Develop Reference

trying to make a drawn line move like a laser in javascript

I made this red line in JavaScript that goes to closest target (balloon 1 to 3) to the player but I need to make it so that it moves like a laser starting from player position into the target position. I thought about multiple ways of implementing this with no luck.
function Tick() {
// Erase the sprite from its current location.
eraseSprite();
for (var i = 0; i < lasers.length; i++) {
lasers[i].x += lasers[i].direction.x * laserSpeed;
lasers[i].y += lasers[i].direction.y * laserSpeed;
//Hit detection here
}
function detectCharClosest() {
var ballon1char = {
x: balloon1X,
y: balloon1Y
};
var ballon2char = {
x: balloon2X,
y: balloon2Y
};
var ballon3char = {
x: balloon3X,
y: balloon3Y,
};
ballon1char.distanceFromPlayer = Math.sqrt((CharX - balloon1X) ** 2 + (CharY - balloon1Y) ** 2);
ballon2char.distanceFromPlayer = Math.sqrt((CharX - balloon2X) ** 2 + (CharY - balloon2Y) ** 2);
ballon3char.distanceFromPlayer = Math.sqrt((CharX - balloon3X) ** 2 + (CharY - balloon3Y) ** 2);
var minDistance = Math.min(
ballon1char.distanceFromPlayer,
ballon2char.distanceFromPlayer,
ballon3char.distanceFromPlayer);
console.log(ballon1char);
console.log(ballon2char);
console.log(ballon3char);
for (let i = 0; i < 3; i++) {
if (minDistance == ballon1char.distanceFromPlayer)
return ballon1char
if (minDistance == ballon2char.distanceFromPlayer)
return ballon2char
if (minDistance == ballon3char.distanceFromPlayer)
return ballon3char
}
}
function loadComplete() {
console.log("Load is complete.");
canvas = document.getElementById("theCanvas");
ctx = canvas.getContext("2d");
myInterval = self.setInterval(function () { Tick() }, INTERVAL);
myInterval = self.setInterval(function () { laserTicker(detectCharClosest()) }, 2000);
function laserTicker(balloon) {
//gets the closest ballon to go to
laserDo(balloon);
}
function laserDo(balloon) {
ctx.beginPath();
ctx.lineWidth = 2;
ctx.strokeStyle = "#F44336"; // "red";
ctx.moveTo(CharX + 16, CharY + 16);
ctx.lineTo(balloon.x, balloon.y);
// lasers.push({x: })
ctx.stroke();
}
I didn't put all of my code here so If something doesn't make sense please tell me. I'm still new to JavaScript and learning it. One way I thought I could make this work was by taking the distance between the player and the target and dividing it by the speed on the x and y axis then changing having it start from the player position and keeps on adding up on both axis until it reaches the target. That didn't work out though. If you have any suggestions then please tell me.
Thanks

From Paperscript to Javascript

I'm trying to convert one of the Paper.js library examples (http://paperjs.org/examples/smoothing/) from PaperScript to Javascript. Following the documentation, I have
Made the scope global
Installed the event handlers onFrame and onResize
Created a tool and installed the event handlers onMouseMove and onMouseDown
But the canvas is not shown. I only see a couple of small blue lines: AFAIK the problem lies in the view.onFrame() function, since commenting that out at least I can see the shape, but not interact with it. The JS console dosen't show any error. What is missing?
// Make the paper scope global, by injecting it into window
paper.install(window);
window.onload = function () {
// Setup directly from canvas id:
paper.setup('myCanvas');
// Create tool
tool = new Tool();
var width, height, center;
var points = 10;
var smooth = true;
var path = new Path();
var mousePos = view.center / 2;
var pathHeight = mousePos.y;
path.fillColor = 'black';
initializePath();
function initializePath() {
center = view.center;
width = view.size.width;
height = view.size.height / 2;
path.segments = [];
path.add(view.bounds.bottomLeft);
for (var i = 1; i < points; i++) {
var point = new Point(width / points * i, center.y);
path.add(point);
}
path.add(view.bounds.bottomRight);
path.fullySelected = true;
}
view.onFrame = function (event) {
pathHeight += (center.y - mousePos.y - pathHeight) / 10;
for (var i = 1; i < points; i++) {
var sinSeed = event.count + (i + i % 10) * 100;
var sinHeight = Math.sin(sinSeed / 200) * pathHeight;
var yPos = Math.sin(sinSeed / 100) * sinHeight + height;
path.segments[i].point.y = yPos;
}
if (smooth)
path.smooth({ type: 'continuous' });
}
tool.onMouseMove = function (event) {
mousePos = event.point;
}
tool.onMouseDown = function (event) {
smooth = !smooth;
if (!smooth) {
// If smooth has been turned off, we need to reset
// the handles of the path:
for (var i = 0, l = path.segments.length; i < l; i++) {
var segment = path.segments[i];
segment.handleIn = segment.handleOut = null;
}
}
}
// Reposition the path whenever the window is resized:
view.onResize = function (event) {
initializePath();
}
}
To try it: https://jsfiddle.net/1rtkbp9s/

