我正在使用 HTML 画布做一些工作，我发现当我画一条线时，我无法清除或绘制它？
所以我在做什么：我有一个带有网格的画布，每个单元格都在其中绘制自己（一个填充的矩形，在页面加载时完成），当鼠标在网格上时，鼠标所在的单元格应该被勾勒出来（画四行在画布上没有其他重绘）。当鼠标移动到不同的单元格时，新的单元格应该被勾勒出来（和以前一样），而之前的被勾勒出来的单元格不应该再被勾勒出来（目的是重新绘制单元格，画出线条）。它是“取消概述”被证明是问题的单元格。
我假设它与画家的算法一起使用，画布上绘制的最后一件事是可见的，但我得到的效果表明填充的形状和线条与更高层上的线条分开处理？
这是一个独立页面的源代码来演示它：
<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
var CANVAS_DIMENSION = 200;
var CANVAS_WIDTH = 10;
var CANVAS_HEIGHT = 10;
var GRID_CELL_WIDTH = CANVAS_DIMENSION/CANVAS_WIDTH;
var GRID_CELL_HEIGHT = CANVAS_DIMENSION/CANVAS_HEIGHT;
var canvas;
var context;
var grid;
var mouseCellColumn = -1;
var mouseCellRow = -1;
function init()
{
canvas = document.getElementById("myCanvas");
context = canvas.getContext("2d");
canvas.addEventListener('mousemove', updateMousePosition, false);
grid = new Array(CANVAS_WIDTH);
for (var i = 0; i < CANVAS_WIDTH; ++i)
{
grid[i] = new Array(CANVAS_HEIGHT);
for (var j = 0; j < CANVAS_HEIGHT; ++j)
{
grid[i][j] = "#0000FF";
}
}
renderScene()
}
function updateMousePosition(event)
{
var initialColumn = mouseCellColumn;
var initialRow = mouseCellRow;
var objectPosition = findPos(this);
var gridPosition = new Point(event.pageX - objectPosition.x, event.pageY - objectPosition.y);
mouseCellColumn = getColumn(gridPosition.x);
mouseCellRow = getRow(gridPosition.y);
var cell_position = getCellPosition(mouseCellColumn, mouseCellRow);
// outline the current cell
drawRectangleOutlineWithColour(cell_position.x,
cell_position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#FF0000");
// if mouse has moved cell redraw
if (((initialColumn != mouseCellColumn) ||
(initialRow != mouseCellRow)) &&
(initialColumn > -1) &&
(initialRow > -1))
{
renderGridCell(initialColumn, initialRow);
}
}
function renderScene()
{
for (var i = 0; i < CANVAS_WIDTH; ++i)
{
for (var j = 0; j < CANVAS_HEIGHT; ++j)
{
renderGridCell(i, j);
}
}
}
function renderGridCell(Column, Row)
{
var position = getCellPosition(Column, Row);
drawRectangleWithColour(position.x,
position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
grid[Column][Row]);
}
function drawRectangleWithColour(minX, minY, width, height, colour)
{
context.fillStyle = colour;
context.fillRect(minX,
minY,
width,
height);
}
function drawRectangleOutlineWithColour(minX, minY, width, height, colour)
{
context.strokeStyle = colour;
context.moveTo(minX, minY);
context.lineTo(minX + width, minY);
context.moveTo(minX + width, minY);
context.lineTo(minX + width, minY + height);
context.moveTo(minX + width, minY + height);
context.lineTo(minX, minY + height);
context.moveTo(minX, minY + height);
context.lineTo(minX, minY);
context.stroke();
}
function Point(x, y)
{
this.x = x;
this.y = y;
}
function getColumn(xPosition)
{
if (xPosition < 0)
{
xPosition = 0;
}
if (xPosition > CANVAS_DIMENSION)
{
xPosition = CANVAS_DIMENSION;
}
return Math.floor(xPosition/GRID_CELL_WIDTH);
}
function getRow(yPosition)
{
if (yPosition < 0)
{
yPosition = 0;
}
if (yPosition > CANVAS_DIMENSION)
{
yPosition = CANVAS_DIMENSION;
}
return Math.floor(yPosition/GRID_CELL_HEIGHT);
}
function getCellPosition(column, row)
{
if (row < 0)
{
row = 0;
}
if (row > CANVAS_HEIGHT)
{
row = CANVAS_HEIGHT - 1;
}
if (column < 0)
{
row = 0;
}
if (column > CANVAS_WIDTH)
{
column = CANVAS_WIDTH - 1;
}
var result = new Point(column * GRID_CELL_WIDTH, row * GRID_CELL_HEIGHT);
return result;
}
function findPos(obj)
{
var result = new Point(0, 0);
if (obj.offsetParent)
{
do
{
result.x += obj.offsetLeft;
result.y += obj.offsetTop;
} while (obj = obj.offsetParent);
}
return result;
}
</script>
</head>
<body onload="init()">
<div id="test" style="width: 200px; height:200px; margin: 0px auto;">
<canvas id="myCanvas" width="200" height="200">
Your browser does not support the canvas element.
</canvas>
</div>
</body>
</html>
违规区域在这里：
// outline the current cell
drawRectangleOutlineWithColour(cell_position.x,
cell_position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#FF0000");
// if mouse has moved cell redraw
if (((initialColumn != mouseCellColumn) ||
(initialRow != mouseCellRow)) &&
(initialColumn > -1) &&
(initialRow > -1))
{
renderGridCell(initialColumn, initialRow);
}
这没有效果，轮廓会累积。
一些深入研究画布重绘建议'clearRect'但这似乎没有帮助，轮廓仍然存在：
// outline the current cell
drawRectangleOutlineWithColour(cell_position.x,
cell_position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#FF0000");
// if mouse has moved cell redraw
if (((initialColumn != mouseCellColumn) ||
(initialRow != mouseCellRow)) &&
(initialColumn > -1) &&
(initialRow > -1))
{
context.clearRect(position.x,
position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT);
renderGridCell(initialColumn, initialRow);
}
让我们用不同的颜色重新绘制轮廓区域？
// outline the current cell
drawRectangleOutlineWithColour(cell_position.x,
cell_position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#FF0000");
// if mouse has moved cell redraw
if (((initialColumn != mouseCellColumn) ||
(initialRow != mouseCellRow)) &&
(initialColumn > -1) &&
(initialRow > -1))
{
var position = getCellPosition(initialColumn, initialRow);
drawRectangleWithColour(position.x,
position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#00FF00");
}
不，网格重绘，但对线条没有影响。重绘整个网格？
renderScene();
// outline the current cell
drawRectangleOutlineWithColour(cell_position.x,
cell_position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#FF0000");
/*
// if mouse has moved cell redraw
if (((initialColumn != mouseCellColumn) ||
(initialRow != mouseCellRow)) &&
(initialColumn > -1) &&
(initialRow > -1))
{
var position = getCellPosition(initialColumn, initialRow);
drawRectangleWithColour(position.x,
position.y,
GRID_CELL_WIDTH,
GRID_CELL_HEIGHT,
"#00FF00");
}
*/
我似乎无法很好地解释画布如何处理这个问题，我在 MacOSX 上的 Safari 和 Windows7 上的 Chrome 和 Firefox 中得到了相同的行为。