Skew a line on canvas using JavaScript - javascript

I am drawing lines on a canvas. The user can select a particular line and be able to skew that line. By skew, I mean they can drag one end point of the line to a desired point on the same x-axis. How can I do this using JavaScript and HTML5 canvas?

The general way to draw a line is this:
ctx.moveTo(line.startX, line.startY);
ctx.lineTo(line.endX, line.endY);
ctx.stroke();
and then you can add EventListeners and check to see if the mouse is near the line...
window.addEventListener("mousemove", function(e)
{
mouse.x = e.layerX || e.offsetX;
mouse.y = e.layerY || e.offsetY;
// check to see if the mouse is near the line(s) here...
// you can change to x/y and start/end
// example:
if (mouse.x <= line.startX + 5 || mouse.x >= line.startX - 5)
{
// mouse is within 5px of first x
}
});

Related

Can we assign class to shapes in canvas in html5

Can we assign a class to shapes in canvas?
I am trying to build a path using lines and want to give a collection of lines a class so as to change their properties specifically.
My code is somewhat like:
ctx.beginPath();
ctx.moveTo(200,450);
ctx.lineTo(200,400);
ctx.lineTo(400,400);
ctx.lineTo(400,450);
ctx.stroke();
I want to assign to a class to all these lines specifically.How is it done?
Any suggestions ?
Canvas is a bitmap board in which you can draw on with no way to track shapes
However...
You can always use this clicking function to interact with your square manually:
var canvas = ...
var ctx = ...
canvas.addEventListener("mousedown", getPosition, false)
function getPosition(event) {
x = event.x;
y = event.y;
x -= canvas.offsetLeft;
y -= canvas.offsetTop;
// Now put code to describe specifically where to click
if (x < 400 && x > 200 && y < 450 && y > 400) {
// Now if you click on your square, you can write code here to interact
}
}
You are also always able to make an array of the squares coordinates and keep track of it that way.
I hope this helped :)

Moving dynamically created grouped object within contained area? (html5canvas - fabric.js)

