canvas fillStyle 间隔透明/不透明

Question

This small code clear old canvas data in interval:

// start interval
ctx.save();
    ctx.fillStyle = "rgba(0, 0, 0, 0.2)";
    ctx.fillRect(0, 0, ctx.width, ctx.height);
ctx.restore();
//some draw code for new graph
...
//end interval

My work area become black, because i set black as fill color ( rgba(0, 0, 0, .2) ), but I need a transparent background, not black.

I tried use globalAlpha and imagePutData but i failed.

How i can do this?

Answer 1

我认为这将解决您的问题

ctx.fillStyle = "rgba(255, 255, 255, 0.5)";

Answer 2

Using an rgba(0,0,0,.2) fillStyle and fillRect() works for me on both chrome and firefox - it paints a semi-transparent black fill. Check to make sure you're not doing something else that's causing a fully opaque paint of some sort.

Answer 3

尝试 ctx.canvas.width，而不是 ctx.width

Answer 4

Has your problem been solved

I encountered this problem while using Windows computer

I used opacity: .99 solved this problem