Develop Reference

Rotate a group of shapes containing text whilst keeping text centered and horizontal

This is probably just maths.
I am using Konva to dynamically generate shapes, which I'm storing as a label. So there's a label which contains a textElement and a rectangle. I want to make sure text in that rectangle is always a) Centered horizontally and vertically and b) facing the right way up.
So a rectangle could have any rotation, but I always want the text centered and facing the right way up.
The code for creation; width, height, rotation, x and y all have values pulled from a database.
var table = new Konva.Label({
x: pos_x,
y: pos_y,
width: tableWidth,
height: tableHeight,
draggable:true
});
table.add(new Konva.Rect({
width: tableWidth,
height: tableHeight,
rotation: rotation,
fill: fillColor,
stroke: strokeColor,
strokeWidth: 4
}));
table.add(new Konva.Text({
width: tableWidth,
height: tableHeight,
x: pos_x, //Defaults to zero
y: pos_y, //Default to zero
text: tableNumber,
verticalAlign: 'middle',
align: 'center',
fontSize: 30,
fontFamily: 'Calibri',
fill: 'black'
}))
tableLayer.add(table);
The problem is, if rotation is in place, text is off center, as in this image:
I do manually correct in some circumstances - for example if rotation = 45 degrees:
pos_x = -tableWidth/2;
pos_y = tableHeight/5;
but that is not a permanent solution. I want the x and y co-ordinates of the text to be at the centerpoint of the shape itself.
I've tried a few approaches (such as applying rotation to the Label itself and then negative rotation value to the text)

