html2canvas/src/renderers/canvas.js

function CanvasRenderer(width, height) {
    Renderer.apply(this, arguments);
    this.canvas = this.document.createElement("canvas");
    this.canvas.width = width;
    this.canvas.height = height;
    this.ctx = this.canvas.getContext("2d");
    this.taintCtx = this.document.createElement("canvas").getContext("2d");
    this.ctx.textBaseline = "bottom";
    this.variables = {};
    log("Initialized CanvasRenderer with size", width, "x", height);
}

CanvasRenderer.prototype = Object.create(Renderer.prototype);

CanvasRenderer.prototype.setFillStyle = function(color) {
    this.ctx.fillStyle = color;
    return this.ctx;
};

CanvasRenderer.prototype.rectangle = function(left, top, width, height, color) {
    this.setFillStyle(color).fillRect(left, top, width, height);
};

CanvasRenderer.prototype.drawShape = function(shape, color) {
    this.shape(shape);
    this.setFillStyle(color).fill();
};

CanvasRenderer.prototype.taints = function(imageContainer) {
    if (imageContainer.tainted === null) {
        this.taintCtx.drawImage(imageContainer.image, 0, 0);
        try {
            this.taintCtx.getImageData(0, 0, 1, 1);
            imageContainer.tainted = false;
        } catch(e) {
            this.taintCtx = document.createElement("canvas").getContext("2d");
            imageContainer.tainted = true;
        }
    }

    return imageContainer.tainted;
};

CanvasRenderer.prototype.drawImage = function(imageContainer, sx, sy, sw, sh, dx, dy, dw, dh) {
    if (!this.taints(imageContainer) || this.options.allowTaint) {
        this.ctx.drawImage(imageContainer.image, sx, sy, sw, sh, dx, dy, dw, dh);
    }
};

CanvasRenderer.prototype.clip = function(shapes, callback, context) {
    this.ctx.save();
    shapes.filter(hasEntries).forEach(function(shape) {
        this.shape(shape).clip();
    }, this);
    callback.call(context);
    this.ctx.restore();
};

CanvasRenderer.prototype.shape = function(shape) {
    this.ctx.beginPath();
    shape.forEach(function(point, index) {
        this.ctx[(index === 0) ? "moveTo" : point[0] + "To" ].apply(this.ctx, point.slice(1));
    }, this);
    this.ctx.closePath();
    return this.ctx;
};

CanvasRenderer.prototype.font = function(color, style, variant, weight, size, family) {
    this.setFillStyle(color).font = [style, variant, weight, size, family].join(" ");
};

CanvasRenderer.prototype.fontShadow = function(color, offsetX, offsetY, blur) {
    this.setVariable("shadowColor", color)
        .setVariable("shadowOffsetY", offsetX)
        .setVariable("shadowOffsetX", offsetY)
        .setVariable("shadowBlur", blur);
};

CanvasRenderer.prototype.clearShadow = function() {
    this.setVariable("shadowColor", "rgba(0,0,0,0)");
};

CanvasRenderer.prototype.setOpacity = function(opacity) {
    this.ctx.globalAlpha = opacity;
};

CanvasRenderer.prototype.setTransform = function(transform) {
    this.ctx.translate(transform.origin[0], transform.origin[1]);
    this.ctx.transform.apply(this.ctx, transform.matrix);
    this.ctx.translate(-transform.origin[0], -transform.origin[1]);
};

CanvasRenderer.prototype.setVariable = function(property, value) {
    if (this.variables[property] !== value) {
        this.variables[property] = this.ctx[property] = value;
    }

    return this;
};

CanvasRenderer.prototype.text = function(text, left, bottom) {
    this.ctx.fillText(text, left, bottom);
};

CanvasRenderer.prototype.backgroundRepeatShape = function(imageContainer, backgroundPosition, size, bounds, left, top, width, height, borderData) {
    var shape = [
        ["line", Math.round(left), Math.round(top)],
        ["line", Math.round(left + width), Math.round(top)],
        ["line", Math.round(left + width), Math.round(height + top)],
        ["line", Math.round(left), Math.round(height + top)]
    ];
    this.clip([shape], function() {
        this.renderBackgroundRepeat(imageContainer, backgroundPosition, size, bounds, borderData[3], borderData[0]);
    }, this);
};

CanvasRenderer.prototype.renderBackgroundRepeat = function(imageContainer, backgroundPosition, size, bounds, borderLeft, borderTop) {
    var offsetX = Math.round(bounds.left + backgroundPosition.left + borderLeft), offsetY = Math.round(bounds.top + backgroundPosition.top + borderTop);
    this.setFillStyle(this.ctx.createPattern(this.resizeImage(imageContainer, size), "repeat"));
    this.ctx.translate(offsetX, offsetY);
    this.ctx.fill();
    this.ctx.translate(-offsetX, -offsetY);
};

CanvasRenderer.prototype.renderBackgroundGradient = function(gradientImage, bounds) {
    if (gradientImage instanceof LinearGradientContainer) {
        var gradient = this.ctx.createLinearGradient(
            bounds.left + bounds.width * gradientImage.x0,
            bounds.top + bounds.height * gradientImage.y0,
            bounds.left +  bounds.width * gradientImage.x1,
            bounds.top +  bounds.height * gradientImage.y1);
        gradientImage.colorStops.forEach(function(colorStop) {
            gradient.addColorStop(colorStop.stop, colorStop.color);
        });
        this.rectangle(bounds.left, bounds.top, bounds.width, bounds.height, gradient);
    }
};

CanvasRenderer.prototype.resizeImage = function(imageContainer, size) {
    var image = imageContainer.image;
    if(image.width === size.width && image.height === size.height) {
        return image;
    }

    var ctx, canvas = document.createElement('canvas');
    canvas.width = size.width;
    canvas.height = size.height;
    ctx = canvas.getContext("2d");
    ctx.drawImage(image, 0, 0, image.width, image.height, 0, 0, size.width, size.height );
    return canvas;
};

function hasEntries(array) {
    return array.length > 0;
}