What I'm trying to create is a small canvas widget that would allow a user to dynamically create a shape onto an image and then place it above an area that caught their interest, effectively it is a highlighter.
The problem is with adding a zoom function, as when I zoom onto the image I would like to ensure that;
There is no possible way for the dynamically created shape to be
dragged anywhere outside the image area. (completed - ish, relies on 2nd step)
You cannot drag the image out of the page view, the canvas area cannot show white space. Part of the image must always be shown, and fill the entire canvas area. (problem)
Here are two examples that I've drawn up, neither of which work correctly;
First example - getBoundingRect does not update and is bound to the image
Second example - getBoundingRect does update and is bound to the grouped object
From the link description you can see that I think I've narrowed the problem down, or at least noticed a key difference between the scripts with how the getBoundingRect behaves.
The first plunk seems to work fine, until you try to zoom in multiple times and at a greater zoom level, then it seems to start bugging out (may take a few clicks and a bit of messing around, it is very inconsistent). The second plunk is very jittery and doesn't work very well.
I've been stuck on this for a week or so now, and I'm at breaking point! So really hoping someone can point out what I'm doing wrong?
Code snippet below for first plunk;
// creates group
var objs = canvas.getObjects();
var group = new fabric.Group(objs, {
status: 'moving'
});
// sets grouped object position
var originalX = active.left,
originalY = active.top,
mouseX = evt.e.pageX,
mouseY = evt.e.pageY;
active.on('moving', function(evt) {
group.left += evt.e.pageX - mouseX;
group.top += evt.e.pageY - mouseY;
active.left = originalX;
active.top = originalY;
originalX = active.left;
originalY = active.top;
mouseX = evt.e.pageX;
mouseY = evt.e.pageY;
// sets boundary area for image when zoomed
// THIS IS THE PART THAT DOESN'T WORK
active.setCoords();
// SET BOUNDING RECT TO 'active'
var boundingRect = active.getBoundingRect();
var zoom = canvas.getZoom();
var viewportMatrix = canvas.viewportTransform;
// scales bounding rect when zoomed
boundingRect.top = (boundingRect.top - viewportMatrix[5]) / zoom;
boundingRect.left = (boundingRect.left - viewportMatrix[4]) / zoom;
boundingRect.width /= zoom;
boundingRect.height /= zoom;
var canvasHeight = canvas.height / zoom,
canvasWidth = canvas.width / zoom,
rTop = boundingRect.top + boundingRect.height,
rLeft = boundingRect.left + boundingRect.width;
// checks top left
if (rTop < canvasHeight || rLeft < canvasWidth) {
group.top = Math.max(group.top, canvasHeight - boundingRect.height);
group.left = Math.max(group.left, canvasWidth - boundingRect.width);
}
// checks bottom right
if (rTop > 0 || rLeft > 0) {
group.top = Math.min(group.top, canvas.height - boundingRect.height + active.top - boundingRect.top);
group.left = Math.min(group.left, canvas.width - boundingRect.width + active.left - boundingRect.left);
}
});
// deactivates all objects on mouseup
active.on('mouseup', function() {
active.off('moving');
canvas.deactivateAll().renderAll();
})
// sets group
canvas.setActiveGroup(group.setCoords()).renderAll();
}
EDIT:
I've added comments and tried to simplify the code in the plunks.
The relevant code starts within the if (active.id == "img") { code block.
I've put irrelevant code as functions at the bottom, they can largely be ignored. ( createNewRect() + preventRectFromLeaving() )
I've removed one of the plunks to avoid confusion.
Let me know if it helps, or If I should try to simplify further.
Thanks!
I think that the grouping was messing with the position of the background image. So, I tried removing the group when the image is moving and manually updating the position of the rect instead.
It sets the last position of the image before moving
var lastLeft = active.left,
lastTop = active.top;
And then it updates those and the position of the rect every time the image moves
rect.left += active.left - lastLeft;
rect.top += active.top - lastTop;
// I think this is needed so the rectangle can be re-selected
rect.setCoords();
lastLeft = active.left;
lastTop = active.top;
Since the image has to stay within the canvas, the rect stays inside the canvas, too, whenever the image moves. The rest of the code you wrote seemed to work fine.
http://plnkr.co/edit/6GGcUxGC7CjcyQzExMoK?p=preview

How can I bind a Raphael element to the mouse cursor?

I want to create an element and then have that element be immediately bound to the cursor. I have tools to move the element, but I don't know how to bind them to the cursor without having to click the element. I thought about simulating the mousedown() event, but I don't know how to do it.
For context, my ultimate goal is to create a line with user defined endpoint. The user clicks a point and 2 small black circles are created. One as a reference point the the first click and the other to be attached to the cursor with a path connect the 2 points. Once the user clicks another point, both small black circles with disappear and only the line will remain.
Any ideas?
Thanks to #Joan Charmant for pointing me in the right direction. Here's my solution thus far. $('#paper') is my canvas and tempPoint is the circle I created to bind to cursor movement.
$("#paper").mousemove(function (event)
{
if(firstLinePointSelected && tempPoint!=null)
{
if (!event) var event = window.event;
var x=0, y=0;
if (event.pageX || event.pageY)
{
x = event.pageX;
y = event.pageY;
}
else if (event.clientX || event.clientY)
{
x = event.clientX + document.body.scrollLeft
+ document.documentElement.scrollLeft;
y = event.clientY + document.body.scrollTop
+ document.documentElement.scrollTop;
}
// subtract paper coords on page
tempPoint.attr("cx", x - $('#paper').offset().left);
tempPoint.attr("cy", y - $('#paper').offset().top);
}
});