Found the solution (credits to Stefan Krüger of the Paper.js Google Group):
var mousePos = view.center / 2;
Should have been:
var mousePos = view.center.divide(2);
The fact is that Math functions should be used instead of operators for Point and Size object... and I didn't realize that view.center IS a Point object: http://paperjs.org/reference/view/#center

Worst quality perspective image on canvas

I have a problem on my project.
I am developing a perspective mockup creating module for designers. Users upload images and i get them for placing in mockups with making some perspective calculations. Then users can download this image. I made all of this on clientside with js.
But there is a problem for images which are drawn on canvas with perspective calculations like this;
Sample img: http://oi62.tinypic.com/2h49dec.jpg
orginal image size: 6500 x 3592 and you can see spread edges on image...
I tried a few technics like ctx.imageSmoothingEnabled true etc.. But result was always same.
What can i do for solve this problem? What do you think about this?
edit
For more detail;
I get an image (Resolution free) from user then crop it for mockup ratio. For example in my sample image, user image was cropped for imac ratio 16:9 then making calculation with four dot of screen. By the way, my mockup image size is 6500 x 3592. so i made scale, transform etc this cropped image and put it in mockup on canvas. And then use blob to download this image to client...
Thanks.

Solved.
I use perspective.js for calculation on canvas. so I made some revisions on this js source.
If you wanna use or check source;
// Copyright 2010 futomi http://www.html5.jp/
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
// perspective.js v0.0.2
// 2010-08-28
/* -------------------------------------------------------------------
* define objects (name space) for this library.
* ----------------------------------------------------------------- */
if (typeof html5jp == 'undefined') {
html5jp = new Object();
}
(function() {
html5jp.perspective = function(ctxd, image) {
// check the arguments
if (!ctxd || !ctxd.strokeStyle) {
return;
}
if (!image || !image.width || !image.height) {
return;
}
// prepare a <canvas> for the image
var cvso = document.createElement('canvas');
cvso.width = parseInt(image.width) * 2;
cvso.height = parseInt(image.height) * 2;
var ctxo = cvso.getContext('2d');
ctxo.drawImage(image, 0, 0, cvso.width, cvso.height);
// prepare a <canvas> for the transformed image
var cvst = document.createElement('canvas');
cvst.width = ctxd.canvas.width;
cvst.height = ctxd.canvas.height;
var ctxt = cvst.getContext('2d');
ctxt.imageSmoothingEnabled = true;
ctxt.mozImageSmoothingEnabled = true;
ctxt.webkitImageSmoothingEnabled = true;
ctxt.msImageSmoothingEnabled = true;
// parameters
this.p = {
ctxd: ctxd,
cvso: cvso,
ctxo: ctxo,
ctxt: ctxt
}
};
var proto = html5jp.perspective.prototype;
proto.draw = function(points) {
var d0x = points[0][0];
var d0y = points[0][1];
var d1x = points[1][0];
var d1y = points[1][1];
var d2x = points[2][0];
var d2y = points[2][1];
