Transforming to bound box with Raphael.js - javascript

I am in need of some math help. I am trying to dynamically transform my Raphael set of elements to a given bound box within my canvas.
For example, say my canvas (paper) is 600 x 300 and is filled with paths. These paths are all in a set.
Now I want fill my canvas with a given bound box. The bound box is in pixel coordinates. e.g. [[50,10], [100,20]]
So the end result would be a function call that would zoom and position the SVG elements. This would cause the canvas to be cropped to the coordinate bounds.
var bbox = [[50,10], [100,20]]
animateToBoundBox(set, bbox, duration);
function animateToBoundBox(set, bbox, duration) { /* beautiful code */ }
I think the way to accomplish this would be by using the element matrix but I'm not sure. What do you think the most elegant way of handling this would be?
Thanks

The other answers are correct -- you want to use setViewBox.
Here's a version that supports animation. It's not entirely beautiful and you'll have to look at the page source to extract the code, but it should do more or less exactly what you want.
Here's the view box animation as a Raphael extension:
Raphael.fn.animateViewBox = function animateViewBox( x, y, w, h, duration, easing_function, callback )
{
var cx = this._viewBox ? this._viewBox[0] : 0,
dx = x - cx,
cy = this._viewBox ? this._viewBox[1] : 0,
dy = y - cy,
cw = this._viewBox ? this._viewBox[2] : this.width,
dw = w - cw,
ch = this._viewBox ? this._viewBox[3] : this.height,
dh = h - ch,
self = this;;
easing_function = easing_function || "linear";
var interval = 25;
var steps = duration / interval;
var current_step = 0;
var easing_formula = Raphael.easing_formulas[easing_function];
var intervalID = setInterval( function()
{
var ratio = current_step / steps;
self.setViewBox( cx + dx * easing_formula( ratio ),
cy + dy * easing_formula( ratio ),
cw + dw * easing_formula( ratio ),
ch + dh * easing_formula( ratio ), false );
if ( current_step++ >= steps )
{
clearInterval( intervalID );
callback && callback();
}
}, interval );
}
And the (not so beautiful) demonstration is here: http://voidblossom.com/tests/easedViewBox.php
If you're really bent on using transform (which could have a few benefits if leveraged well, but will in general be fragile compared to viewbox manipulation), there's another example using transform located at http://voidblossom.com/tests/zoomByTransform.php.

Not sure if I completely understand what you are looking for, but it sounds like you want to zoom the view to a specific bounding box. Have you looked at the setViewBox function? Basically your function would call it like this:
setViewBox(bbox[0][0], bbox[0][1], bbox[1][0] - bbox[0][1], bbox[1][1] - bbox[0][0])

From what I can tell, everything you want to accomplish would be better handled with Paper.setViewBox() in an animation. See http://raphaeljs.com/reference.html#Paper.setViewBox

Related

How to fix HTML canvas element and its coordinates across different screen sizes?

