题
When you put a picture named logo.png in the same directory as this html file and try to run it in a web browser the picture only appears 1 times out of 10 refreshes in IE and doesn't appear the first time in Firefox but does appear after further refreshes.
What the heck is going on ?
(drawImage() method is called in the showIntro() function)
https://stackoverflow.com/questions/22758019
italiano
english
français
española
中国
日本の
العربية
Deutsch
한국어
Português
Russian<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title>Example 1 - Title Screen</title>
<script>
window.onload = function () {
var canvas = document.getElementById('myCanvas');
var c = canvas.getContext('2d');
var State = {
_current: 0,
INTRO: 0,
LOADING: 1,
LOADED: 2
}
window.addEventListener('click', handleClick, false);
window.addEventListener('resize', doResize, false);
doResize();
function handleClick() {
State._current = State.LOADING;
fadeToWhite();
}
function doResize() {
canvas.width = document.body.clientWidth;
canvas.height = document.body.clientHeight;
switch (State._current) {
case State.INTRO:
showIntro();
break;
}
}
function fadeToWhite(alphaVal) {
// If the function hasn't received any parameters, start with 0.02
var alphaVal = (alphaVal == undefined) ? 0.02 : parseFloat(alphaVal) + 0.02;
// Set the color to white
c.fillStyle = '#FFFFFF';
// Set the Global Alpha
c.globalAlpha = alphaVal;
// Make a rectangle as big as the canvas
c.fillRect(0, 0, canvas.width, canvas.height);
if (alphaVal < 1.0) {
setTimeout(function () {
fadeToWhite(alphaVal);
}, 30);
}
}
function showIntro() {
var phrase = "Click or tap the screen to start the game";
// Clear the canvas
c.clearRect(0, 0, canvas.width, canvas.height);
// Make a nice blue gradient
var grd = c.createLinearGradient(0, canvas.height, canvas.width, 0);
grd.addColorStop(0, '#ceefff');
grd.addColorStop(1, '#52bcff');
c.fillStyle = grd;
c.fillRect(0, 0, canvas.width, canvas.height);
var logoImg = new Image();
logoImg.src = './logo.png';
// Store the original width value so that we can keep
// the same width/height ratio later
var originalWidth = logoImg.width;
// Compute the new width and height values
logoImg.width = Math.round((50 * document.body.clientWidth) / 100);
logoImg.height = Math.round((logoImg.width * logoImg.height) / originalWidth);
// Create an small utility object
var logo = {
img: logoImg,
x: (canvas.width / 2) - (logoImg.width / 2),
y: (canvas.height / 2) - (logoImg.height / 2)
}
// Present the image
c.drawImage(logo.img, logo.x, logo.y, logo.img.width, logo.img.height);
// Change the color to black
c.fillStyle = '#000000';
c.font = 'bold 16px Arial, sans-serif';
var textSize = c.measureText(phrase);
var xCoord = (canvas.width / 2) - (textSize.width / 2);
c.fillText(phrase, xCoord, (logo.y + logo.img.height) + 50);
}
}
</script>
<style type="text/css" media="screen">
html { height: 100%; overflow: hidden }
body {
margin: 0px;
padding: 0px;
height: 100%;
}
</style>
</head>
<body>
<canvas id="myCanvas" width="100" height="100">
Your browser doesn't include support for the canvas tag.
</canvas>
</body>
</html>
解决方案
The problem is that you aren't waiting for the image to load when you call drawImage().
You could use something like:
logo.img.onload = function(){
c.drawImage(logo.img, logo.x, logo.y, logo.img.width, logo.img.height);
};
Although make sure you don't start modifying the canvas until this has happened.