var d3x = points[3][0];
var d3y = points[3][1];
// compute the dimension of each side
var dims = [
Math.sqrt(Math.pow(d0x - d1x, 2) + Math.pow(d0y - d1y, 2)), // top side
Math.sqrt(Math.pow(d1x - d2x, 2) + Math.pow(d1y - d2y, 2)), // right side
Math.sqrt(Math.pow(d2x - d3x, 2) + Math.pow(d2y - d3y, 2)), // bottom side
Math.sqrt(Math.pow(d3x - d0x, 2) + Math.pow(d3y - d0y, 2)) // left side
];
//
var ow = this.p.cvso.width;
var oh = this.p.cvso.height;
// specify the index of which dimension is longest
var base_index = 0;
var max_scale_rate = 0;
var zero_num = 0;
for (var i = 0; i < 4; i++) {
var rate = 0;
if (i % 2) {
rate = dims[i] / ow;
} else {
rate = dims[i] / oh;
}
if (rate > max_scale_rate) {
base_index = i;
max_scale_rate = rate;
}
if (dims[i] == 0) {
zero_num++;
}
}
if (zero_num > 1) {
return;
}
//
var step = 0.10;
var cover_step = step * 250;
//
var ctxo = this.p.ctxo;
var ctxt = this.p.ctxt;
//*** ctxt.clearRect(0, 0, ctxt.canvas.width, ctxt.canvas.height);
if (base_index % 2 == 0) { // top or bottom side
var ctxl = this.create_canvas_context(ow, cover_step);
var cvsl = ctxl.canvas;
for (var y = 0; y < oh; y += step) {
var r = y / oh;
var sx = d0x + (d3x - d0x) * r;
var sy = d0y + (d3y - d0y) * r;
var ex = d1x + (d2x - d1x) * r;
var ey = d1y + (d2y - d1y) * r;
var ag = Math.atan((ey - sy) / (ex - sx));
var sc = Math.sqrt(Math.pow(ex - sx, 2) + Math.pow(ey - sy, 2)) / ow;
ctxl.setTransform(1, 0, 0, 1, 0, -y);
ctxl.drawImage(ctxo.canvas, 0, 0);
//
ctxt.translate(sx, sy);
ctxt.rotate(ag);
ctxt.scale(sc, sc);
ctxt.drawImage(cvsl, 0, 0);
//
ctxt.setTransform(1, 0, 0, 1, 0, 0);
}
} else if (base_index % 2 == 1) { // right or left side
var ctxl = this.create_canvas_context(cover_step, oh);
var cvsl = ctxl.canvas;
for (var x = 0; x < ow; x += step) {
var r = x / ow;
var sx = d0x + (d1x - d0x) * r;
var sy = d0y + (d1y - d0y) * r;
var ex = d3x + (d2x - d3x) * r;
var ey = d3y + (d2y - d3y) * r;
var ag = Math.atan((sx - ex) / (ey - sy));
var sc = Math.sqrt(Math.pow(ex - sx, 2) + Math.pow(ey - sy, 2)) / oh;
ctxl.setTransform(1, 0, 0, 1, -x, 0);
ctxl.drawImage(ctxo.canvas, 0, 0);
//
ctxt.translate(sx, sy);
ctxt.rotate(ag);
ctxt.scale(sc, sc);
ctxt.drawImage(cvsl, 0, 0);
//
ctxt.setTransform(1, 0, 0, 1, 0, 0);
}
}
// set a clipping path and draw the transformed image on the destination canvas.
this.p.ctxd.save();
this.set_clipping_path(this.p.ctxd, [
[d0x, d0y],
[d1x, d1y],
[d2x, d2y],
[d3x, d3y]
]);
this.p.ctxd.drawImage(ctxt.canvas, 0, 0);
this.p.ctxd.restore();
}
proto.create_canvas_context = function(w, h) {
var canvas = document.createElement('canvas');
canvas.width = w;
canvas.height = h;
var ctx = canvas.getContext('2d');
ctx.imageSmoothingEnabled = true;
ctx.mozImageSmoothingEnabled = true;
ctx.webkitImageSmoothingEnabled = true;
ctx.msImageSmoothingEnabled = true;
return ctx;
};
proto.set_clipping_path = function(ctx, points) {
ctx.beginPath();
ctx.moveTo(points[0][0], points[0][1]);
for (var i = 1; i < points.length; i++) {
ctx.lineTo(points[i][0], points[i][1]);
}
ctx.closePath();
ctx.clip();
};
})();