I've spent the past two weeks making a game with a few friends using the HTML canvas tag and JavaScript. None of us have any prior experience with a project of this scale, so considerations of browser/screen-size compatibility wasn't on our minds. The game runs and looks fine on our laptops (all have similar screen sizes), but it looked bad when we sent a link to another friend whose screen size differs greatly from what we had in mind.
To discuss the layout of the game in greater detail, it's set up with the canvas element acting as the actual game with a series of divs sitting below the canvas to represent things like a dialogue box or the pause menu (only one of these divs is shown at a time with the others being hidden).
The game is gridbased in that every object, from the wall tiles to enemies, has a size relative to some constant blockWidth (defined below) which itself is relative to the desired amount of squares on-screen, numSquares (also defined below).
Changing the canvas's height and width properties in JavaScript did successfully fix a ratio of the canvas size and ensure that the wall and floor textures loaded in their proper place. The player and other NPCs, however, appear at odd places onscreen, sometimes not showing up onload at all.
I'm not quite sure what to attribute this problem to, but I think it has something to do with canvas' coordinate system not mixing well with the admittedly poorly executed block system we put in place.
//some relevant misc variables
var numSquares = 30;
const blockWidth = 1132 / numSquares * 0.75;
screen.width = 1132;
screen.height = 600;
//some relevant player variables
stats.x = 678;
stats.y = 600;
stats.width = blockWidth * 3 * 0.37;
stats.height = blockWidth * 3;
Again, I suspect that the problem has something to do with the fact that the tiles that render correctly (i.e. wall and floor textures) have their coordinates in terms of blockWidth whereas the tiles that render incorrectly (i.e. the player) have their coordinates as regular numbers.
Is there a way to go about adjusting our game for different monitor sizes other than revamping the entire coordinate system?
try this meta which will solve cross platform screen (laptop, Computer, Tab, Mobile)problem:
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>
Your main problem is using hard coded values for your variables. If stats.x = 678;and your screen is 480px wide (for example), the stats will fall out the screen.
Also: screen.width and screen.height is a hardcoded number. Probably what you need is something like: screen.width = window.innerWidth and screen.height = window.innerHeight
What I'm missing from your relevant misc variables: In your code you have this:
var numSquares = 30;
const blockWidth = 1132 / numSquares * 0.75;
Where 1132 is in fact the screen.width. This means that you don't have a grid as you say. This means you have only 1 (one) row of squares. Alternatively you have 30 squares per row, and in this case you have a grid.
Also, it would be nice to see how you draw your grid. I would do it like this:
for (let y = 0; y < ch; y += cw / numSquares) {
for (let x = 0; x < cw; x += cw / numSquares) {
let o = { x: x, y: y, w: blockWidth, h: blockWidth };
drawRect(o);
}
}
Where drawRect is a custom function to draw a rect. Since your blockWidth = screen.width / numSquares * 0.75; I'm assuming you let a gap between rects, but this is assuming.
As I've commented before you can't give your stats hard coded values. You will need to calculate these values in function of your grid cells. For example your stats.x may be the x of the 5'th column of your grid and the stats.y may be the y of the 3-rd row. But this is again assuming and I may be wrong.
Next comes a code example. Please take a look and let me know if this is what you need.
const screen = document.getElementById("canvas");
const ctx = screen.getContext("2d");
let cw = (screen.width = window.innerWidth);
let ch = (screen.height = window.innerHeight);
let stats = {};
let numSquares = 30;
let blockWidth;
generateGrid();
function Init() {
// a function to re-generate the grid and re-draw the stats when screen.width and/or screen.height changed
cw = screen.width = window.innerWidth;
ch = screen.height = window.innerHeight;
generateGrid();
drawStats(5, 3);
}
// recalculate everything on resize
setTimeout(function() {
Init();
addEventListener("resize", Init, false);
}, 15);
// some useful functions
function drawRect(o) {
ctx.beginPath();
ctx.fillStyle = "rgba(0,0,0,.05)";
ctx.fillRect(o.x, o.y, o.w, o.h);
}
function drawStats(x, y) {
stats.x = x * cw / numSquares;
stats.y = y * cw / numSquares;
stats.width = blockWidth * 3 * 0.37;
stats.height = blockWidth * 3;
ctx.fillStyle = "red";
ctx.beginPath();
ctx.fillRect(stats.x, stats.y, stats.width, stats.height);
}
function generateGrid() {
blockWidth = cw / numSquares * 0.75;
for (let y = 0; y < ch; y += cw / numSquares) {
for (let x = 0; x < cw; x += cw / numSquares) {
let o = { x: x, y: y, w: blockWidth, h: blockWidth };
drawRect(o);
}
}
}
*{margin:0;padding:0}
<canvas id="canvas"></canvas>
If this is not what you need, please update your question, and add more explanations and more code.

