Compositing 示例
这个案例程序演示了一些图像合成操作，其结果如下所示：
合成示例
下面的代码定义了一些全局变量，可用于程序的其他部分。
js
var canvas1 = document.createElement("canvas");
var canvas2 = document.createElement("canvas");
var gco = [
"source-over",
"source-in",
"source-out",
"source-atop",
"destination-over",
"destination-in",
"destination-out",
"destination-atop",
"lighter",
"copy",
"xor",
"multiply",
"screen",
"overlay",
"darken",
"lighten",
"color-dodge",
"color-burn",
"hard-light",
"soft-light",
"difference",
"exclusion",
"hue",
"saturation",
"color",
"luminosity",
].reverse();
var gcoText = [
"这是默认设置，并在现有画布上下文之上绘制新图形。",
"新图形只在新图形和目标画布重叠的地方绘制。其他的都是透明的。",
"在不与现有画布内容重叠的地方绘制新图形。",
"新图形只在与现有画布内容重叠的地方绘制。",
"在现有的画布内容后面绘制新的图形。",
"现有的画布内容保持在新图形和现有画布内容重叠的位置。其他的都是透明的。",
"现有内容保持在新图形不重叠的地方。",
"现有的画布只保留与新图形重叠的部分，新的图形是在画布内容后面绘制的。",
"两个重叠图形的颜色是通过颜色值相加来确定的。",
"只显示新图形。",
"图像中，那些重叠和正常绘制之外的其他地方是透明的。",
"将顶层像素与底层相应像素相乘，结果是一幅更黑暗的图片。",
"像素被倒转，相乘，再倒转，结果是一幅更明亮的图片。",
"multiply 和 screen 的结合，原本暗的地方更暗，原本亮的地方更亮。",
"保留两个图层中最暗的像素。",
"保留两个图层中最亮的像素。",
"将底层除以顶层的反置。",
"将反置的底层除以顶层，然后将结果反过来。",
"屏幕相乘（A combination of multiply and screen）类似于叠加，但上下图层互换了。",
"用顶层减去底层或者相反来得到一个正值。",
"一个柔和版本的强光（hard-light）。纯黑或纯白不会导致纯黑或纯白。",
"和 difference 相似，但对比度较低。",
"保留了底层的亮度（luma）和色度（chroma），同时采用了顶层的色调（hue）。",
"保留底层的亮度（luma）和色调（hue），同时采用顶层的色度（chroma）。",
"保留了底层的亮度（luma），同时采用了顶层的色调 (hue) 和色度 (chroma)。",
"保持底层的色调（hue）和色度（chroma），同时采用顶层的亮度（luma）。",
].reverse();
var width = 320;
var height = 340;
主程序
当页面加载时，这部分程序会运行，并执行示例代码：
js
window.onload = function () {
// lum in sRGB
var lum = {
r: 0.33,
g: 0.33,
b: 0.33,
};
// resize canvas
canvas1.width = width;
canvas1.height = height;
canvas2.width = width;
canvas2.height = height;
lightMix();
colorSphere();
runComposite();
return;
};
这部分代码，runComposite()，处理了大部分的工作，但需要依赖于许多效用函数来完成复杂的处理工作。
js
function createCanvas() {
var canvas = document.createElement("canvas");
canvas.style.background = "url(" + op_8x8.data + ")";
canvas.style.border = "1px solid #000";
canvas.style.margin = "5px";
canvas.width = width / 2;
canvas.height = height / 2;
return canvas;
}
function runComposite() {
var dl = document.createElement("dl");
document.body.appendChild(dl);
while (gco.length) {
var pop = gco.pop();
var dt = document.createElement("dt");
dt.textContent = pop;
dl.appendChild(dt);
var dd = document.createElement("dd");
var p = document.createElement("p");
p.textContent = gcoText.pop();
dd.appendChild(p);
var canvasToDrawOn = createCanvas();
var canvasToDrawFrom = createCanvas();
var canvasToDrawResult = createCanvas();
var ctx = canvasToDrawResult.getContext("2d");
ctx.clearRect(0, 0, width, height);
ctx.save();
ctx.drawImage(canvas1, 0, 0, width / 2, height / 2);
ctx.globalCompositeOperation = pop;
ctx.drawImage(canvas2, 0, 0, width / 2, height / 2);
ctx.globalCompositeOperation = "source-over";
ctx.fillStyle = "rgba(0,0,0,0.8)";
ctx.fillRect(0, height / 2 - 20, width / 2, 20);
ctx.fillStyle = "#FFF";
ctx.font = "14px arial";
ctx.fillText(pop, 5, height / 2 - 5);
ctx.restore();
var ctx = canvasToDrawOn.getContext("2d");
ctx.clearRect(0, 0, width, height);
ctx.save();
ctx.drawImage(canvas1, 0, 0, width / 2, height / 2);
ctx.fillStyle = "rgba(0,0,0,0.8)";
ctx.fillRect(0, height / 2 - 20, width / 2, 20);
ctx.fillStyle = "#FFF";
ctx.font = "14px arial";
ctx.fillText("existing content", 5, height / 2 - 5);
ctx.restore();
var ctx = canvasToDrawFrom.getContext("2d");
ctx.clearRect(0, 0, width, height);
