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我有一個簡單的函數，旨在使用HTML中的畫布標籤創建模式更容易。我有嵌入該代碼在網站上的圖像：畫布createPattern函數不工作
<style type="text/css">
.invisible {
display: none;
}
</style>
<img class="invisible" id="image" src="Images/test.png"></img>和我的功能，使一個模式是這樣的：
//Function that allows you to create a pattern
this.pattern = function(image, mode) {
var pat = this.ctx.createPattern(image, mode);
return pat;
};但是當我嘗試用此代碼調用該函數：
var testImage = document.getElementById("image");
myCanvas.fill(myCanvas.pattern(testImage, "repeat"));似乎是說，這在日誌中的錯誤：
Uncaught TypeError: Failed to execute 'createPattern' on 'CanvasRenderingContext2D': The provided value is not of type '(CSSImageValue or HTMLImageElement or SVGImageElement or HTMLVideoElement or HTMLCanvasElement or ImageBitmap or OffscreenCanvas)'
現在，我敢肯定，我把是一個HTMLImageElement變量，但錯誤的是還在那兒。
全碼：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Canvas Library</title>
</head>
<style type="text/css">
.invisible {
display: none;
}
</style>
<body>
<p id="text">Unknown</p>
<canvas width="400" height="400" id="canvas"></canvas>
<script type="text/javascript" src="Canvas.js"></script>
<img class="invisible" id="image" src="Images/test.png"></img>
</body>
</html>// JavaScript File
var Canvas = document.getElementById("canvas");
var label = document.getElementById("text");
var testImage = document.getElementById("image");
var distance = function(x1, y1, x2, y2) {
var a = Math.pow(x2 - x1, 2);
var b = Math.pow(y2 - y1, 2);
return Math.sqrt(a + b);
};
//Declare the Canvas Object
var canvas = function(src) {
//Function to see if a line is colliding with a certain point Has an accuracy of about 1 pixel
this.lineIsColliding = function(startX, startY, endX, endY, testX, testY) {
const v1 = {
x: endX - startX,
y: endY - startY
};
const l2 = v1.x * v1.x + v1.y * v1.y;
if (l2 === 0) {
return false;
} // line has no length so can't be near anything
const v2 = {
x: testX - startX,
y: testY - startY
};
const u = (v1.x * v2.x + v1.y * v2.y)/l2;
return u >= 0 && u <= 1 && Math.abs((v1.x * v2.y - v1.y * v2.x)/Math.sqrt(l2)) < 1;
};
//The Canvas to draw on
this.src = src;
//The context of source(used for drawing)
this.ctx = this.src.getContext("2d");
//The Mouse Move Function
this.showCoordinates = function(e) {
console.log(e);
label.innerHTML = "<b>x: </b>" + e.offsetX + " <b>y: </b>" + e.offsetY + ", " + myCanvas.lineIsColliding(358, 277, 365, 268, e.offsetX, e.offsetY);
};
//Show coordinates variable
this.showCoordinatesBool = true;
//The boolean to tell if we should use stroke
var useStroke = true;
//The fill style and stroke style(can be color, pattern, or gradient)
this.fillStyle = "#000000";
this.strokeStyle = "#000000";
//The Line cap style (can be butt, square, or round)
this.lineCap = "butt";
//The Stroke Weight (how wide the strokes are)
this.strokeWeightVar = "default";
//The corner style (how the corners are drawn)
this.cornerStyle = "miter";
//The Shadow Color
this.shadowColorVar = "#000000";
//The shadow Blur
this.shadowBlurVar = 0;
//The shadow Offsets
this.shadowOffsetX = 0;
this.shadowOffsetY = 0;
//Function to set the fill style
this.fill = function(style) {
this.fillStyle = style;
this.ctx.fillStyle = style;
};
//Function to set the stroke style
this.stroke = function(style) {
this.useStroke = true;
this.strokeStyle = style;
this.ctx.strokeStyle = style;
};
//Function to delete the stroke
this.noStroke = function() {
this.useStroke = false;
};
//Function to draw a rectangle
this.rect = function(x, y, width, height) {
this.ctx.fillRect(x, y, width, height);
if (this.useStroke) {
this.ctx.strokeRect(x, y, width, height);
}
};
//Function to draw a corner
this.corner = function(style, centerX, centerY, x1, y1, x2, y2) {
this.ctx.lineJoin = style;
this.cornerStyle = style;
this.ctx.beginPath();
this.ctx.moveTo(x1, y1);
this.ctx.lineTo(centerX, centerY);
this.ctx.lineTo(x2, y2);
this.ctx.stroke();
};
//Function to draw a hollow rectangle
this.hollowRect = function(x, y, width, height) {
this.ctx.strokeRect(x, y, width, height);
};
//Function to set the canvas background
this.background = function(style) {
this.fillStyle = style;
this.ctx.fillStyle = style;
this.ctx.fillRect(0, 0, this.src.width, this.src.height);
};
//Function to draw a line
this.line = function(startX, startY, endX, endY) {
this.ctx.beginPath();
this.ctx.moveTo(startX, startY);
this.ctx.lineTo(endX, endY);
this.ctx.stroke();
};
//Function to change line style
this.lineCap = function(mode) {
this.ctx.lineCap = mode;
this.lineCap = mode;
};
//Function to change stroke weight
this.strokeWeight = function(weight) {