Image jumping at the beginning of scrolling

I wrote some code to zoom in my image, but when I scroll at the very beginning this picture jumps a little. How to fix the problem?
Full page view.
Editor view.
HTML
<canvas id="canvas"></canvas>
JS
function draw(scroll) {
scroll = (window.scrollY || window.pageYOffset) / (document.body.clientHeight - window.innerHeight) * 3000;
canvas.setAttribute('width', window.innerWidth);
canvas.setAttribute('height', window.innerHeight);
//The main formula that draws and zooms the picture
drawImageProp(ctx, forest, 0, (-scroll * 3.9) / 4, canvas.width, canvas.height + (scroll * 3.9) / 2);
}
Not a bug fix
I had a look at the Codepen example and it does jump at the top (sometimes). I have a fix for you but I did not have the time to locate the source of your code problem. I did notice that the jump involved a aspect change so it must be in the scaling that your error is. (look out for negatives)
GPU is a better clipper
Also your code is actually doing unnecessary work, because you are calculating the image clipping region. Canvas context does the clipping for you and is especially good at clipping images. Even though you provide the clip area the image will still go through clip as that is part of the render pipeline. The only time you should be concerned about the clipped display of an image is whether or not any part of the image is visible so that you don't send a draw call, and it only really matters if you are pushing the image render count (ie game sprite counts 500+)
Code example
Anyway I digress. Below is my code. You can add the checks and balances. (argument vetting, scaling max min, etc).
Calling function.
// get a normalised scale 0-1 from the scroll postion
var scale = (window.scrollY || window.pageYOffset) / (document.body.clientHeight - window.innerHeight);
// call the draw function
// scale 0-1 where 0 is min scale and 1 is max scale (min max determined in function
// X and y offset are clamped but are ranged
// 0 - image.width and 0 - image.height
// where 0,0 shows top left and width,height show bottom right
drawImage(ctx, forest, scale, xOffset, yOffset);
The function.
The comments should cover what you need to know. You will notice that all I am concerned with is how big the image should be and where the top left corner will be. The GPU will do the clipping for you, and will not cost you processing time (even for unaccelerated displays). I personally like to work with normalised values 0-1, it is a little extra work but my brain likes the simplicity, it also reduces the need for magic numbers (magics number are a sign that code is not adaptable) . Function will work for any size display and any size image. Oh and I like divide rather than multiply, (a bad coding habit that comes from a good math habit) replacing the / 2 and needed brackets with * 0.5 will make it more readable.
function drawImage(ctx, img, scale, x, y){
const MAX_SCALE = 4;
const MIN_SCALE = 1;
var w = canvas.width; // set vars just for source clarity
var h = canvas.height;
var iw = img.width;
var ih = img.height;
var fit = Math.max(w / iw, h / ih); // get the scale to fill the avalible display area
// Scale is a normalised value from 0-1 as input arg Convert to range
scale = (MAX_SCALE - MIN_SCALE) * scale + MIN_SCALE;
var idw = iw * fit * scale; // get image total display size;
var idh = ih * fit * scale;
x /= iw; // normalise offsets
y /= ih; //
x = - (idw - w) * x; // transform offsets to display coords
y = - (idh - h) * y;
x = Math.min( 0, Math.max( - (idw - w), x) ); // clamp image to display area
y = Math.min( 0, Math.max( - (idh - h), y) );
// use set transform to scale and translate
ctx.setTransform(scale, 0, 0, scale, idw / 2 + x, idh / 2 + y);
// display the image to fit;
ctx.drawImage(img, ( - iw / 2 ) * fit, (- ih / 2 ) * fit);
// restore transform.
ctx.setTransform(1, 0, 0, 1, 0, 0)
}
Sorry I did not solve the problem directly, but hopefully this will help you redesign your approch.
I recently added a similar answer involving zooming and panning (and rotation) with the mouse which you may be interested in How to pan the canvas? Its a bit messy still "note to self (my clean it up)" and has no bounds clamping. But shows how to set a zoom origin, and convert from screen space to world space. (find where a screen pixel is on a pan/scale/rotated display).
Good luck with your project.

Calculate the bounding box's X, Y, Height and Width of a rotated element via JavaScript

