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 !
Related
Ok, so i am making a 3D rendering engine in pure javascript, as a challenge of course - to test my linear algebra skills. I am not using webgl, so please do not say "use webgl".
Anyways, the software will take in triangles, a camera and local transformations, and render the data onto the screen (i even made it interactive)
There are only 6 lines of rendering code, however, which are:
// some shading and math calculations then this:
context.fillStyle = color;
context.strokeStyle = color;
context.beginPath();
context.moveTo(x0, y0);
context.lineTo(x1, y1);
context.lineTo(x2, y2);
context.lineTo(x0, y0);
context.closePath();
context.fill();
context.stroke();
And while that works, it drops to 10fps with 4k+ faces on my Chromebook. (60fps on a regular computer)
Anyways, that outputs this:
But to make it faster, and because canvas state changes are slow, i removed the stroke, making the rendering code:
// some shading and math calculations then this:
context.fillStyle = color;
//context.strokeStyle = color;
context.beginPath();
context.moveTo(x0, y0);
context.lineTo(x1, y1);
context.lineTo(x2, y2);
context.lineTo(x0, y0);
context.closePath();
context.fill();
//context.stroke();
which runs twice as fast, but the resulting thing that gets rendered to the screen is this: (different model)
which has ugly lines everywhere at the edges of the triangles (which get removed when I re-add the stroke)
However, the fps doubles and performance gains are great...
So i believe the lines are caused because the canvas fill doesnt include the area where it would have stroked (the outline, as you may say).
I have tried to fix it with math, and although it works there are some edge cases where it doesn't
So my question is as follows:
Is there a way to make the context fill include the stroke area without stroking, because it is very expensive?
Using both stroke and fill will force the rasterization twice which explains the approximate double time.
The reason why you get glitches between the triangles is because of rounding errors and anti-aliasing. There is not a straight-forward solution to this; the stroke will cover the glitches of course, but to do it without the stroke will require you to offset and expand at least every other triangle.
However, you could use a small trick to cover up the gap and that is to redraw the entire image (as bitmap) on top offset just a single pixel (you might get away with 0.5 pixel but then anti-aliasing is needed). This adds to the time, but far less than rasterization or recalculation of the paths.
Say that the result on the left is what you have (simulated here) with a clear gap. Redrawing it on top as shown in the right will cover the gap without too much distortion.
Simply use:
ctx.drawImage(sourceCanvas, 1, 1);
Tip: when only calling fill() you don't need closePath() as it is called implicit, saving one op. Microscopic gain perhaps but still (with more complex geometry it even might have an influence :) ).
Note: drawing to itself will cause an internal allocation of a temporary bitmap copy. However, you will only need to do one extra drawImage() operation. The option is to use off-canvas render but draw twice to a main displayed canvas. Either way...
var ctx = c.getContext("2d");
ctx.fillStyle = "#777";
tri(10,10, 72,17, 40.2, 100);
// simulates gap
ctx.fillStyle = "#222";
tri(72.5,17.5, 40.7,100.5, 90,25);
// fill entire image back again, drawn twice here for demo
ctx.drawImage(c, 100, 0);
ctx.drawImage(c, 0, 0, 100, 150, 101, 1, 100, 150);
ctx.fillText("Raster", 5, 8);
ctx.fillText("Offset self", 105, 8);
function tri(x0,y0,x1,y1,x2,y2) {
ctx.beginPath();
ctx.moveTo(x0, y0);
ctx.lineTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.fill();
}
<canvas id=c></canvas>
Basically, I want to have a large rectangle and cut smaller rectangles out of it. I can draw the rectangle, then do clearRect(x, y, w, h) but it removes stuff I've drawn on previous to even creating the rectangle.
So basically, can I draw a rectangle over an image and then cut the rectangle without affecting the image underneath it? I have a small idea of using ctx.save and restore, but I can't quite get it to work - and I'm not 100% sure that's the right answer.
Any help would be greatly appreciated!
Thanks,
Cam
An alternative might be to use paths:
Call context.beginPath()
Define the outer rect with moveTo and lineTo and closePath, going clockwise
Cut out the holes the same way but, going counter-clockwise
Call context.fill() to fill the rectangle without the holes.
