I have some colored lines in a canvas which I scan using a mousemove event, which returns the line's hex color code.
Is there anyway I could make this canvas invisible to the eye, but when the pointer goes over the canvas, still returns the correct color code?
I've tried setting the context's globalAlpha to transparent or near transparent (0, 0.1...) with mixed results in the following fiddle:
In Firefox, the collected hex returns an altered color due to transparency,
In Chrome, it returns the correct color regardless of transparency (this is actually the behavior I want).
And oddly, in my original code, even in Chrome, canvas lines with globalAlpha=0 no longer return their original color.
Can someone kindly explain what the expected behavior for globalAlpha is? More importantly, is there another way to make a canvas invisible to the eye but which still allows collecting the colors with a mouse event? Any help appreciated!
Set CSS property opacity to 0.
When you apply the property to your canvas like
#examplecanvas {
opacity: 0;
}
It will vanish from the screen but still detect color when you mouse over it (or click it).
Related
I've been having difficulty getting the expected results with setChildIndex().
In this example I have 2 MovieClip instances named redDot and yellowDot, and a black square Shape. I would expect it to place yellowDot on bottom, then square, then redDot.
//make black rectangle shape
var square = new createjs.Shape(new createjs.Graphics().f("#000000").dr(100,100,100,100));
this.addChild(square);
this.setChildIndex(this.yellowDot, 0); //set z-index towards background
this.setChildIndex(square,1);
this.setChildIndex(this.redDot, 2);//towards foreground
Instead I get redDot, yellowDot, then square. Adding this.stop() to the end seems to change it back to the expected order. It's not clear to me what is causing this discrepancy. Looping is disabled in the publish settings. Am I misunderstanding how this function and the Animate timeline work?
I wonder if the stage is not being updated? What happens if you use stage.update() at the end of your code. (or however you update the stage in an animate script).
I believe calling this.stop() is re-rendering the initial state of the clip, which uses the original z-index definition. Probably makes sense to call it before you change the contents programmatically.
I have application made using PIXI.js and it uses a WebGLRenderer.
I'm preserving the drawing buffer and not clearing before render:
{ preserveDrawingBuffer: true, clearBeforeRender: false }
This allows me to create trails as objects move around.
I want the trails to fade out over time, so I apply a transparent black rectangle on top over every rendering. This works, but the fade out eventually rounds off to gray. I want a complete fade to black.
I've tried using a ColorMatrixFilter filter with a lowered brightness on my root container, hoping it would cause a fade effect. It didn't cause any fade effect, instead just causing everything to be slightly darker. If that had worked, then a custom filter could help to do the job.
How can I get a slow gradual fade to complete black for the trails of my rendered objects?
EDIT: Here are a few examples of what I've tried:
// `this` being my app object.
this.fadeGraphics = new PIXI.Graphics()
this.root.addChild(this.fadeGraphics)
// Blend Mode
this.fadeGraphics.blendMode = PIXI.BLEND_MODES.MULTIPLY
this.fadeGraphics.beginFill(0xf0f0f0)
this.fadeGraphics.drawRect(0, 0, this.screenWidth, this.screenHeight)
this.fadeGraphics.endFill()
// Transparent black rectangle.
this.fadeGraphics.beginFill(0x000000, .05)
this.fadeGraphics.drawRect(0, 0, this.screenWidth, this.screenHeight)
this.fadeGraphics.endFill()
Both these methods leave me with a gray trail, the trail goes away if my values are strong enough. Though, I want a very long-term trail so I have to use small values, and possibly also apply them every nth frame.
I think a SUBTRACT blend mode might be able to do what I need.
Unfortunately it doesn't seem available in Pixi.js.
I eventually figured out that fading to white works wonderfully.
Thus, one solution is to fade to white, then invert color, and rotate hue 180 degrees.
You can do this with a CSS filter on your canvas, though it doesn't work in all browsers, has a performance hit, and all your color intensities get inverted.
I would like to create an element, that shows a red circle. Once the user clicks on it, she can record her voice. In order to show the LIVE mode, I'd like to make the circle "breath" according to the incoming frequencies.
I'm experimenting with a <canvas> element. That means it creates a circle that gets bigger and smaller, depending on the variable arcrad. However, the lines are being drawn correctly, but they do not disappear afterwards. I tried to apply .clip() but can't get it to work...
if (arcrad <= 10) arcrad = 10;
analyserContext.beginPath();
analyserContext.arc(100,120,arcrad,0,2*Math.PI);
analyserContext.closePath();
analyserContext.lineWidth = 2;
analyserContext.strokeStyle = 'red';
analyserContext.stroke();
Any ideas - or completely different strategies for this use case?
Canvas will overdraw by default. For your animation you’ll need to clean the canvas at the start of each frame. Use something the following at the start of your drawing function:
analyserContext.clearRect(0,0,200,200);
assuming your canvas is 200 pixels wide and high. It’s worth pointing out that sometimes you don’t want to completely clear the animation field every frame. For example, if you were to draw a semi transparent rectangle over the frame at the beginning (instead of clearing it) then you’d end up with a basic ‘bullet time’ style effect.
It's a normal behavior. Once something it's drawn on the canvas, it's there forever. You have to think like if you were painting something: what has been done cannot be undone.