The problem is (most likely, but no code shows so..) that the image is actually too big.
The canvas typically uses bi-linear interpolation (2x2 samples) rather than bi-cubic (4x4 samples). That means if you scale it down a large percentage in one chunk the algorithm will skip some pixels that otherwise should have been sampled, resulting in a more pixelated look.
The solution do is to resize the image in steps, ie. 50% of itself repeatably until a suitable size is achieved. Then use perspective calculations on it. The exact destination size is something you need to find by trial and error, but a good starting point is to use the largest side of the resulting perspective image.
Here is one way to step-down rescale an image in steps.

Fill to x-labels affects the color of stacked bar chart in Flot

I am using FlotChart and Flot Tick Rotor [jquery.flot.tickrotor] plugins.
I tried to tweak the rotor to provide some lines between my x-axis labels by drawing some rectangles.
However, the last bar on my stacked graph is also filled with the color I set to my fills.
Can anyone help me?
Here's the image :
Here's my tweaked code :
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this file,
* You can obtain one at http://mozilla.org/MPL/2.0/. */
/*
* flot-tickrotor: flot plugin to display angled X-axis tick labels.
*
* Requires flot 0.7 or higher and a browser supporting <canvas>.
*
* To activate, just set xaxis.rotateTicks to an angle in degrees. Labels
* are rotated clockwise, so if you want the labels to angle up and to the
* right (/) you need to provide an angle > 90. The text will be flipped so
* that it is still right-side-up.
* Angles greater than or equal to 180 are ignored.
*/
(function ($) {
var options = { };
function init(plot) {
// Taken from flot-axislabels.
// This is kind of a hack. There are no hooks in Flot between
// the creation and measuring of the ticks (setTicks, measureTickLabels
// in setupGrid() ) and the drawing of the ticks and plot box
// (insertAxisLabels in setupGrid() ).
//
// Therefore, we use a trick where we run the draw routine twice:
// the first time to get the tick measurements, so that we can change
// them, and then have it draw it again.
var ticks = []; // preserve between draw() calls.
var font;
var secondPass = false;
var rotateTicks, rotateTicksRads, radsAboveHoriz;
plot.hooks.draw.push(function (plot, ctx) {
var xaxis; // for convenience
if (!secondPass) {
var opts = plot.getAxes().xaxis.options;
if (opts.rotateTicks === undefined) {
return;
}
rotateTicks = parseInt(opts.rotateTicks, 10);
if (rotateTicks.toString() != opts.rotateTicks || rotateTicks >= 180) { // || rotateTicks == 0
return;
}
rotateTicksRads = rotateTicks * Math.PI/180;
if (rotateTicks > 90) {
radsAboveHoriz = Math.PI - rotateTicksRads;
} else {
radsAboveHoriz = Math.PI/2 - rotateTicksRads;
}
font = opts.rotateTicksFont;
if (!font) {
font = $('.tickLabel').css('font');
}
if (!font) {
font = 'arial';
}
var elem, maxLabelWidth = 0, maxLabelHeight = 0, minX = 0, maxX = 0;
// We have to clear the ticks option so that flot core
// doesn't draw ticks superimposed with ours, but we preserve
// the tick data as xaxis.rotatedTicks so that external code
// can still get to it.
// FIXME: It would obviously be better to just interrupt
// the drawing of the ticks and preserve the 'ticks'
// property. That probably requires another hook.
xaxis = plot.getAxes().xaxis;
ticks = plot.getAxes().xaxis.ticks;
xaxis.rotatedTicks = ticks;
opts.ticks = []; // we'll make our own
var x;
for (var i = 0; i < ticks.length; i++) {
var raber = ticks[i].label.split(" ");
elem = $('<span style="font-size:11pt; font:' + font + '">' + ticks[i].label + '</span>');
plot.getPlaceholder().append(elem);
ticks[i].height = elem.outerHeight(true);
ticks[i].width = elem.outerWidth(true);
elem.remove();
if (ticks[i].height > maxLabelHeight) {
maxLabelHeight = ticks[i].height;
}
if (ticks[i].width > maxLabelWidth) {
maxLabelWidth = ticks[i].width;
}
var tick = ticks[i];
// See second-draw code below for explanation of offsets.
if (rotateTicks > 90) {
// See if any labels are too long and require increased left
// padding.
x = Math.round(plot.getPlotOffset().left + xaxis.p2c(tick.v))
- Math.ceil(Math.cos(radsAboveHoriz) * tick.height)
- Math.ceil(Math.cos(radsAboveHoriz) * tick.width);
if (x < minX) {
minX = x;
}
} else {
// See if any labels are too long and require increased right
// padding.
x = Math.round(plot.getPlotOffset().left + xaxis.p2c(tick.v))
+ Math.ceil(Math.cos(radsAboveHoriz) * tick.height)
+ Math.ceil(Math.cos(radsAboveHoriz) * tick.width);
if (x > maxX) {
maxX = x;
}
}
}
// Calculate maximum label height after rotating.
if (rotateTicks > 90) {
var acuteRads = rotateTicksRads - Math.PI/2;
opts.labelHeight = Math.ceil(Math.sin(acuteRads) * maxLabelWidth)
+ Math.ceil(Math.sin(acuteRads) * maxLabelHeight) + 20;
} else {
var acuteRads = Math.PI/2 - rotateTicksRads;
// Center such that the top of the label is at the center of the tick.
opts.labelHeight = Math.ceil(Math.sin(rotateTicksRads) * maxLabelWidth)
+ Math.ceil(Math.sin(acuteRads) * maxLabelHeight) + 20;
}
if (minX < 0) {
plot.getAxes().yaxis.options.labelWidth = -1 * minX;
}
// Doesn't seem to work if there are no values using the
// second y axis.
//if (maxX > xaxis.box.left + xaxis.box.width) {
// plot.getAxes().y2axis.options.labelWidth = maxX - xaxis.box.left - xaxis.box.width;
//}
// re-draw with new label widths and heights
secondPass = true;
plot.setupGrid();
plot.draw();
} else {
if (ticks.length == 0) {
return;
}
xaxis = plot.getAxes().xaxis;
var box = xaxis.box;
var tick, label, xoffset, yoffset;
var showWeek = false;
for (var i = 0; i < ticks.length; i++) {
tick = ticks[i];
if (!tick.label) {
continue;
}
ctx.save();
ctx.font = font;
if (rotateTicks <= 90) {
// Center such that the top of the label is at the center of the tick.
xoffset = -Math.ceil(Math.cos(radsAboveHoriz) * tick.height) - 10;
yoffset = Math.ceil(Math.sin(radsAboveHoriz) * tick.height) - 10;
ctx.translate(Math.round(plot.getPlotOffset().left + xaxis.p2c(tick.v)) + xoffset,
box.top + box.padding + plot.getOptions().grid.labelMargin + yoffset);
ctx.rotate(rotateTicksRads);
} else {
// We want the text to facing up, so we have to
// rotate counterclockwise, which means the label
// has to *end* at the center of the tick.
xoffset = Math.ceil(Math.cos(radsAboveHoriz) * tick.height)
- Math.ceil(Math.cos(radsAboveHoriz) * tick.width);
yoffset = Math.ceil(Math.sin(radsAboveHoriz) * tick.width)
+ Math.ceil(Math.sin(radsAboveHoriz) * tick.height);
ctx.translate(Math.round(plot.getPlotOffset().left + xaxis.p2c(tick.v) + xoffset),
box.top + box.padding + plot.getOptions().grid.labelMargin + yoffset);
ctx.rotate(-radsAboveHoriz);
}
var ticksMe = tick.label.split(" ");
// draw labels
var absXoffset = Math.abs(xoffset);
var leftPad = 5;
ctx.fillText(ticksMe[0], absXoffset - leftPad, 0);
if(showWeek){
ctx.fillText(ticksMe[1], (xoffset + leftPad) , yoffset * 2);
showWeek = false;
if(i == ticks.length - 1){
var offset = Math.abs(xoffset * 3);
ctx.rect(offset - 2, -10, 2 ,(yoffset * 4));
ctx.fillStyle = "#868686";
ctx.fill();
}
}
else{
showWeek = true;
ctx.rect(absXoffset - (leftPad * 2) + 2, -10, -2,(yoffset * 4));
ctx.fillStyle = "#868686";
ctx.fill();
}
ctx.restore();
}
}
});
}
$.plot.plugins.push({
init: init,
options: options,
name: 'tickRotor',
version: '1.0'
});
})(jQuery);

