为什么HTML 5画布中没有矩形黑色？ [英] Why isn't rectangle black in HTML 5 canvas?

问题描述

下面是一个带有canvas标签的简单HTML 5页面。在画布上，矩形以黑色绘制，并显示黑色文本。但由于某种原因，矩形实际上是灰色的。为了使它变黑，我必须在它上面画2到3次。这似乎表明存在某种α问题，但我不知道为什么会这样。

Below is a simple HTML 5 page with a canvas tag. On the canvas a rectangle is drawn in black, and black text is shown. But for some reason, the rectangle is actually grey. To make it black, I have to draw it 2 or 3 times on top of itself. That seems to indicate there is some sort of alpha issue going on, but I don't know why that would be.

另外，线宽看起来比1px宽......？

ALSO, the line width looks quite a bit more than 1px wide.... ?

任何人都可以告诉我我做错了什么？

Can anyone show me what I'm doing wrong?

function draw()
{
    var canvas = document.getElementById('tutorial');
    if (canvas.getContext)
    {
        var ctx = canvas.getContext('2d');
        //ctx.globalAlpha = 1;
        //ctx.globalCompositeOperation = "source-over";

        ctx.lineWidth = "1";
        ctx.strokeStyle = "#000000";
        ctx.strokeRect(100, 100, 50, 50); //appears grey

        ctx.font = "22px Arial";
        ctx.fillStyle = "#000000";
        ctx.fillText("test", 120, 120); //appears black as expected
    }
}

推荐答案

你的线看起来很胖和灰色，因为它在绘制时会重叠像素 - 即你的线跨越了两个像素。如果您将代码更改为：

Your line looks fat and grey as it is overlapping pixels when it is drawn - i.e. your line straddles two pixels. If you change your code to:

ctx.strokeRect(100.5, 100.5, 50, 50);

你会看到一条黑线。

查看此页面了解更多信息： http://diveintohtml5.info/canvas.html#paths

See this page for more information: http://diveintohtml5.info/canvas.html#paths

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