Luckily, you still have solutions:
1) redraw another circle on top of the first one with the background color. It's really not the recommend way, but it still can be useful
2) use clearRect method (see How to clear the canvas for redrawing)
There are numerous ways to clear a canvas pre drawing to create animation:
How to clear the canvas for redrawing
simplest in my mind:
canvas.width=canvas.width;
though can equally use clearRect (which is actually quicker and won't reset the entire canvas if that is an issue regarding transforms etc!) over the region or whole canvas.
Get the likes of:
http://jsfiddle.net/dw17jxee/
So I know that context.clearRect makes pixels transparent, but I'm wondering, is there a function to make pixels translucent?
For example, say I have a canvas with these colors (fourth one in each color is alpha):
#ffff #feef #abff
#5f6f #000f #ffff
Running clearRect would resolve into this (or something, just make them all transparent):
#fff0 #fee0 #abf0
#5f60 #0000 #fff0
I want to remove opacity, but not make it transparent (kind of like globalAlpha for clearRect), so that it can end up like this (lets say I set the globalAlpha equivalent to 0.5):
#fff8 #fee8 #abf8
#5f68 #0008 #fff8
Is this possible? Or would it be simpler just to draw everything on an off-screen canvas, then draw that canvas (with globalAlpha set) on an on-screen one?
Let me know if this isn't clear in any way.
The answer above gets the job done, however getImageData is super slow and if you have a lot of other stuff going on it will slow down your animation immensely. If you create a second off screen canvas element you can set its global alpha to .9 and shuffle them back and forth and get the same effect with much greater speed.
context2.clearRect(0,0,width,height);
context2.globalAlpha = .9;
context2.drawImage(canvas1,0,0);
context1.clearRect(0,0,width,height);
context1.drawImage(canvas2,0,0);
context1.the rest of the drawing that you are doing goes here.
I just tried to figure this out too, and I've decided to count through the pixels, setting the alpha channel of each one manually. This is a bit more work, because I can't just cover the entire canvas with a rect, but it's working so far.
I'm doing this so that I can set a background image for the webpage and put my canvas animation over it, without having to draw the background in the canvas element.
var oldArray = context.getImageData(0,0,canvas.width,canvas.height);
//count through only the alpha pixels
for(var d=3;d<oldArray.data.length;d+=4){
//dim it with some feedback, I'm using .9
oldArray.data[d] = Math.floor(oldArray.data[d]*.9);
}
sw.context.putImageData(oldArray,0,0);
I just created a fancy canvas effect using cheap motion blur
ctx.fillStyle = "rgba(255,255,255,0.2)";
ctx.fillRect(0,0,canvas.width,canvas.height);
Now i want to do the same, but with transparent background. Is there any way to do something like that? I'm playing with globalAlpha, but this is probably a wrong way.
PS: Google really don't like me today
Here's a more performance friendly way of doing it, it requires an invisible buffer and a visible canvas.
buffer.save();
buffer.globalCompositeOperation = 'copy';
buffer.globalAlpha = 0.2;
buffer.drawImage(screen.canvas, 0, 0, screen.canvas.width, screen.canvas.height);
buffer.restore();
Basically you draw your objs to the buffer, which being invisible is very fast, then draw it to the screen. Then you replace clearing the buffer with copying the last frame onto the buffer using the global alpha, and globalCompositeOperation 'copy' to make the buffer into a semi-transparent version of the previous frame.
You can create an effect like this by using globalAlpha and two different canvas objects: one for the foreground, and one for the background. For example, with the following canvas elements:
<canvas id="bg" width="256" height="256"></canvas>
<canvas id="fg" width="256" height="256"></canvas>
You could copy draw both a background texture and a motion blurred copied of foreground like so:
bg.globalAlpha = 0.1;
bg.fillStyle = bgPattern;
bg.fillRect(0, 0, bgCanvas.width, bgCanvas.height);
bg.globalAlpha = 0.3;
bg.drawImage(fgCanvas, 0, 0);
Here is a jsFiddle example of this.
OP asked how to do this with an HTML background. Since you can't keep a copy of the background, you have to hold onto copies of previous frames, and draw all of them at various alphas each frame. Nostalgia: the old 3dfx Voodoo 5 video card had a hardware feature called a "t-buffer", which basically let you do this technique with hardware acceleration.
Here is a jsFiddle example of that style. This is nowhere near as performant as the previous method, though.
What you are doing in the example is partially clear the screen with a semi transparent color, but as it is, you will always gonna to "add" to the alpha channel up to 1 (no transparency).
To have this working with transparent canvas (so you can see what lies below) you should subtract the alpha value instead of adding, but I don't know a way to do this with the available tools, except running all the pixels one by one and decrease the alpha value, but this will be really, really slow.
If you are keeping track of the entities on screen you can do this by spawning new entities as the mouse moves and then setting their alpha level in a tween down to zero. Once they reach zero alpha, remove the entity from memory.
This requires multiple drawing and will slow down rendering if you crank it up too much. Obviously the two-canvas approach is the simplest and cheapest from a render performance perspective but it doesn't allow you to control other features like making the "particles" move erratically or apply physics to them!