Question

我正在使用HTML画布进行一些工作，我发现当我绘制一条线时，我无法清除或绘制它？

所以我正在做的事情：我有一个带有网格的画布，每个单元格绘制自己（填充矩形，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（在画布上绘制四条线，没有其他重绘）。当鼠标移动到不同的单元格时，应该勾勒出新单元格（如前所述），并且不再勾画出前面列出的单元格（目的是重绘单元格，绘制线条）。它没有“概述”被证明是问题的细胞。

我认为它适用于画家的算法，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是用更高层的线条分开处理的？

以下是展示它的独立页面的来源：

<!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，轮廓累积。

一些挖掘画布重绘表明'clearRect'，但这似乎没有帮助，大纲仍然存在：

// 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");
    }
*/

我似乎无法找到关于画布如何处理这个的很好的解释，我在MacOSX上的Safari和Windows7上的Chrome和Firefox中获得相同的行为。

Answer 1

这里有两件事情。

第一个也是最重要的是你永远不会打电话给context.beginPath()

每当您认为自己正在绘制一个新的红色轮廓时，实际上是在添加上下文的当前路径并使其更长。第一次上下文只有一个rect，然后它有两个，然后它有三个，等等。

每次调用笔划时，您都会抚摸您绘制的每个矩形，因为上下文的当前路径包含它们。要解决此问题，您必须通过调用beginPath来重置当前路径。

第二个问题是您的线条没有在完美像素上绘制，因此当您去除它们时，您会看到由于抗锯齿造成的图形问题。要解决此问题，您需要绘制完美的像素，并注意要擦除的矩形。有关这方面的更多信息，请参阅Loktar的回答：HTML5 Canvas and Line Width

这是您的代码，使用这两个修复程序实现：

http://jsfiddle.net/QjKAp/

HTML画布清晰绘制的线条

1 个答案: