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w3c-canvas
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b00-阅读笔记/w3c-canvas翻译/w3c-canvas.md
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# HTML Canvas 2D Context
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[英文原版地址](https://www.w3.org/TR/2dcontext/#pixel-manipulation)
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## 摘要
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该规范定义了 HTML canvas 元素的 2D 上下文。2D 上下文提供对象，方法和属性，以在画布绘图表面上绘制和操纵图形。
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## 本文件的状态
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略...
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## 目录
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1. [符合标准](#符合标准)
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2. [canvas状态](#canvas状态)
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3. [线样式](#线样式)
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4. [文本样式](#文本样式)
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5. [建立路径](#建立路径)
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6. [变换](#变换)
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7. [2D渲染上下文的图像源](#2D渲染上下文的图像源)
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8. [填充和描边样式](#填充和描边样式)
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9. [在画布上绘制矩形](#在画布上绘制矩形)
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10. [在画布上绘制文字](#在画布上绘制文字)
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11. [在画布上绘制路径](#在画布上绘制路径)
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12. [在画布上绘制图像](#在画布上绘制图像)
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13. [热区](#热区)
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14. [像素操作](#像素操作)
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15. [合成](#合成)
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16. [阴影](#阴影)
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17. [绘制模型](#绘制模型)
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18. [最佳实践](#最佳实践)
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19. [例子](#例子)
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## 符合标准
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该规范是 HTML 规范。 HTML5 核心规范中描述的所有一致性要求，一致性类，定义，依赖项，术语和印刷约定均适用于此规范。
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接口是根据[Web IDL](https://www.w3.org/TR/WebIDL-1/)定义的。
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该规范定义了 2d 上下文类型，其 2D API 使用[CanvasRenderingContext2D](https://www.w3.org/TR/2dcontext/#canvasrenderingcontext2d)接口实现。
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当 [canvas](https://html.spec.whatwg.org/multipage/canvas.html#the-canvas-element) 元素的 [getContext()](https://html.spec.whatwg.org/multipage/canvas.html#dom-canvas-getcontext) 方法返回 contextId 2d 的新对象时，用户代理必须返回一个新的 CanvasRenderingContext2D 对象。任何其他参数都将被忽略。
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2D 上下文采用笛卡尔坐标系，其原点（0,0）位于左上角，坐标空间的 x 值在向右时增加，而 y 值在向下时增加。
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IDL定义：
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```
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typedef (HTMLImageElement or
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HTMLVideoElement or
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HTMLCanvasElement) CanvasImageSource;
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interface CanvasRenderingContext2D {
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// back-reference to the canvas
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readonly attribute HTMLCanvasElement canvas;
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// state
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void save(); // push state on state stack
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void restore(); // pop state stack and restore state
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// transformations (default: transform is the identity matrix)
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void scale(unrestricted double x, unrestricted double y);
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void rotate(unrestricted double angle);
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void translate(unrestricted double x, unrestricted double y);
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void transform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
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void setTransform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
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// compositing
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attribute unrestricted double globalAlpha; // (default: 1.0)
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attribute DOMString globalCompositeOperation; // (default: "source-over")
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// colors and styles (see also the CanvasDrawingStyles interface)
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attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default: "black")
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attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default: "black")
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CanvasGradient createLinearGradient(double x0, double y0, double x1, double y1);
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CanvasGradient createRadialGradient(double x0, double y0, double r0, double x1, double y1, double r1);
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CanvasPattern createPattern(CanvasImageSource image, [TreatNullAs=EmptyString] DOMString repetition);
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// shadows
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attribute unrestricted double shadowOffsetX; // (default: 0)
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attribute unrestricted double shadowOffsetY; // (default: 0)
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attribute unrestricted double shadowBlur; // (default: 0)
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attribute DOMString shadowColor; // (default: "transparent black")
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// rects
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void clearRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
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void fillRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
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void strokeRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
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// path API (see also CanvasPathMethods)
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void beginPath();
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void fill();
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void stroke();
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void drawFocusIfNeeded(Element element);
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void clip();
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boolean isPointInPath(unrestricted double x, unrestricted double y);
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// text (see also the CanvasDrawingStyles interface)
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void fillText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
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void strokeText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
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TextMetrics measureText(DOMString text);
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// drawing images
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void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy);
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void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh);
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void drawImage(CanvasImageSource image, unrestricted double sx, unrestricted double sy, unrestricted double sw, unrestricted double sh, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh);
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// hit regions
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void addHitRegion(HitRegionOptions options);
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void removeHitRegion(DOMString id);
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void clearHitRegions();
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// pixel manipulation
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ImageData createImageData(unrestricted double sw, unrestricted double sh);
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ImageData createImageData(ImageData imagedata);
