javascript - 使用图案覆盖 Canvas 中的路径

Question

我有这样的功能:

function fillSlide(slideId){
        var context_background = new Image();
        context_background.src = './images/'+slideId+'.png'; 
        context_background.onload = function(){
            var canvas = document.getElementById(slideId);

            window.addEventListener('resize', resizeCanvas, false);

            if(canvas.getContext){
                var context = canvas.getContext('2d');
                var pattern = context.createPattern(this,'no-repeat');

                function resizeCanvas() {
                    var width = window.innerWidth;
                    var height = document.getElementsByClassName('slidecont')[0].offsetHeight;
                    height += 50;
                    canvas.width = width;
                    canvas.height = height;

                    drawStuff(context,width,pattern);
                }
                resizeCanvas();

                function drawStuff(ctx,w,p) {
                    var l = w/2 - 120;
                    var r = w/2 + 120;

                    context.fillStyle = p;
                    ctx.save();
                    ctx.beginPath();
                    ctx.moveTo(0,50);
                    ctx.lineTo(0,1924.06925);
                    ctx.lineTo(w,1924.06925);
                    ctx.lineTo(w,50);
                    ctx.lineTo(r,50);
                    ctx.bezierCurveTo((r-35),50,(r-70),0,(r-120),0);
                    ctx.bezierCurveTo((l+70),0,(l+49),50,l,50);
                    ctx.lineTo(0,50);
                    ctx.closePath();
                    ctx.fill();
                    ctx.restore();
                }

            }
        };
    }

如您所见，我正在使用 Image() 元素创建图案并用它填充上下文。我想用这个图案作为背景。但是 Canvas 中没有背景大小样式。

如我所见，我需要在填充 Canvas 之前剪切图案元素。我该如何管理？谢谢。

更新:JSFIDDLE

Answer 1

也许我的回复不值得添加答案，但我还不能添加评论。

我认为this可能对你有帮助。

编辑

您不需要 fillStyle 模式来用图像填充形状。另一种方法是:

1. 画出路径
2. 在上下文对象上调用clip 方法(这将裁剪每个绘制的对象以匹配路径指定的形状)
3. 使用drawImage 方法代替图案，这样您就可以设置合适的尺寸。

看看我的jsfiddle snippet

var canvas = document.getElementById('slide');
var ctx = canvas.getContext('2d');
var img = new Image();

function draw() {
    var w = canvas.width;
    var h = canvas.height;
    var l = w/2 - 120;
    var r = w/2 + 120; 
    ctx.save();
    ctx.beginPath();
    ctx.moveTo(0,50);
    ctx.lineTo(0,h);
    ctx.lineTo(w,h);
    ctx.lineTo(w,50);
    ctx.lineTo(r,50);
    ctx.bezierCurveTo((r-35),50,(r-70),0,(r-120),0);
    ctx.bezierCurveTo((l+70),0,(l+49),50,l,50);
    ctx.lineTo(0,50);
    ctx.closePath();
    ctx.clip();
    ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
    ctx.restore();
}

function fitCanvasSize() {
    canvas.width = document.body.clientWidth;
    canvas.height = document.body.clientHeight;
}

window.addEventListener('resize', function () {
    fitCanvasSize();
    draw();
});

img.onload = function () {
    draw();
}
fitCanvasSize();
img.src = 'https://placeholdit.imgix.net/~text?txtsize=28&txt=300%C3%97300&w=300&h=300';

编辑2，保持图片纵横比

为了保持图像的纵横比图像可以被切割和居中like here .为此，我们需要额外的变量:

var sx, sy, sWidth, sHeight;
var imgAspectRatio = img.width / img.height;
var areaAspectRatio = w / h;

sx, sx, sWidth, sHeight 的用途在 MDN 中有解释。

sx: The X coordinate of the top left corner of the sub-rectangle of the source image to draw into the destination context.

sy: The Y coordinate of the top left corner of the sub-rectangle of the source image to draw into the destination context.

sWidth: The width of the sub-rectangle of the source image to draw into the destination context. If not specified, the entire rectangle from the coordinates specified by sx and sy to the bottom-right corner of the image is used.

sHeight: The height of the sub-rectangle of the source image to draw into the destination context.

以下是默认值，如果 imgAspectRatio 等于 areaAspectRatio，这些值将保持不变:

sx = 0;
sy = 0;
sWidth = img.width;
sHeight = img.height;

否则:

if (imgAspectRatio > areaAspectRatio) {
    // image is centered horizontally
    sWidth = (areaAspectRatio / imgAspectRatio) * img.width;
    sx = (img.width - sWidth ) / 2;
} else if (imgAspectRatio < areaAspectRatio) {
    // image is centered vertically
    sHeight = (imgAspectRatio / areaAspectRatio) * img.height;
    sy = (img.height - sHeight) / 2;
}

下一张画图:

ctx.drawImage(img, sx, sy, sWidth, sHeight, 0, 0, canvas.width, canvas.height);

最终代码:

var canvas = document.getElementById('slide');
var ctx = canvas.getContext('2d');
var img = new Image();

function draw() {
    var w = canvas.width;
    var h = canvas.height;
    var sx, sy, sWidth, sHeight;
    var imgAspectRatio = img.width / img.height;
    var areaAspectRatio = w / h;

    // below values remains unchanged if 
    // image aspect ratio === area aspect ratio
    sx = 0;
    sy = 0;
    sWidth = img.width;
    sHeight = img.height;

    if (imgAspectRatio > areaAspectRatio) {
        // image is centered horizontally
        sWidth = (areaAspectRatio / imgAspectRatio) * img.width;
        sx = (img.width - sWidth ) / 2;
    } else if (imgAspectRatio < areaAspectRatio) {
        // image is centered vertically
        sHeight = (imgAspectRatio / areaAspectRatio) * img.height;
        sy = (img.height - sHeight) / 2;
    }
    var l = w/2 - 120;
    var r = w/2 + 120; 
    ctx.save();
    ctx.beginPath();
    ctx.moveTo(0,50);
    ctx.lineTo(0,h);
    ctx.lineTo(w,h);
    ctx.lineTo(w,50);
    ctx.lineTo(r,50);
    ctx.bezierCurveTo((r-35),50,(r-70),0,(r-120),0);
    ctx.bezierCurveTo((l+70),0,(l+49),50,l,50);
    ctx.lineTo(0,50);
    ctx.closePath();
    ctx.clip();
    ctx.drawImage(img, sx, sy, sWidth, sHeight, 0, 0, canvas.width, canvas.height);
    ctx.restore();
}

function fitCanvasSize() {
    canvas.width = document.body.clientWidth;
    canvas.height = document.body.clientHeight;
}

window.addEventListener('resize', function () {
    fitCanvasSize();
    draw();
});

img.onload = function () {
    draw();
}
fitCanvasSize();
img.src = 'http://upload.wikimedia.org/wikipedia/commons/3/3f/Brown-bear-in-spring.jpg';

更简单的方法是保持 Canvas 纵横比而不是剪切图像。

此示例有效，因为 Canvas 是裁剪区域的边界框。如果裁剪区域与 Canvas 相比较小，则:

the image will "float" around the clipped area (see markE comment)

如果有人要求，我可以写出这个问题的解决方案。