ctx.save();
ctx.drawImage(canvas2, 0, 0, width / 2, height / 2);
ctx.fillStyle = "rgba(0,0,0,0.8)";
ctx.fillRect(0, height / 2 - 20, width / 2, 20);
ctx.fillStyle = "#FFF";
ctx.font = "14px arial";
ctx.fillText("new content", 5, height / 2 - 5);
ctx.restore();
dd.appendChild(canvasToDrawOn);
dd.appendChild(canvasToDrawFrom);
dd.appendChild(canvasToDrawResult);
dl.appendChild(dd);
}
}
效用函数 (Utility functions)
程序需要依赖许多效用函数。
js
var lightMix = function () {
var ctx = canvas2.getContext("2d");
ctx.save();
ctx.globalCompositeOperation = "lighter";
ctx.beginPath();
ctx.fillStyle = "rgba(255,0,0,1)";
ctx.arc(100, 200, 100, Math.PI * 2, 0, false);
ctx.fill();
ctx.beginPath();
ctx.fillStyle = "rgba(0,0,255,1)";
ctx.arc(220, 200, 100, Math.PI * 2, 0, false);
ctx.fill();
ctx.beginPath();
ctx.fillStyle = "rgba(0,255,0,1)";
ctx.arc(160, 100, 100, Math.PI * 2, 0, false);
ctx.fill();
ctx.restore();
ctx.beginPath();
ctx.fillStyle = "#f00";
ctx.fillRect(0, 0, 30, 30);
ctx.fill();
};
js
var colorSphere = function (element) {
var ctx = canvas1.getContext("2d");
var width = 360;
var halfWidth = width / 2;
var rotate = (1 / 360) * Math.PI * 2; // per degree
var offset = 0; // scrollbar offset
var oleft = -20;
var otop = -20;
for (var n = 0; n <= 359; n++) {
var gradient = ctx.createLinearGradient(
oleft + halfWidth,
otop,
oleft + halfWidth,
otop + halfWidth,
);
var color = Color.HSV_RGB({ H: (n + 300) % 360, S: 100, V: 100 });
gradient.addColorStop(0, "rgba(0,0,0,0)");
gradient.addColorStop(
0.7,
"rgba(" + color.R + "," + color.G + "," + color.B + ",1)",
);
gradient.addColorStop(1, "rgba(255,255,255,1)");
ctx.beginPath();
ctx.moveTo(oleft + halfWidth, otop);
ctx.lineTo(oleft + halfWidth, otop + halfWidth);
ctx.lineTo(oleft + halfWidth + 6, otop);
ctx.fillStyle = gradient;
ctx.fill();
ctx.translate(oleft + halfWidth, otop + halfWidth);
ctx.rotate(rotate);
ctx.translate(-(oleft + halfWidth), -(otop + halfWidth));
}
ctx.beginPath();
ctx.fillStyle = "#00f";
ctx.fillRect(15, 15, 30, 30);
ctx.fill();
return ctx.canvas;
};
js
// HSV (1978) = H: Hue / S: Saturation / V: Value
Color = {};
Color.HSV_RGB = function (o) {
var H = o.H / 360,
S = o.S / 100,
V = o.V / 100,
R,
G,
B;
var A, B, C, D;
if (S == 0) {
R = G = B = Math.round(V * 255);
} else {
if (H >= 1) H = 0;
H = 6 * H;
D = H - Math.floor(H);
A = Math.round(255 * V * (1 - S));
B = Math.round(255 * V * (1 - S * D));
C = Math.round(255 * V * (1 - S * (1 - D)));
V = Math.round(255 * V);
switch (Math.floor(H)) {
case 0:
R = V;
G = C;
B = A;
break;
case 1:
R = B;
G = V;
B = A;
break;
case 2:
R = A;
G = V;
B = C;
break;
case 3:
R = A;
G = B;
B = V;
break;
case 4:
R = C;
G = A;
B = V;
break;
case 5:
R = V;
G = A;
B = B;
break;
}
}
return {
R: R,
G: G,
B: B,
};
};
var createInterlace = function (size, color1, color2) {
var proto = document.createElement("canvas").getContext("2d");
proto.canvas.width = size * 2;
proto.canvas.height = size * 2;
proto.fillStyle = color1; // top-left
proto.fillRect(0, 0, size, size);
proto.fillStyle = color2; // top-right
proto.fillRect(size, 0, size, size);
proto.fillStyle = color2; // bottom-left
proto.fillRect(0, size, size, size);
proto.fillStyle = color1; // bottom-right
proto.fillRect(size, size, size, size);
var pattern = proto.createPattern(proto.canvas, "repeat");
pattern.data = proto.canvas.toDataURL();
return pattern;
};
var op_8x8 = createInterlace(8, "#FFF", "#eee");