Basically I'm asking this question for JavaScript: Calculate Bounding box coordinates from a rotated rectangle
In this case:
iX = Width of rotated (blue) HTML element
iY = Height of rotated (blue) HTML element
bx = Width of Bounding Box (red)
by = Height of Bounding Box (red)
x = X coord of Bounding Box (red)
y = Y coord of Bounding Box (red)
iAngle/t = Angle of rotation of HTML element (blue; not shown but
used in code below), FYI: It's 37 degrees in this example (not that it matters for the example)
How does one calculate the X, Y, Height and Width of a bounding box (all the red numbers) surrounding a rotated HTML element (given its width, height, and Angle of rotation) via JavaScript? A sticky bit to this will be getting the rotated HTML element (blue box)'s original X/Y coords to use as an offset somehow (this is not represented in the code below). This may well have to look at CSS3's transform-origin to determine the center point.
I've got a partial solution, but the calculation of the X/Y coords is not functioning properly...
var boundingBox = function (iX, iY, iAngle) {
var x, y, bx, by, t;
//# Allow for negetive iAngle's that rotate counter clockwise while always ensuring iAngle's < 360
t = ((iAngle < 0 ? 360 - iAngle : iAngle) % 360);
//# Calculate the width (bx) and height (by) of the .boundingBox
//# NOTE: See https://stackoverflow.com/questions/3231176/how-to-get-size-of-a-rotated-rectangle
bx = (iX * Math.sin(iAngle) + iY * Math.cos(iAngle));
by = (iX * Math.cos(iAngle) + iY * Math.sin(iAngle));
//# This part is wrong, as it's re-calculating the iX/iY of the rotated element (blue)
//# we want the x/y of the bounding box (red)
//# NOTE: See https://stackoverflow.com/questions/9971230/calculate-rotated-rectangle-size-from-known-bounding-box-coordinates
x = (1 / (Math.pow(Math.cos(t), 2) - Math.pow(Math.sin(t), 2))) * (bx * Math.cos(t) - by * Math.sin(t));
y = (1 / (Math.pow(Math.cos(t), 2) - Math.pow(Math.sin(t), 2))) * (-bx * Math.sin(t) + by * Math.cos(t));
//# Return an object to the caller representing the x/y and width/height of the calculated .boundingBox
return {
x: parseInt(x), width: parseInt(bx),
y: parseInt(y), height: parseInt(by)
}
};
I feel like I am so close, and yet so far...
Many thanks for any help you can provide!
TO HELP THE NON-JAVASCRIPTERS...
Once the HTML element is rotated, the browser returns a "matrix transform" or "rotation matrix" which seems to be this: rotate(Xdeg) = matrix(cos(X), sin(X), -sin(X), cos(X), 0, 0); See this page for more info.
I have a feeling this will enlighten us on how to get the X,Y of the bounding box (red) based solely on the Width, Height and Angle of the rotated element (blue).
New Info
Humm... interesting...
Each browser seems to handle the rotation differently from an X/Y perspective! FF ignores it completely, IE & Opera draw the bounding box (but its properties are not exposed, ie: bx & by) and Chrome & Safari rotate the rectangle! All are properly reporting the X/Y except FF. So... the X/Y issue seems to exist for FF only! How very odd!
Also of note, it seems that $(document).ready(function () {...}); fires too early for the rotated X/Y to be recognized (which was part of my original problem!). I am rotating the elements directly before the X/Y interrogation calls in $(document).ready(function () {...}); but they don't seem to update until some time after(!?).
When I get a little more time, I will toss up a jFiddle with the example, but I'm using a modified form of "jquery-css-transform.js" so I have a tiny bit of tinkering before the jFiddle...
So... what's up, FireFox? That ain't cool, man!
The Plot Thickens...
Well, FF12 seems to fix the issue with FF11, and now acts like IE and Opera. But now I am back to square one with the X/Y, but at least I think I know why now...
It seems that even though the X/Y is being reported correctly by the browsers for the rotated object, a "ghost" X/Y still exists on the un-rotated version. It seems as though this is the order of operations:
Starting with an un-rotated element at an X,Y of 20,20
Rotate said element, resulting in the reporting of X,Y as 15,35
Move said element via JavaScript/CSS to X,Y 10,10
Browser logically un-rotates element back to 20,20, moves to 10,10 then re-rotates, resulting in an X,Y of 5,25