This code snippet illustrates a solution. It is copied & modified from my other self-answer when I was looking for a robust approach to rotation around an arbitrary point - note that I consider this a slightly different question than my original so I have not suggested this is a dup. The difference is the need to work with a more complex grouped shape and to keep some element within that group unrotated.
Not in the OP's question, but I set a background rectangle into the text by making the text a group. The purpose of this was to show that the text rectangle will extend outside the label rectangle in some points of rotation. This is not a critical issue but it is useful to see it happen.
The fundamental challenge for the coder is to understand how the shapes move when rotated since we usually want to spin them around their centre but the fundamental 2D canvas pattern that Konva (and all HTML5 canvas wrappers) follow is to rotate from the top-left corner, al least for rectangles as per shapes in the question. It 'is' possible to move the rotation point (known as the offset) but again that is a conceptual challenge for the dev and a nice trap for anyone trying to support the code later.
There's a lot of code in this answer that is here to set up something dynamic that you can use to visualise what is going on. However, the crux is in this:
// This is the important call ! Cross is the rotation point as illustrated by crosshairs.
rotateAroundPoint(shape, rotateBy, {x: cross.x(), y: cross.y()});
// The label is a special case because we need to keep the text unrotated.
if (shape.name() === 'label'){
let text = shape.find('.text')[0];
rotateAroundPoint(text, -1 * rotateBy, {x: text.getClientRect().width/2, y: text.getClientRect().height/2});
}
The rotateAroundPoint() function takes as parameters the Konva shape to rotate, the clockwise rotation angle (not radians, good ole degrees), and the x & y position of the rotation point on the canvas / parent.
I constructed a group of shapes as my label, composing it from a rectangle and a text shape. I named this 'label'. Actually I switched the text shape to be another group of rect + text to that I could show the rectangle the text sits within. You could leave out the extra group. I named this 'text'.
The first call to rotateAroundPoint() rotates the group named 'label'. So the group rotates on the canvas. Since the 'text' is a child of the 'label' group, that would leave the 'text' rotated, so the next line checks if we are working with the 'label' group, and if so we need to get hold of the 'text' shape which is what this line does:
let text = shape.find('.text')[0];
In Konva the result of a find() is a list so we take the first in the list. Now all that remains for me to do is rotate the text on the 'label' group back again by applying the negative rotation degrees to its center point. The line below achieves this.
rotateAroundPoint(text, -1 * rotateBy, {x: text.getClientRect().width/2, y: text.getClientRect().height/2});
One note worthy of mention - I used a group for my 'text' shape. A Konva group does not naturally have a width or height - it is more of a means to collect shapes together but without a 'physical' container. So to get its width and height for the centre point calculations I use the group.getClientRect() method which gives the size of the minimum bounding box that would contain all shapes in the group, and yields an object formed as {width: , height: }.
Second note - the first use of rotateAroundPoint() affects the 'label' group which has as its parent the canvas. The second use of that function affects the 'text' group which has the 'label' group as its parent. Its subtle but worth knowing.
Here is the snippet. I urge you to run it fullscreen and spin a few shapes around a few different points.
// Code to illustrate rotation of a shape around any given point. The important functions here is rotateAroundPoint() which does the rotation and movement math !
let
angle = 0, // display value of angle
startPos = {x: 80, y: 45},
shapes = [], // array of shape ghosts / tails
rotateBy = 20, // per-step angle of rotation
shapeName = $('#shapeName').val(), // what shape are we drawing
shape = null,
ghostLimit = 10,
// Set up a stage
stage = new Konva.Stage({
container: 'container',
width: window.innerWidth,
height: window.innerHeight
}),
// add a layer to draw on
layer = new Konva.Layer(),
// create the rotation target point cross-hair marker
lineV = new Konva.Line({points: [0, -20, 0, 20], stroke: 'lime', strokeWidth: 1}),
lineH = new Konva.Line({points: [-20, 0, 20, 0], stroke: 'lime', strokeWidth: 1}),
circle = new Konva.Circle({x: 0, y: 0, radius: 10, fill: 'transparent', stroke: 'lime', strokeWidth: 1}),
cross = new Konva.Group({draggable: true, x: startPos.x, y: startPos.y}),
labelRect, labelText;
// Add the elements to the cross-hair group
cross.add(lineV, lineH, circle);
layer.add(cross);
// Add the layer to the stage
stage.add(layer);
$('#shapeName').on('change', function(){
shapeName = $('#shapeName').val();
shape.destroy();
shape = null;
reset();
})
// Draw whatever shape the user selected
function drawShape(){
// Add a shape to rotate
if (shape !== null){
shape.destroy();
}
switch (shapeName){
case "rectangle":
shape = new Konva.Rect({x: startPos.x, y: startPos.y, width: 120, height: 80, fill: 'magenta', stroke: 'black', strokeWidth: 4});
break;
case "hexagon":
shape = new Konva.RegularPolygon({x: startPos.x, y: startPos.y, sides: 6, radius: 40, fill: 'magenta', stroke: 'black', strokeWidth: 4});
break;
case "ellipse":
shape = new Konva.Ellipse({x: startPos.x, y: startPos.y, radiusX: 40, radiusY: 20, fill: 'magenta', stroke: 'black', strokeWidth: 4});
break;
case "circle":
shape = new Konva.Ellipse({x: startPos.x, y: startPos.y, radiusX: 40, radiusY: 40, fill: 'magenta', stroke: 'black', strokeWidth: 4});
break;
case "star":
shape = new Konva.Star({x: startPos.x, y: startPos.y, numPoints: 5, innerRadius: 20, outerRadius: 40, fill: 'magenta', stroke: 'black', strokeWidth: 4});
break;
case "label":
shape = new Konva.Group({name: 'label'});
labelRect = new Konva.Rect({x: 0, y: 0, width: 120, height: 80, fill: 'magenta', stroke: 'black', strokeWidth: 4, name: 'rect'})
shape.add(labelRect);
labelText = new Konva.Group({name: 'text'});
labelText.add(new Konva.Rect({x: 0, y: 0, width: 100, height: 40, fill: 'cyan', stroke: 'black', strokeWidth: 2}))
labelText.add(new Konva.Text({x: 0, y: 0, width: 100, height: 40, text: 'Wombat',fontSize: 20, fontFamily: 'Calibri', align: 'center', padding: 10}))
shape.add(labelText)
labelText.position({x: (labelRect.width() - labelText.getClientRect().width) /2, y: (labelRect.height() - labelText.getClientRect().height) /2})
break;
};
layer.add(shape);
cross.moveToTop();
}
// Reset the shape position etc.
function reset(){
drawShape(); // draw the current shape
// Set to starting position, etc.
shape.position(startPos)
cross.position(startPos);
angle = 0;
$('#angle').html(angle);
$('#position').html('(' + shape.x() + ', ' + shape.y() + ')');
clearTails(); // clear the tail shapes
stage.draw(); // refresh / draw the stage.
}
// Click the stage to move the rotation point
stage.on('click', function (e) {
cross.position(stage.getPointerPosition());
stage.draw();
});
// Rotate a shape around any point.
// shape is a Konva shape
// angleRadians is the angle to rotate by, in radians
// point is an object {x: posX, y: posY}
function rotateAroundPoint(shape, angleDegrees, point) {
let angleRadians = angleDegrees * Math.PI / 180; // sin + cos require radians
const x =
point.x +
(shape.x() - point.x) * Math.cos(angleRadians) -
(shape.y() - point.y) * Math.sin(angleRadians);
const y =
point.y +
(shape.x() - point.x) * Math.sin(angleRadians) +
(shape.y() - point.y) * Math.cos(angleRadians);
shape.rotation(shape.rotation() + angleDegrees); // rotate the shape in place
shape.x(x); // move the rotated shape in relation to the rotation point.
shape.y(y);
shape.moveToTop(); //
}
$('#rotate').on('click', function(){
let newShape = shape.clone();
shapes.push(newShape);
layer.add(newShape);
// This ghost / tails stuff is just for fun.
if (shapes.length >= ghostLimit){
shapes[0].destroy();
shapes = shapes.slice(1);
}
for (var i = shapes.length - 1; i >= 0; i--){
shapes[i].opacity((i + 1) * (1/(shapes.length + 2)))
};
// This is the important call ! Cross is the rotation point as illustrated by crosshairs.
rotateAroundPoint(shape, rotateBy, {x: cross.x(), y: cross.y()});
// The label is a special case because we need to keep the text unrotated.
if (shape.name() === 'label'){
let text = shape.find('.text')[0];
rotateAroundPoint(text, -1 * rotateBy, {x: text.getClientRect().width/2, y: text.getClientRect().height/2});
}
cross.moveToTop();
stage.draw();
angle = angle + 10;
$('#angle').html(angle);
$('#position').html('(' + Math.round(shape.x() * 10) / 10 + ', ' + Math.round(shape.y() * 10) / 10 + ')');
})
// Function to clear the ghost / tail shapes
function clearTails(){
for (var i = shapes.length - 1; i >= 0; i--){
shapes[i].destroy();
};
shapes = [];
}
// User cicks the reset button.
$('#reset').on('click', function(){
reset();
})
// Force first draw!
reset();
body {
margin: 10;
padding: 10;
overflow: hidden;
background-color: #f0f0f0;
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://unpkg.com/konva#^3/konva.min.js"></script>
<p>1. Click the rotate button to see what happens when rotating around shape origin.</p>
<p>2. Reset then click stage to move rotation point and click rotate button again - rinse & repeat</p>
<p>
<button id = 'rotate'>Rotate</button>
<button id = 'reset'>Reset</button>
<select id='shapeName'>
<option value='label' selected='selected'>Label</option>
<option value='rectangle'>Rectangle</option>
<option value='hexagon'>Polygon</option>
<option value='ellipse' >Ellipse</option>
<option value='circle' >Circle</option>
<option value='star'>Star</option>
</select>
Angle : <span id='angle'>0</span>
Position : <span id='position'></span>
</p>
<div id="container"></div>