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ImageData getImageData(double sx, double sy, double sw, double sh);
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void putImageData(ImageData imagedata, double dx, double dy);
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void putImageData(ImageData imagedata, double dx, double dy, double dirtyX, double dirtyY, double dirtyWidth, double dirtyHeight);
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};
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CanvasRenderingContext2D implements CanvasDrawingStyles;
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CanvasRenderingContext2D implements CanvasPathMethods;
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[NoInterfaceObject]
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interface CanvasDrawingStyles {
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// line caps/joins
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attribute unrestricted double lineWidth; // (default: 1)
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attribute DOMString lineCap; // "butt", "round", "square" (default: "butt")
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attribute DOMString lineJoin; // "round", "bevel", "miter" (default: "miter")
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attribute unrestricted double miterLimit; // (default: 10)
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// dashed lines
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void setLineDash(sequence<unrestricted double> segments); // (default: empty)
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sequence<unrestricted double> getLineDash();
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attribute unrestricted double lineDashOffset;
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// text
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attribute DOMString font; // (default: "10px sans-serif")
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attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start")
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attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic")
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};
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[NoInterfaceObject]
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interface CanvasPathMethods {
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// shared path API methods
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void closePath();
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void moveTo(unrestricted double x, unrestricted double y);
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void lineTo(unrestricted double x, unrestricted double y);
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void quadraticCurveTo(unrestricted double cpx, unrestricted double cpy, unrestricted double x, unrestricted double y);
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void bezierCurveTo(unrestricted double cp1x, unrestricted double cp1y, unrestricted double cp2x, unrestricted double cp2y, unrestricted double x, unrestricted double y);
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void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radius);
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void rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
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void arc(unrestricted double x, unrestricted double y, unrestricted double radius, unrestricted double startAngle, unrestricted double endAngle, optional boolean counterclockwise = false);
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};
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interface CanvasGradient {
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// opaque object
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void addColorStop(double offset, DOMString color);
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};
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interface CanvasPattern {
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// opaque object
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};
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interface TextMetrics {
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readonly attribute double width;
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};
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dictionary HitRegionOptions {
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// dictionary to allow expansion on Hit Regions in Canvas Context 2D Level 2
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DOMString id = "";
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// for control-backed regions:
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Element? control = null;
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};
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interface ImageData {
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readonly attribute unsigned long width;
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readonly attribute unsigned long height;
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readonly attribute Uint8ClampedArray data;
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};
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```
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`context.canvas` 属性必须返回在当前上下文上绘制的 canvas 元素。
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除非另有说明，否则对于 2D 上下文接口，任何带有数值参数（其值为无穷大或 NaN 值）的方法调用都必须被忽略。
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每当在此 API 中将 CSS 值 [currentColor](https://www.zhangxinxu.com/wordpress/2014/10/currentcolor-css3-powerful-css-keyword/) 用作颜色时，currentColor 是该元素的 color 计算值，如果在特定情况下 color 属性的计算值未定义，currentColor 关键字是完全不透明的黑色。
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如果是 CanvasGradient 上的 addColorStop() 方法，用于确定 currentColor 关键字的计算值的“'color'属性的计算值”始终是完全不透明的黑色（因为没有相关的元素）。
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> NOTE: 这是因为 CanvasGradient 对象与画布无关,由一个画布创建的 CanvasGradient 对象可以被另一个画布使用，因此，在指定颜色时无法知道哪个是“相关元素”。
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> NOTE: 与字体相关的属性也存在类似的问题。这些规则将在下面的相关部分中详细介绍。
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## canvas状态
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每个上下文维护一堆绘图状态。绘图状态（Drawing states）包括：
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* 当前的[变换矩阵](https://www.w3.org/TR/2dcontext/#transformations)。
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* 当前的[裁剪区域](https://www.w3.org/TR/2dcontext/#clipping-region)。
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* 以下属性的当前值：`strokeStyle`，`fillStyle`，`globalAlpha`，`lineWidth`，`lineCap`，`lineJoin`，`miterLimit`，`shadowOffsetX`，`shadowOffsetY`，`shadowBlur`，`shadowColor`，`globalCompositeOperation`，`font`，`textAlign`，`textBaseline`。
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> NOTE: 当前路径和当前位图不属于绘图状态。当前路径是持久路径，只能使用 beginPath() 方法进行重置。当前位图是画布的属性，而不是上下文。
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> NOTE:
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context.save() 将当前状态压入堆栈。
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context.restore() 在栈顶弹出状态，将上下文恢复到该状态。
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`save()`方法必须将当前图形状态的副本推入图形状态堆栈。
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`restore()`方法必须在绘图状态堆栈中弹出顶部条目，并重置其描述的绘图状态。如果没有保存状态，则该方法将不执行任何操作。
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## 线样式
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## 文本样式
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## 建立路径
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## 变换
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## 2D渲染上下文的图像源
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## 填充和描边样式
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## 在画布上绘制矩形
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## 在画布上绘制文字
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## 在画布上绘制路径
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## 在画布上绘制图像
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## 热区
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## 像素操作
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## 合成
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## 阴影
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## 绘制模型
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## 最佳实践
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## 例子
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## 参考
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[canvasapi](https://www.canvasapi.cn/)
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