画布 - 在完全透明的所有区域填充矩形 [英] Canvas - Fill a rectangle in all areas that are fully transparent
问题描述
我在使用HTML5 canvas编写一个简单的2D游戏引擎。我来添加一个照明引擎。每个光源具有半径值和强度值(0-1,例如1将非常亮)。还有一个环境光值,用于点亮世界上没有光源(0-1,例如0.1将是月光)附近的一切。照明的过程在主画布上方的单独画布上完成:
- 对于每个光源,在该位置绘制径向渐变具有与光源相同的半径。梯度给出两个停止:中心是黑色的,具有1-强度的光的α,并且末端/边缘是黑色的,具有1-环境光值的α。
- 这是错误的地方:/我需要用黑色和1-环境光值的alpha填充整个画布,在这一刻,我这样做将context.globalCompositeOperation设置为source-out,然后fillRecting整个画布。
我的代码为:
var amb ='rgba(0,0,0,'+(1-f.ambientLight)+')';
for(i in f.entities){
var e = f.entities [i],p = f.toScreenPoint(e.Position.x,e.position.y),radius = e。光半径
if(radius> 0){
var g = cxLighting.createRadialGradient(p.x,p.y,0,p.x,p.y,radius);
g.addColorStop(0,'rgba(0,0,0,'+(1-e.light.intensity)+')');
g.addColorStop(1,amb);
cxLighting.fillStyle = g;
cxLighting.beginPath();
cxLighting.arc(p.x,p.y,radius,0,Math.PI * 2,true);
cxLighting.closePath();
cxLighting.fill();
}
}
//环境光
cxLighting.globalCompositeOperation ='source-out';
cxLighting.fillStyle = amb;
cxLighting.fillRect(0,0,f.width,f.height);
cxLighting.globalCompositeOperation ='source-over';
然而,不是得到我从引擎(左)反向梯度(右)。我认为这是因为当我用 source-out
复合操作绘制矩形时,它会影响渐变本身的颜色,因为它们是半透明的。
有什么办法不同或更好?
此外,我修改了Mozilla开发中心的
额外优化:您可以根本不使用任何路径实现效果,简单地绘制完全相同的径向渐变到矩形,而不是圆形路径:
http://jsfiddle.net/a2Age/2/
希望有所帮助!
I'm writing a simple 2D game engine using the HTML5 canvas. I've come to adding a lighting engine. Each light source has a radius value and an intensity value (0-1, eg 1 would be very bright). There's also an ambient light value that is used to light everything else in the world that isn't near a light source (0-1, eg 0.1 would be moonlight). The process of lighting is done on a separate canvas above the main canvas:
- For each light source, a radial gradient is drawn at that position with the same radius as the light source. The gradient is given two stops: the center is black with an alpha of 1-intensity of the light and the end/edge is black with alpha of 1-ambient light value. That all works fine.
- This is where it goes wrong :/ I need to fill the whole canvas with black with and alpha of 1-ambient light value and at the moment I do this by setting the context.globalCompositeOperation to source-out then fillRecting the whole canvas.
My code for this stuff is:
var amb = 'rgba(0,0,0,' + (1-f.ambientLight) + ')';
for(i in f.entities) {
var e = f.entities[i], p = f.toScreenPoint(e.position.x, e.position.y), radius = e.light.radius;
if(radius > 0) {
var g = cxLighting.createRadialGradient(p.x, p.y, 0, p.x, p.y, radius);
g.addColorStop(0, 'rgba(0,0,0,' + (1-e.light.intensity) + ')');
g.addColorStop(1, amb);
cxLighting.fillStyle = g;
cxLighting.beginPath();
cxLighting.arc(p.x, p.y, radius, 0, Math.PI*2, true);
cxLighting.closePath();
cxLighting.fill();
}
}
//Ambient light
cxLighting.globalCompositeOperation = 'source-out';
cxLighting.fillStyle = amb;
cxLighting.fillRect(0, 0, f.width, f.height);
cxLighting.globalCompositeOperation = 'source-over';
However instead of getting what I wan't out of the engine (left) I get a kind of reversed gradient (right). I think this is because when I draw the rectangle with the source-out
composite operation it affects the colours of the gradient itself because they are semi-transparent.
Is there a way to do this differently or better? Using clipping maybe, or drawing the rect over everything first?
Also, I modified the Mozila Dev Centre's example on composting to replicate what I need to do and none of the composite modes seemed to work, check that out if it would help.
Thanks very much, any answer would be great :)
One trivial way would be to use imageData
but that would be painfully slow. It's an option, but not a good one for a game engine.
Another way would be to think of the ambient light and the light-source as if they were one path. That would make it very easy to do:
Or see it with an image behind: http://jsfiddle.net/HADky/10/
The thing you're taking advantage of here is the fact that any intersection of a path on canvas is always only unioned and never compounded. So you're using a single gradient brush to draw the whole thing.
But it gets a bit trickier than that if there's more than one light-source. I'm not too sure how to cover that in an efficient way, especially if you plan for two light-sources to intersect.
What you should probably do instead is devise an alpha channel instead of this overlay thing, but I can't currently think of a good way to get it to work. I'll revisit this if I think of anything else.
EDIT: Hey! So I've done a bit of thinking and came up with a good solution.
What you need to do is draw a sort of alpha channel, where the dark spots mark the places where you want light to be. So if you had three light sources it would look like this:
Then you want to set the fill style to your ambient color and set the globalCompositeOperation to xor and xor the whole thing.
ctx.fillStyle = amb;
ctx.globalCompositeOperation = 'xor';
ctx.fillRect(0,0,500,500);
That will leave you with the "opposite" image except the transparent parts will be correctly ambient:
Here's a working example of the code:
Extra optimization: You can actually achieve the effect without using any paths at all, by simply drawing the exact same radial gradients onto rects instead of circular paths:
http://jsfiddle.net/a2Age/2/ Hope that helps!
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