我向帮助提出了一个问题。我按照说明创建水波纹效果，我用我的图像替换了背景。效果运行得很好。但它有一个问题。第一次加载时，浏览器无法加载我的背景（我的图像），您必须重新加载浏览器（F5）或再次在地址栏上输入以查看效果。请告诉我如何解决它。谢谢所有!!
（抱歉我的英文，我使用谷歌翻译）
我的代码：
<!DOCTYPE html>
<html>
<head>
<title>Water Ripple HTML5</title>
<meta name="author" content="Brent Dingle">
<meta name="description" content="HTML5 canvas example of water ripple effect">
<style >
.waterCanvasStyle
{
border-width: 1px;
border-style: solid;
border-color:#a1a1d0;
border-radius: 8px;
box-shadow: #c6c6d0 4px 4px 10px;
}
</style>
</head>
<body>
<canvas id="waterCanvas0" width="400" height="400" >
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
var canvas = document.getElementById('waterCanvas0');
var ctx = canvas.getContext('2d');
var width = canvas.width;
var height = canvas.height;
var halfWidth = width >> 1;
var halfHeight = height >> 1;
var size = width * (height + 2) * 2; // space for 2 images (old and new), +2 to cover ripple radius <= 3
var delay = 30; // delay is desired FPS
var oldIdx = width;
var newIdx = width * (height + 3); // +2 from above size calc +1 more to get to 2nd image
var rippleRad = 3;
var rippleMap = [];
var lastMap = [];
var mapIdx;
// texture and ripple will hold the image data to be displayed
var ripple;
var texture;
// Any image can be used, but we will create a simple pattern instead
// So need some variables to create the background/underwater image
var stripeWidth = 25;
var step = stripeWidth * 2;
var count = height / stripeWidth;
canvas.width = width;
canvas.height = height;
var img = new Image();
img.src = "sea.jpg";
// Here is a neat trick so you don't have to type ctx.blah over and over again
with (ctx)
{
drawImage(img,0,0);
save();
restore();
}
// Initialize the texture and ripple image data
// Texture will never be changed
// Ripple is what will be altered and displayed --> see run() function
texture = ctx.getImageData(0, 0, width, height);
ripple = ctx.getImageData(0, 0, width, height);
// Initialize the maps
for (var i = 0; i < size; i++)
{
lastMap[i] = 0;
rippleMap[i] = 0;
}
// -------------------------------------------------------
// --------------------- Main Run Loop --------------
// -------------------------------------------------------
function run()
{
newframe();
ctx.putImageData(ripple, 0, 0);
}
// -------------------------------------------------------
// Drop something in the water at location: dx, dy
// -------------------------------------------------------
function dropAt(dx, dy)
{
// Make certain dx and dy are integers
// Shifting left 0 is slightly faster than parseInt and math.* (or used to be)
dx <<= 0;
dy <<= 0;
// Our ripple effect area is actually a square, not a circle
for (var j = dy - rippleRad; j < dy + rippleRad; j++)
{
for (var k = dx - rippleRad; k < dx + rippleRad; k++)
{
rippleMap[oldIdx + (j * width) + k] += 512;
}
}
}
// -------------------------------------------------------
// Create the next frame of the ripple effect
// -------------------------------------------------------
function newframe()
{
var i;
var a, b;
var data, oldData;
var curPixel, newPixel;
// Store indexes - old and new may be misleading/confusing
// - current and next is slightly more accurate
// - previous and current may also help in thinking
i = oldIdx;
oldIdx = newIdx;
newIdx = i;
// Initialize the looping values - each will be incremented
i = 0;
mapIdx = oldIdx;
for (var y = 0; y < height; y++)
{
for (var x = 0; x < width; x++)
{
// Use rippleMap to set data value, mapIdx = oldIdx
// Use averaged values of pixels: above, below, left and right of current
data = (
rippleMap[mapIdx - width] +
rippleMap[mapIdx + width] +
rippleMap[mapIdx - 1] +
rippleMap[mapIdx + 1]) >> 1; // right shift 1 is same as divide by 2
// Subtract 'previous' value (we are about to overwrite rippleMap[newIdx+i])
data -= rippleMap[newIdx + i];
// Reduce value more -- for damping
// data = data - (data / 32)
data -= data >> 5;
// Set new value
rippleMap[newIdx + i] = data;
// If data = 0 then water is flat/still,
// If data > 0 then water has a wave
data = 1024 - data;
oldData = lastMap[i];
lastMap[i] = data;
if (oldData != data) // if no change no need to alter image
{
// Recall using "<< 0" forces integer value
// Calculate pixel offsets
a = (((x - halfWidth) * data / 1024) << 0) + halfWidth;
b = (((y - halfHeight) * data / 1024) << 0) + halfHeight;
// Don't go outside the image (i.e. boundary check)
if (a >= width) a = width - 1;
if (a < 0) a = 0;
if (b >= height) b = height - 1;
if (b < 0) b = 0;
// Set indexes
newPixel = (a + (b * width)) * 4;
curPixel = i * 4;
// Apply values
ripple.data[curPixel] = texture.data[newPixel];
ripple.data[curPixel + 1] = texture.data[newPixel + 1];
ripple.data[curPixel + 2] = texture.data[newPixel + 2];
}
mapIdx++;
i++;
}
}
}
// -------------------------------------------------------
// Select random location to create drops
// So if user is doing nothing, water still
// gets ripples.
// -------------------------------------------------------
function randomDrop()
{
// Make it a little, irregular in timing
if ( Math.random() > 0.3 )
{
dropAt(Math.random() * width, Math.random() * height);
}
}
// -------------------------------------------------------
// Event handler for mouse motion
// -------------------------------------------------------
canvas.onmousemove = function(/* Event */ evt)
{
dropAt(evt.offsetX || evt.layerX, evt.offsetY || evt.layerY);
}
// -------------------------------------------------------
// Begin our infinite loop
// For user interaction and display updates
// -------------------------------------------------------
setInterval(run, delay);
// -------------------------------------------------------
// Create random ripples
// Note: this is NOT at same rate as display refresh
// -------------------------------------------------------
setInterval(randomDrop, 1250);
</script>
</body>
答案 0 :(得分：1)
发生此类问题时常见的错误是您尝试在加载前绘制图像。尝试使用img.onload函数等待图像加载完成所有绘图，然后开始运行主循环。