As I figured out, I just need to put on ctx.beginPath() and my problem got solved. :(

Math to find edge between two boxes

I am building prototype tool to draw simple diagrams.
I need to draw an arrow between two boxes, the problem is i have to find edges of two boxes so that the arrow line does not intersect with the box.
This is the drawing that visualize my problem:
How to find x1,y1 and x2,y2 ?
-- UPDATE --
After 2 days finding solution, this is example & function that i use:
var box1 = { x:1,y:10,w:30,h:30 };
var box2 = { x:100,y:110,w:30,h:30 };
var edge1 = findBoxEdge(box1,box2,1,0);
var edge2 = findBoxEdge(box1,box2,2,0);
function findBoxEdge(box1,box2,box,distant) {
var c1 = box1.x + box1.w/2;
var d1 = box1.y + box1.h/2;
var c2 = box2.x + box2.w/2;
var d2 = box2.y + box2.h/2;
var w,h,delta_x,delta_y,s,c,e,ox,oy,d;
if (box == 1) {
w = box1.w/2;
h = box1.h/2;
} else {
w = box2.w/2;
h = box2.h/2;
}
if (box == 1) {
delta_x = c2-c1;
delta_y = d2-d1;
} else {
delta_x = c1-c2;
delta_y = d1-d2;
}
w+=5;
h+=5;
//intersection is on the top or bottom
if (w*Math.abs(delta_y) > h * Math.abs(delta_x)) {
if (delta_y > 0) {
s = [h*delta_x/delta_y,h];
c = "top";
}
else {
s = [-1*h*delta_x/delta_y,-1*h];
c = "bottom";
}
}
else {
//intersection is on the left or right
if (delta_x > 0) {
s = [w,w*delta_y/delta_x];
c = "right";
}
else {
s = [-1*w,-1*delta_y/delta_x];
c = "left";
}
}
if (typeof(distant) != "undefined") {
//for 2 paralel distant of 2e
e = distant;
if (delta_y == 0) ox = 0;
else ox = e*Math.sqrt(1+Math.pow(delta_x/delta_y,2))
if (delta_x == 0) oy = 0;
else oy = e*Math.sqrt(1+Math.pow(delta_y/delta_x,2))
if (delta_y != 0 && Math.abs(ox + h * (delta_x/delta_y)) <= w) {
d = [sgn(delta_y)*(ox + h * (delta_x/delta_y)),sgn(delta_y)*h];
}
else if (Math.abs(-1*oy + (w * delta_y/delta_x)) <= h) {
d = [sgn(delta_x)*w,sgn(delta_x)*(-1*oy + w * (delta_y/delta_x))];
}
if (delta_y != 0 && Math.abs(-1*ox+(h * (delta_x/delta_y))) <= w) {
d = [sgn(delta_y)*(-1*ox + h * (delta_x/delta_y)),sgn(delta_y)*h];
}
else if (Math.abs(oy + (w * delta_y/delta_x)) <= h) {
d = [sgn(delta_x)*w,sgn(delta_x)*(oy + w * (delta_y/delta_x))];
}
if (box == 1) {
return [Math.round(c1 +d[0]),Math.round(d1 +d[1]),c];
} else {
return [Math.round(c2 +d[0]),Math.round(d2 +d[1]),c];
}
} else {
if (box == 1) {
return [Math.round(c1 +s[0]),Math.round(d1 +s[1]),c];
} else {
return [Math.round(c2 +s[0]),Math.round(d2 +s[1]),c];
}
}