Fabric.js Clip Canvas (or object group) To Polygon

I have a canvas drawn in Fabric.js that i am adding a group of rectangles to, i want to limit the edges of those rectangles as a group to not go outside a certain area.
Imagine making a stripy t-shirt, the stripes are make by using a series of rectangles and i need to keep them to the shape of the t-shirt.
I think its better to clip the entire canvas to the shape of the t shirt, so anything i add to it remains within the t-shirt but i am stuck. So far i am only clip to basic circles and rectangles.
Thanks

You can just render a shape inside canvas.clipTo :)
I just loaded a random SVG shape in kitchensink and did this:
var shape = canvas.item(0);
canvas.remove(shape);
canvas.clipTo = function(ctx) {
shape.render(ctx);
};
As you can see, entire canvas is now clipped by that SVG shape.

You may also try this one: http://jsfiddle.net/ZxYCP/198/
var clipPoly = new fabric.Polygon([
{ x: 180, y: 10 },
{ x: 300, y: 50 },
{ x: 300, y: 180 },
{ x: 180, y: 220 }
], {
originX: 'left',
originY: 'top',
left: 180,
top: 10,
width: 200,
height: 200,
fill: '#DDD', /* use transparent for no fill */
strokeWidth: 0,
selectable: false
});
You can simply use Polygon to clip. Answer is based on #natchiketa idea in this question Multiple clipping areas on Fabric.js canvas

divide/cutting SVG in boxes, RaphaelJS

I am using RaphaelJS to draw 4 boxes inside a canvas.
var paper = new Raphael(document.getElementById('canvas_container'), 512, 256);
var rectangle = paper.rect(50, 50, 50, 50),
rectangle2 = paper.rect(400, 50, 50, 50);
rectangle3 = paper.rect(50, 200, 50, 50);
rectangle4 = paper.rect(400, 200, 50, 50);
rectangle2.attr("fill", "red");
rectangle3.attr("fill", "green");
rectangle4.attr("fill", "blue");
jsfiddle: http://jsfiddle.net/SFRWj/1/
--
My problem: I want to loop and cut/divide my svg canvas in 4: top left, top right, bottom left, bottom right
Purpose: I want to have 4 separate elements I can then try to convert to png or base64 or just do something with them.
any ideas?
UPDATE 1:
with two for loops and
paper.setViewBox(i*viewStep,j*viewStep,128,128,false);
I got something like this: http://jsfiddle.net/SFRWj/3/