this.useStroke = true;
this.ctx.lineWidth = weight;
this.strokeWeightVar = weight;
};
//Function to clear a certain area
this.clearArea = function(x, y, width, height) {
this.ctx.clearRect(x, y, width, height);
};
//Turn the show coordinate function on
this.enableCoordinates = function() {
this.showCoordinatesBool = true;
this.src.addEventListener("mousemove", this.showCoordinates);
};
/*Shadows*/
//Set the shadow color
this.shadowColor = function(color) {
this.shadowColorVar = color;
this.ctx.shadowColor = color;
};
//Set the shadow blur
this.shadowBlur = function(blur) {
this.shadowBlurVar = blur;
this.ctx.shadowBlur = blur;
};
//Set the shadow offset
this.shadowOffset = function(offsetX, offsetY) {
this.shadowOffsetX = offsetX;
this.shadowOffsetY = offsetY;
this.ctx.shadowOffsetX = offsetX;
this.ctx.shadowOffsetY = offsetY;
};
//Remove shadows
this.noShadow = function() {
this.shadowOffset(0, 0);
this.shadowColor("#000000");
this.shadowBlur(0);
};
//Function to see if a rectangle is colliding with a specific point
this.rectIsColliding = function(rectX, rectY, rectWidth, rectHeight, testX, testY) {
this.ctx.rect(rectX, rectY, rectWidth, rectHeight);
return this.ctx.isPointInPath(testX, testY);
};
//Function that returns a custom linear gradient
this.linearGradient = function(startX, startY, endX, endY, colorStops) {
var gradient = this.ctx.createLinearGradient(startX, startY, endX, endY);
for (var i = 0; i < colorStops.length; i++) {
gradient.addColorStop(colorStops[i].location, colorStops[i].color);
}
return gradient;
};
//Function that returns a custom radial gradient
this.radialGradient = function(x0, y0, r0, x1, y1, r1, colorStops) {
var radialGradientVar = this.ctx.createRadialGradient(x0, y0, r0, x1, y1, r1);
for (var i = 0; i < colorStops.length; i++) {
radialGradientVar.addColorStop(colorStops[i].location, colorStops[i].color);
}
return radialGradientVar;
};
//Function that allows you to create a pattern
this.pattern = function(image, mode) {
var pat = this.ctx.createPattern(image, mode);
return pat;
};
};
//The following code is for testing purposes ONLY!
{
//Create a new canvas
var myCanvas = new canvas(Canvas);
//Set the Background Color
myCanvas.background("#ff0000");
//Set the fill color
myCanvas.fill("#0000ff");
//Set the Stroke Color
myCanvas.stroke("#00ff00");
//Draw a rectangle
myCanvas.rect(164, 153, 50, 100);
//Draw a hollow rectangle
myCanvas.hollowRect(300, 300, 50, 50);
//Disable the Stroke
myCanvas.noStroke();
//Draw a rectangle with no stroke
myCanvas.rect(21, 18, 50, 50);
//Change the Stroke color
myCanvas.stroke("#ffff00");
//Change the stroke weight
myCanvas.strokeWeight(10);
//Change the line cap
myCanvas.lineCap("round");
//Draw a line
myCanvas.line(350, 30, 250, 80);
//Draw a corner
myCanvas.corner("miter", 50, 135, 100, 185, 100, 110);
//Enable the Coordinates
myCanvas.enableCoordinates();
//Clear a space from the canvas
myCanvas.clearArea(6, 245, 100, 100);
//Set the Shadow Color
myCanvas.shadowColor("black");
//Set the shadow Blur
myCanvas.shadowBlur(20);
//Set the shadow offset
myCanvas.shadowOffset(10, 0);
//Set the stroke
myCanvas.noStroke();
//Set the fill color
myCanvas.fill("orange");
//Draw a rectangle
myCanvas.rect(268, 167, 30, 30);
//Remove the shadow
myCanvas.noShadow();
//Test if the rectangle is colliding with a specific point
if (myCanvas.rectIsColliding(268, 167, 30, 30, 290, 170)) {
myCanvas.fill("green");
myCanvas.rect(358, 0, 50, 46);
}
else {
myCanvas.fill("yellow");
myCanvas.rect(362, 0, 50, 46);
}
//Test if a line is colliding with a certain point
console.log("function returned: " + myCanvas.lineIsColliding(358, 277, 365, 268, 362, 271));
if (myCanvas.lineIsColliding(358, 277, 365, 268, 362, 271)) {
console.log("line is colliding!");
myCanvas.line(358, 277, 365, 268);
}
else {
console.log("line is not colliding!");
}
myCanvas.line(0, 0, 50, 10);
//Color stop used for creating gradients
var colorStop = function(location, color) {
this.location = location;
this.color = color;
};
console.log(distance(0, 0, 50, 10));
myCanvas.fill(myCanvas.linearGradient(259, 77, 359, 127, [new colorStop(0.0, "green"), new colorStop(1.0, "blue")]));
myCanvas.rect(259, 87, 100, 50);
myCanvas.fill(myCanvas.radialGradient(309, 225, 10, 309, 225, 50, [new colorStop(0.0, "yellow"), new colorStop(1.0, "green")]));
myCanvas.rect(259, 200, 100, 50);
myCanvas.fill(myCanvas.pattern(testImage, "repeat"));
myCanvas.rect(132, 81, 50, 100);
}
當您的腳本執行時'![]()
'不被解析，因此不在DOM中。因此，document.getElementBy（'image'）將返回null。只有當頁面已經加載（最好）時，才執行腳本，或者將它插入到你的html標記的底部（但是你將不得不等待你的圖像能夠從它創建一個模式）。 –
Kaiido