeof canvas == "string") { canvas = document.getElementById(canvas); }
			if (type == undefined) { type = 'png'; }
			type = fixType(type);

			if (/bmp/.test(type)) {
				var data = getImageData(scaleCanvas(canvas, width, height));
				var strData = genBitmapImage(data);
				return genImage(makeURI(strData, 'image/bmp'));
			} else {
				var strData = getDataURL(canvas, type, width, height);
				return genImage(strData);
			}
		}
	};



	return {
		saveAsImage: saveAsImage,
		saveAsPNG: function (canvas, width, height) {
			return saveAsImage(canvas, width, height, 'png');
		},
		saveAsJPEG: function (canvas, width, height) {
			return saveAsImage(canvas, width, height, 'jpeg');
		},
		saveAsGIF: function (canvas, width, height) {
			return saveAsImage(canvas, width, height, 'gif');
		},
		saveAsBMP: function (canvas, width, height) {
			return saveAsImage(canvas, width, height, 'bmp');
		},

		convertToImage: convertToImage,
		convertToPNG: function (canvas, width, height) {
			return convertToImage(canvas, width, height, 'png');
		},
		convertToJPEG: function (canvas, width, height) {
			return convertToImage(canvas, width, height, 'jpeg');
		},
		convertToGIF: function (canvas, width, height) {
			return convertToImage(canvas, width, height, 'gif');
		},
		convertToBMP: function (canvas, width, height) {
			return convertToImage(canvas, width, height, 'bmp');
		}
	};

}();