tl;dr -> Look at the jsbin code-example
It is our goal to draw a line from the edges of two Rectangles A & B that would be drawn through their centers.
Therefore we'll have to determine where the line pierces through the edge of a Rect.
We can assume that our Rect is an object containing x and y as offset from the upper left edge and width and height as dimension offset.
This can be done by the following code. The Method you should look at closely is pointOnEdge.
// starting with Point and Rectangle Types, as they ease calculation
var Point = function(x, y) {
return { x: x, y: y };
};
var Rect = function(x, y, w, h) {
return { x: x, y: y, width: w, height: h };
};
var isLeftOf = function(pt1, pt2) { return pt1.x < pt2.x; };
var isAbove = function(pt1, pt2) { return pt1.y < pt2.y; };
var centerOf = function(rect) {
return Point(
rect.x + rect.width / 2,
rect.y + rect.height / 2
);
};
var gradient = function(pt1, pt2) {
return (pt2.y - pt1.y) / (pt2.x - pt1.x);
};
var aspectRatio = function(rect) { return rect.height / rect.width; };
// now, this is where the fun takes place
var pointOnEdge = function(fromRect, toRect) {
var centerA = centerOf(fromRect),
centerB = centerOf(toRect),
// calculate the gradient from rectA to rectB
gradA2B = gradient(centerA, centerB),
// grab the aspectRatio of rectA
// as we want any dimensions to work with the script
aspectA = aspectRatio(fromRect),
// grab the half values, as they are used for the additional point
h05 = fromRect.width / 2,
w05 = fromRect.height / 2,
// the norm is the normalized gradient honoring the aspect Ratio of rectA
normA2B = Math.abs(gradA2B / aspectA),
// the additional point
add = Point(
// when the rectA is left of rectB we move right, else left
(isLeftOf(centerA, centerB) ? 1 : -1) * h05,
// when the rectA is below
(isAbove(centerA, centerB) ? 1 : -1) * w05
);
// norm values are absolute, thus we can compare whether they are
// greater or less than 1
if (normA2B < 1) {
// when they are less then 1 multiply the y component with the norm
add.y *= normA2B;
} else {
// otherwise divide the x component by the norm
add.x /= normA2B;
}
// this way we will stay on the edge with at least one component of the result
// while the other component is shifted towards the center
return Point(centerA.x + add.x, centerA.y + add.y);
};
I wrote a jsbin, you can use to test with some boxes (lower part, in the ready method):
You might want to take a look at a little Geometry helper I wrote some time ago on top of prototype.js
I really hope, that this helps you with your problem ;)

To draw a line between those boxes, you'd first have to define where you want the line to be.
Apparently you want to draw the lines/arrows from the right edge of Rect A to the left edge of
Rect B, somewhat like this:
Assuming your know the origin (upper left Point as { x, y } of a Rect) and its Size (width and height), you first want to determine the position of the center of the edges:
var rectA, rectB; // I assume you have those data
var rectARightEdgeCenter = {
// x is simply the origin's x plus the width
x: rectA.origin.x + rectA.size.width,
// for y you need to add only half the height to origin.y
y: rectA.origin.y + rectA.size.height / 2.0
}
var rectBLeftEdgeCenter = {
// x will be simply the origin's x
x: rectB.origin.x,
// y is half the height added to the origin's y, just as before
y: rectB.origin.y + rectB.size.height / 2.0
}
The more interesting question would be how to determine, from which edge to which other edge you might want to draw the lines in a more dynamic scenario.
If your boxes just pile up from left to right the given solution will fit,
but you might want to check for minimum distances of the edges, to determine a possible best arrow.