Here is an example with svg.js but you can probably achieve the same with Raphael:
var src = 'http://distilleryimage11.s3.amazonaws.com/89ac2e90d9b111e297bf22000a1f9263_7.jpg'
/* create canvases */
var draw1 = SVG('canvas1').viewbox(0,0,75,75)
var draw2 = SVG('canvas2').viewbox(75,0,75,75)
var draw3 = SVG('canvas3').viewbox(0,75,75,75)
var draw4 = SVG('canvas4').viewbox(75,75,75,75)
/* draw image */
var image = draw1.image(src).size(150,150)
/* clone image to other canvases */
draw2.add(image.clone())
draw3.add(image.clone())
draw4.add(image.clone())
http://jsfiddle.net/wout/rgzG6/
This example basically created four svg canvases, clones the content from the first to the other canvases and sets a viewbox for everyone of them. If you have multiple elements you could consider putting everything in a group before cloning them.

Drawing in particular area of HTML5 canvas

If a lot of my drawings are going to be within a particular area of my larger canvas (in this case, in the center), is there a way to just say that you're working within that particular 'sub-canvas' instead of having to add/subtract the margins every time you want to draw? It just makes my code look a lot more complicated every time I'm specifying coordinates.

You can change the coordinates' origin using translate().
First, save the original origin using save(). Then, find the origin that suits the centre of your screen's drawing area and call translate(x, y). Do your drawing, and then use restore() to get your previous origin back.
jsFiddle.

Kinetic.js, a popular library for Canvas allows you to create a Group layer. You can specify the x, y coordinates, height and width of this Group. You can also add shapes and draw other things within this group.
Here's and example:
var stage = new Kinetic.Stage({
container: 'container',
width: 578,
height: 200
});
var shapesLayer = new Kinetic.Layer();
/*
* create a group which will be used to combine
* multiple simple shapes. Transforming the group will
* transform all of the simple shapes together as
* one unit
*/
var group = new Kinetic.Group({
x: 220,
y: 40,
rotationDeg: 20
});
var colors = ['red', 'orange', 'yellow'];
for(var n = 0; n < 3; n++) {
// anonymous function to induce scope
(function() {
var i = n;
var box = new Kinetic.Rect({
x: i * 30,
y: i * 18,
width: 100,
height: 50,
name: colors[i],
fill: colors[i],
stroke: 'black',
strokeWidth: 4
});
group.add(box);
})();
}
shapesLayer.add(group);
stage.add(shapesLayer);
Here's a tutorial on how to add Groups

You can use drawimage to draw an offscreen canvas to a certain part of another canvas.
Create a new canvas object and draw all your stuff to that. In the end draw that canvas to your onscreen canvas with drawimage at some coordinates.

Drawing shapes and lines with HTML5 Canvas and jQuery

I have an upcoming project that I would like to use the HTML5 canvas element to accomplish what would have had to be done in the past with either images and absolutely paced div's or Flash. Here is a basic example of the concept
Here are my concerns:
I am ok with using div's with corner radius to create the circles as they will be styled, and I'm not sure if I can do that with a mix of svg and the canvas element.
My main concern is the stroke that joins the outer circles to the inner, I would like to do this with canvas but am not sure A) How to get multiple canvas elements on one page in one containing element (a wrapper div) and B) how to figure out the starting points, I would assume the ending point would just be the center of the wrapper div (IE if its 600x600 = x=300, y=300)
Can anyone shed some light on this and offer any suggestions? Is there an advantage to using any of the jQuery canvas plugiins over vanilla JS?
thank you!

The canvas API consists of some functions which seem to do the job just fine:
.moveTo/.lineTo for a line path
.arc for a circle path
.stroke to stroke a path (line)
.fill to fill a path (circle)
Here's a very trivial proof of concept: http://jsfiddle.net/eGjak/275/.
I've used (x, y) for both the lines and the circles, which means the lines go from and to the midpoint of two circles. r is the radius of a circle.
var ctx = $('#cv').get(0).getContext('2d');
var circles = [ // smaller circles
{ x: 50, y: 50, r: 25 },
{ x: 250, y: 50, r: 25 },
{ x: 250, y: 250, r: 25 },
{ x: 50, y: 250, r: 25 },
];
var mainCircle = { x: 150, y: 150, r: 50 }; // big circle
function drawCircle(data) {
ctx.beginPath();
ctx.arc(data.x, data.y, data.r, 0, Math.PI * 2); // 0 - 2pi is a full circle
ctx.fill();
}
function drawLine(from, to) {
ctx.beginPath();
ctx.moveTo(from.x, from.y);
ctx.lineTo(to.x, to.y);
ctx.stroke();
}
drawCircle(mainCircle); // draw big circle
$.each(circles, function() { // draw small circles and lines to them
drawCircle(this);
drawLine(mainCircle, this);
});​

You could just do all of these circles in CSS. Get some divs, style them as you like in CSS, and then apply border-radius: 100; to the object, and done. I hope this helped.