I'm trying to use a canvas for two things in my application. First is to display a video on canvas (which I already did). After that, I need to draw squares over this video, and it's in that part that I've got some trouble. When I try to use one canvas over another, the video always stays on top of the squares I'm creating. If i use only one canvas the same thing happens. Can someone help me please?
With canvas it's all about order of operations, in your example you are drawing a video and want the sqaures on top, so you need to draw the squares AFTER you draw the video.
Simple example:
https://jsfiddle.net/a3z5hb45/
ctx.fillStyle = "red"
ctx.fillRect(x, y, 100, 100);
ctx.fillStyle = "blue"
ctx.fillRect(x + 50, y + 50, 100, 100);
Here you see that red goes under blue. Imagine that red is your video, so if you drew something after it will appear above.
If you reverse the order you get this:
https://jsfiddle.net/a3z5hb45/1/
Related
I am working on a real time canvas drawing webapp using socket.io, node.js, and p5.js. I am having trouble creating a smooth line when the mouse is dragged. If the mouse is dragged too fast there is a trail of empty space in between each ellipse. The end goal here is to create a smooth path. Here are the things I have tried so far:
Attempt 1:
noStroke();
fill(lineColor[0],lineColor[1],lineColor[2]);
ellipse(mouseX, mouseY, lineThickness, lineThickness);
Attempt 2:
strokeWeight(lineThickness);
line(mouseX,mouseY);
stroke(lineColor[0],lineColor[1],lineColor[2]);
Here is a picture of what the issue looks like:
canvas drawing incomplete path image
any feedback is welcome; thanks!
Kevin's answer is great because it will be more efficient to draw lines instead of many ellipses. You should also look into:
beginShape()/endShape()
bezierVertex()
curveVertex()
curvePoint()
The above should help you draw a smooth path and setting a thicker stroke will looks as it many filled ellipses are connected forming the path.
If for some reason you do want to draw many ellipses, you can interpolate position when the mouse move faster and create gaps to fill those gaps.
For more information and p5.js example, check out this answer:
Processing: Draw vector instead of pixels
You could use the pmouseX and pmouseY variables, which hold the previous position of the cursor. Use that to draw a line from the previous position to the current position to fill in the blank space between mouse events.
From the reference:
// Move the mouse across the canvas to leave a trail
function setup() {
//slow down the frameRate to make it more visible
frameRate(10);
}
function draw() {
background(244, 248, 252);
line(mouseX, mouseY, pmouseX, pmouseY);
print(pmouseX + " -> " + mouseX);
}
<script src="https://github.com/processing/p5.js/releases/download/0.5.11/p5.js"></script>
I am making a 3d game and to make a cube, an image needed to rotate more than to it's 0 point:
the last image is like seeing the back of the image as it was transparent. It has rotated round......
I tried this, I think it should work, or didn't the javascript developers think of it? :
var ctx = document.getElementById("canvas").getContext("2d");
ctx.beginPath();
ctx.fillStyle = "red";
ctx.fillRect(100, 100, -100, - 100);
Here is my 3d thingy for those who are interested : Just for fun, try to use it also with sphere, in your notepad
Instead of using negative widths, you can use negative scale factors, e.g. if you use ctx.scale(-1, 1) then all of your coordinates will draw from right to left instead of left to right.
See https://jsfiddle.net/alnitak/eqyqmdk3/ for a demo.
I'm just started working with Leap Motion (it is so much fun). The Leap works mainly with vectors. And now I want to create a program where I can visualise where is a vector pointing. The only way I can imagine doing this is by using a small image which appears when this fuction is on and positioning by using the img.style.left , img.style.top instructions. Any other ideas?
If your goal is to represent 2D Vectors,
You can use canvas to draw lines.
A canvas is like a div but you can draw whatever you want in it, I don't know anything about Leap Motion but if you want to draw lines and circles at precise coordinates, it may be a good solution instead of working with the DOM itself.
The JS part looks like this :
var canvas = document.getElementById('my-canvas');
var ctx = canvas.getContext('2d');
//For exemple here is how to draw a rectangle
//fillStyle support all valid css color
ctx.fillStyle = "rgba(50, 255, 24, 0.7)";
//Create the rectangle, with (startX, startY, height, width)
ctx.fillRect(20, 15, 50, 50);
ctx.beginPath(); //Tells canvas we want to draw
ctx.moveTo(250,250); //Moves the cursor to the coordinates (250, 250);
ctx.lineTo(75, 84); //Draws a line from the cursor (250, 250) to (75, 84);
ctx.closePath(); //Tells canvas to 'close' the drawing
ctx.strokeStyle = "red";
ctx.stroke(); //Draws the line stroke
And the HTML is simply :
<canvas id="my-canvas" height="500px" width="500px">
Here is the text displayed when the browser doesnt support canvas.