Canvas accessing coordinate of translated position of panning background image

Cannot figure this out, how to find the translated position of the background relative to the canvas. I have the characters coordinates, and I have the coordinates from a mouse click within the canvas, but can't figure out how to find the offset.
In the canvas, when I click somewhere, I get an (x,y) value from (0,0) - (650,575), the size of the window, no matter where my character is. If the character is at (2000, 1500) on the canvas, my click/touch input will always send the character up and left towards 0,0 on the background coordinate.
At first I thought I should subtract the player X position from the max width, then add an offset half the width of the screen, and do the same for the Y position, but that didn't work.
Then I tried subtracting half the width/height of the screen from the current player x,y values but that doesn't work.
Anyone point me in the right direction, it seems elementary but I can't figure it out it's been years since math class???? Thanks
var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d");
canvas.width = 650;
canvas.height = 575;
var WIDTH=5000; //level width
var HEIGHT=3750; //level height
ctx.translate(-WIDTH*.5,-HEIGHT*.5); //starts in center of background
Where my player begins on load:
hero.x = WIDTH*.5+325; //offset half canvas width
hero.y = HEIGHT*.5+275; //offset half canvas height
For the Background:
ctx.drawImage(bgImage, BGsrcX , BGsrcY, 1250 , 938 ,-150, -150, BGdestW, BGdestH); `//image is stretched to 5000x3750`
This is the mouse input I'm using
if(navigator.userAgent.match(/(iPhone)|(iPod)|(iPad)/i)){
document.addEventListener('touchstart', function(e) {
if(e.touches.length == 1){ // Only deal with one finger
var touch = e.touches[0]; // Get the information for finger #1
var x = touch.pageX - canvas.offsetLeft;
var y = touch.pageY - canvas.offsetTop;
//clickEvent(x,y); //call your function to manage tweets
}
},false);
}
else{
document.addEventListener('mousedown',function(e) {
var x = e.pageX - canvas.offsetLeft;
var y = e.pageY - canvas.offsetTop;
console.log(x+":"+y);
clickEvent(x,y); //call your function to manage tweets
},false);
}
For the keyboard input to actually pan the background:
if(16 in keysDown && 38 in keysDown && hero.y > 200) {ctx.translate(0,12); }
Don't work with half-translated and non-translated coordinates, translate your mouse click coordinates AND your canvas coordinates.
Then you can just use simple subtraction to find the offset, and to find the distance, you you use the distance formula.

Getting mouse location in canvas [duplicate]