This only works if you know all the holes from the start, though. This answer has more details for this approach:
Polygon with a hole in the middle with HTML5's canvas
Thanks to markE and Stefan for giving a solution. However, after hours of searching I found my own answer based on this answer. I have no idea why it works, but check out what code I used below:
ctx.fillStyle = "grey";
ctx.beginPath();
ctx.rect(100, 100, 50, 50);
ctx.rect(400, 0, -400, 400);
ctx.fill();
For some reason, if you declare what shape you want to cut out first, and then draw the overlaying shape BACKWARDS (?!) over your area, it cuts out the shape. WTF Javascript? Anyways, it works :)
I am making annotations on images using a jpeg image and rectangles that I am drawing onto the image. I can then transfer the image to a canvas, and, using a for-loop, I can grab the bounds of the rectangular divs that I had drawn on the image to re-draw on the canvas.
The problem arises when I have multiple rectangles because the opacity decreases on each subsequent rectangle due to the fact that they are being layered, as such: `
function drawRectangleToCanvas(left, top, width, height, canvas, context, opacity) {
context.globalCompositeOperation='destination-over';
context.strokeStyle = 'rgba(0,255,130,0.7)';
context.fillStyle = 'rgba(0,0,255,'+opacity+')';
context.rect(left, top, width, height);
context.setLineDash([2,1]);
context.lineWidth = 2;
context.fill();
context.stroke();
}
From my understanding, the context.globalCompositeOperation='destination-over' causes the rectangles to be placed onto the image like slices of bread. With each rectangle that is drawn on the div, the opacities overlap, causing the opacity to increase by a factor of, in this case, 0.1. This is what the issue looks like: Canvas with layered rectangles and opacity problems.
How can I just add all the rectangles without this opacity issue? I call this method for each and every rectangle that I have, so I didn't know if I could put all the rectangles in an array or what. Any suggestions to fix this would be helpful.
EDIT: The darkest rectangle is the first one to be drawn, just to add some information.
Not completely sure what you want but you can leave out the calls to the stroke and fill methods until you have defined all the rectangles.
// Not much left to do in the function but just here to illustrate
// that creating the rectangles should be put together
function drawRectangleToCanvas(left, top, width, height, canvas, context){
context.rect(left, top, width, height);
}
context.globalCompositeOperation='destination-over';
context.strokeStyle = 'rgba(0,255,130,0.7)';
context.fillStyle = 'rgba(0,0,255,'+opacity+')';
context.setLineDash([2,1]);
context.lineWidth = 2;
context.beginPath();
while(??? ){
// loop and define all the rectangles
drawRectangleToCanvas(... //
}
// once all the rectangles are defined
// call the fill and stroke to render them
context.fill();
context.stroke();
This will stop them compounding the alpha values
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'm trying to draw a circle and then have an image follow the circle around. Later I want to rotate and move the image around with respect to the circle drawn. The problem I'm facing is that when I try to rotate the image it won't rotate. It also doesn't show me an error in the console. I have functions allowing me to move the circle around and the image moves with it, I just can't seem to rotate the image.
Here is the code:
draw: function(){
//draw self on canvas;
//intended only to be called from update, should never
//need to be deliberately called
ctx = this.context;
ctx.save();
ctx.fillStyle="#000000";
ctx.beginPath();
//void arc(double x, double y,
// double radius, double startAngle, double endAngle,
// optional boolean anticlockwise = false);
ctx.arc(this.x,this.y,this.size,0,Math.PI*2,true);
ctx.closePath();
ctx.fill();
//ctx.translate(this.x, this.y);
ctx.rotate(this.imgAngle);
//draw the hammer
ctx.drawImage(this.hammer,this.hammerX,this.hammerY,100,100)
ctx.rotate(Math.PI/2);
ctx.restore();
},
Live Demo
Try changing your code to the following. You need to perform the rotation before drawing. You can translate the canvas to the entities position and then draw the image at x:0,y:0 to get the effect you desire. Note I did 0-50,0-50 because that puts the point of origin in the center since the width and height are 100. Meaning your image will rotate around its center rather than around its corner.
//draw the hammer
ctx.translate(this.hammerX, this.hammerY);
ctx.rotate(this.imgAngle);
ctx.drawImage(this.hammer,0-50,0-50,100,100);
The rotation will only affect drawings made AFTER the rotation is done.
You could try moving the rotate calls to just before the object that needs to be rotated?