</canvas>
I made a jsfiddle to show you what simple things we can do with canvas.
http://jsfiddle.net/pq8g0bf0/1/
A nice website to learn canvas : http://www.html5canvastutorials.com/tutorials/html5-canvas-element/
Since it's javascript, you are free to do calculations for your vectors coordinates, addding eventListeners etc ...
I'm trying to understand the combination of HTML5/CSS3 and Javascript more and more.
That's why I thought, make a little project so you learn all about that more.
In short, I like the new iOS7 wallpaper and use it on my website (http://www.betadevelops.com). Then I thought, let's make this more lightweight and draw it with pure Javascript.
I started and managed to get quite far (http://www.betadevelops.com/jOS7.html). But now I face a stupid problem I can't seem to get fixed.
I draw circles on the canvas, and dynamically assign colors to it. But each time a new circle (and so a new color gets chosen) it automatically recolors the old circles...
So let's say, 10 circles:
1: blue circle, draw's it and done
2: yellow circle, draw's it and done, but it also colors the first blue one to yellow
I also wanted to add opacity and blurring. The opacity kinda works in the sense it has opacity on only 2-3 circles from the 20 I draw. I think this is not possible because I use Math.Random the calculate a random opacity.
Considered the blurring, I can add blurring to the whole canvas with follow code
canvas.style.webkitFilter = "blur(3px)";
but that's not what I want. I want the blur on the circle itself and to be more precisely, the outline. I read about it and it's not possible, but you can mimmick the looks with using CSS box-shadow.
So I tried
canvas.style.webkitFilter = "box-shadow(10px 10px 5px #888)";
but this also doesn't work it seems...
So, you website guru's. What am I doing wrong and can you help me out?
You can find the code by clicking on the second link. Uploaded it there.
EDIT:
Nevermind the blur, managed to solve it partially with this code
if (blurred) {
ctx.shadowColor = color;
ctx.shadowBlur = 15;
ctx.shadowOffsetX = 0;
ctx.shadowOffsetY = 0;
}
#Stig Runar Vangen has the correct answer.
I would just add that if you don't intend the circles to "run", you could use ctx.closePath after drawing each ctx.arc.
ctx.beginPath();
ctx.arc(centerX, centerY, diameter, 0, 2 * Math.PI, false);
ctx.closePath();
color = color.replace('opacity', Math.random().toString());
ctx.fillStyle = color;
ctx.fill();
The reason why you see that all your circles gets the same color, might be because you join all circles into one draw operation. To separate each circle draw operation, start each circle placement with:
ctx.beginPath();
Each arc should then also be drawn with a call to either ctx.stoke() or ctx.fill() after the definition of each single circle.
This is purely guesswork as I haven't seen your code.
I am using an HTML canvas and javascript and I need to clear all of the pixels underneath a shape created by closing a path (for example, I am using flot, and I want to make rounded corners, and to do this, I first need to remove the square corners by drawing a curve on top of the corner to remove the desired pixels).
Right now, I am doing this by just filling the shape with the same color as the background, which can imitate what I want to do, but, it is not ideal as it makes it impossible to place the chart on top of non-solid backgrounds without seeing the square corners. I know that there is a clearRect method that would do what I want to do, but with only rectangles, I need to do it with any closed shape. Is it possible, and if so, how would I do it?
brainjam's code was heading in the right direction, but didn't fully solve the problem. Here's the solution:
context.save();
context.globalCompositeOperation = 'copy';
context.fillStyle = 'rgba(0,0,0,0)';
//draw shape to cover up stuff underneath
context.fill();
context.restore();
Here's an example of a function that will clear a circle from a canvas:
var clearCircle = function(x, y, radius)
{
context.save();
context.globalCompositeOperation = 'destination-out';
context.beginPath();
context.arc(x, y, radius, 0, 2 * Math.PI, false);
context.fill();
context.restore();
};
I think what you want is a clipping region, defined by the clip() function. The latter takes a bunch of paths. Here's an example.
This is a little different from what you are specifically asking (which is to remove pixels after drawing them), but actually not drawing the pixels in the first place is probably better, if I understand your requirements correctly.
Edit: I now think I understand that what you want to do is clear pixels to transparent black. To do that, after having defined your paths, do something like this:
context.fillStyle = 'rgba(0,0,0,0)';
context.fill();
The first statement sets the fill color to transparent black.
Use globalCompositeOperation = 'destination-out' instead of 'copy', it will erase all pixels of the shape in the canvas.
See all kinds of composition here
very usefull !