This question already has answers here:
How do I get the coordinates of a mouse click on a canvas element? [duplicate]
(22 answers)
Closed 3 years ago.
Is there a way to get the location mouse inside a <canvas> tag? I want the location relative to to the upper right corner of the <canvas>, not the entire page.
The accepted answer will not work every time. If you don't use relative position the attributes offsetX and offsetY can be misleading.
You should use the function: canvas.getBoundingClientRect() from the canvas API.
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
canvas.addEventListener('mousemove', function(evt) {
var mousePos = getMousePos(canvas, evt);
console.log('Mouse position: ' + mousePos.x + ',' + mousePos.y);
}, false);
Easiest way is probably to add a onmousemove event listener to the canvas element, and then you can get the coordinates relative to the canvas from the event itself.
This is trivial to accomplish if you only need to support specific browsers, but there are differences between f.ex. Opera and Firefox.
Something like this should work for those two:
function mouseMove(e)
{
var mouseX, mouseY;
if(e.offsetX) {
mouseX = e.offsetX;
mouseY = e.offsetY;
}
else if(e.layerX) {
mouseX = e.layerX;
mouseY = e.layerY;
}
/* do something with mouseX/mouseY */
}
Also note that you'll need CSS:
position: relative;
set to your canvas tag, in order to get the relative mouse position inside the canvas.
And the offset changes if there's a border
I'll share the most bulletproof mouse code that I have created thus far. It works on all browsers will all manner of padding, margin, border, and add-ons (like the stumbleupon top bar)
// Creates an object with x and y defined,
// set to the mouse position relative to the state's canvas
// If you wanna be super-correct this can be tricky,
// we have to worry about padding and borders
// takes an event and a reference to the canvas
function getMouse = function(e, canvas) {
var element = canvas, offsetX = 0, offsetY = 0, mx, my;
// Compute the total offset. It's possible to cache this if you want
if (element.offsetParent !== undefined) {
do {
offsetX += element.offsetLeft;
offsetY += element.offsetTop;
} while ((element = element.offsetParent));
}
// Add padding and border style widths to offset
// Also add the <html> offsets in case there's a position:fixed bar (like the stumbleupon bar)
// This part is not strictly necessary, it depends on your styling
offsetX += stylePaddingLeft + styleBorderLeft + htmlLeft;
offsetY += stylePaddingTop + styleBorderTop + htmlTop;
mx = e.pageX - offsetX;
my = e.pageY - offsetY;
// We return a simple javascript object with x and y defined
return {x: mx, y: my};
}
You'll notice that I use some (optional) variables that are undefined in the function. They are:
stylePaddingLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingLeft'], 10) || 0;
stylePaddingTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['paddingTop'], 10) || 0;
styleBorderLeft = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderLeftWidth'], 10) || 0;
styleBorderTop = parseInt(document.defaultView.getComputedStyle(canvas, null)['borderTopWidth'], 10) || 0;
// Some pages have fixed-position bars (like the stumbleupon bar) at the top or left of the page
// They will mess up mouse coordinates and this fixes that
var html = document.body.parentNode;
htmlTop = html.offsetTop;
htmlLeft = html.offsetLeft;
I'd recommend only computing those once, which is why they are not in the getMouse function.
For mouse position, I usually use jQuery since it normalizes some of the event attributes.
function getPosition(e) {
//this section is from http://www.quirksmode.org/js/events_properties.html
var targ;
if (!e)
e = window.event;
if (e.target)
targ = e.target;
else if (e.srcElement)
targ = e.srcElement;
if (targ.nodeType == 3) // defeat Safari bug
targ = targ.parentNode;
// jQuery normalizes the pageX and pageY
// pageX,Y are the mouse positions relative to the document
// offset() returns the position of the element relative to the document
var x = e.pageX - $(targ).offset().left;
var y = e.pageY - $(targ).offset().top;
return {"x": x, "y": y};
};
// now just make sure you use this with jQuery
// obviously you can use other events other than click
$(elm).click(function(event) {
// jQuery would normalize the event
position = getPosition(event);
//now you can use the x and y positions
alert("X: " + position.x + " Y: " + position.y);
});
This works for me in all the browsers.
EDIT:
I copied the code from one of my classes I was using, so the jQuery call to this.canvas was wrong. The updated function figures out which DOM element (targ) caused the event and then uses that element's offset to figure out the correct position.
GEE is an endlessly helpful library for smoothing out troubles with canvas, including mouse location.
Simple approach using mouse event and canvas properties:
JSFiddle demo of functionality http://jsfiddle.net/Dwqy7/5/
(Note: borders are not accounted for, resulting in off-by-one):
Add a mouse event to your canvas
canvas.addEventListener("mousemove", mouseMoved);
Adjust event.clientX and event.clientY based on:
canvas.offsetLeft
window.pageXOffset
window.pageYOffset
canvas.offsetTop
Thus:
canvasMouseX = event.clientX - (canvas.offsetLeft - window.pageXOffset);
canvasMouseY = event.clientY - (canvas.offsetTop - window.pageYOffset);
The original question asked for coordinates from the upper right (second function).
These functions will need to be within a scope where they can access the canvas element.
0,0 at upper left:
function mouseMoved(event){
var canvasMouseX = event.clientX - (canvas.offsetLeft - window.pageXOffset);
var canvasMouseY = event.clientY - (canvas.offsetTop - window.pageYOffset);
}
0,0 at upper right:
function mouseMoved(event){
var canvasMouseX = canvas.width - (event.clientX - canvas.offsetLeft)- window.pageXOffset;
var canvasMouseY = event.clientY - (canvas.offsetTop - window.pageYOffset);
}
I'd use jQuery.
$(document).ready(function() {
$("#canvas_id").bind( "mousedown", function(e){ canvasClick(e); } );
}
function canvasClick( e ){
var x = e.offsetX;
var y = e.offsetY;
}
This way your canvas can be anywhere on your page, relative or absolute.
Subtract the X and Y offsets of the canvas DOM element from the mouse position to get the local position inside the canvas.

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