So... I want the element to end up at 10,10 post rotation, but thanks to the fact that the element is (seemingly) re-rotated post move, the resulting X,Y differs from the set X,Y.
This is my problem! So what I really need is a function to take the desired destination coords (10,10), and work backwards from there to get the starting X,Y coords that will result in the element being rotated into 10,10. At least I know what my problem is now, as thanks to the inner workings of the browsers, it seems with a rotated element 10=5!
I know this is a bit late, but I've written a fiddle for exactly this problem, on an HTML5 canvas:
http://jsfiddle.net/oscarpalacious/ZdQKg/
I hope somebody finds it useful!
I'm actually not calculating your x,y for the upper left corner of the container. It's calculated as a result of the offset (code from the fiddle example):
this.w = Math.sin(this.angulo) * rotador.h + Math.cos(this.angulo) * rotador.w;
this.h = Math.sin(this.angulo) * rotador.w + Math.cos(this.angulo) * rotador.h;
// The offset on a canvas for the upper left corner (x, y) is
// given by the first two parameters for the rect() method:
contexto.rect(-(this.w/2), -(this.h/2), this.w, this.h);
Cheers
Have you tried using getBoundingClientRect() ?
This method returns an object with current values of "bottom, height, left, right, top, width" considering rotations
Turn the four corners into vectors from the center, rotate them, and get the new min/max width/height from them.
EDIT:
I see where you're having problems now. You're doing the calculations using the entire side when you need to be doing them with the offsets from the center of rotation. Yes, this results in four rotated points (which, strangely enough, is exactly as many points as you started with). Between them there will be one minimum X, one maximum X, one minimum Y, and one maximum Y. Those are your bounds.
My gist can help you
Bounding box of a polygon (rectangle, triangle, etc.):
Live demo https://jsfiddle.net/Kolosovsky/tdqv6pk2/
let points = [
{ x: 125, y: 50 },
{ x: 250, y: 65 },
{ x: 300, y: 125 },
{ x: 175, y: 175 },
{ x: 100, y: 125 },
];
let minX = Math.min(...points.map(point => point.x));
let minY = Math.min(...points.map(point => point.y));
let maxX = Math.max(...points.map(point => point.x));
let maxY = Math.max(...points.map(point => point.y));
let pivot = {
x: maxX - ((maxX - minX) / 2),
y: maxY - ((maxY - minY) / 2)
};
let degrees = 90;
let radians = degrees * (Math.PI / 180);
let cos = Math.cos(radians);
let sin = Math.sin(radians);
function rotatePoint(pivot, point, cos, sin) {
return {
x: (cos * (point.x - pivot.x)) - (sin * (point.y - pivot.y)) + pivot.x,
y: (sin * (point.x - pivot.x)) + (cos * (point.y - pivot.y)) + pivot.y
};
}
let boundingBox = {
x1: Number.POSITIVE_INFINITY,
y1: Number.POSITIVE_INFINITY,
x2: Number.NEGATIVE_INFINITY,
y2: Number.NEGATIVE_INFINITY,
};
points.forEach((point) => {
let rotatedPoint = rotatePoint(pivot, point, cos, sin);
boundingBox.x1 = Math.min(boundingBox.x1, rotatedPoint.x);
boundingBox.y1 = Math.min(boundingBox.y1, rotatedPoint.y);
boundingBox.x2 = Math.max(boundingBox.x2, rotatedPoint.x);
boundingBox.y2 = Math.max(boundingBox.y2, rotatedPoint.y);
});
Bounding box of an ellipse:
Live demo https://jsfiddle.net/Kolosovsky/sLc7ynd1/
let centerX = 350;
let centerY = 100;
let radiusX = 100;
let radiusY = 50;
let degrees = 200;
let radians = degrees * (Math.PI / 180);
let radians90 = radians + Math.PI / 2;
let ux = radiusX * Math.cos(radians);
let uy = radiusX * Math.sin(radians);
let vx = radiusY * Math.cos(radians90);
let vy = radiusY * Math.sin(radians90);
let width = Math.sqrt(ux * ux + vx * vx) * 2;
let height = Math.sqrt(uy * uy + vy * vy) * 2;
let x = centerX - (width / 2);
let y = centerY - (height / 2);

Get absolute screen bounds of a DOM element from a Firefox extension

I have a firefox extension that needs to get the exact screen co-ordinates of a DOM element and passes it to a native DLL via js/c-types.
Now I have it mostly covered :
var gDomWindowUtils = window.QueryInterface(Components.interfaces.nsIInterfaceRequestor)
.getInterface(nsIDOMWindowUtils);
function getScreenRect(oElem)
{
var rc =
{
x : 0,
y : 0,
w : 0,
h : 0
};
var o = oElement;
while(o != null)
{
rc.y += o.offsetTop;
rc.x += o.offsetLeft;
o = o.offsetParent;
}
var x = {}, y = {};
gDomWindowUtils.getScrollXY(false, x, y);
rc.x -= x.value;
rc.y -= y.value;
var scale = gDomWindowUtils.screenPixelsPerCSSPixel;
rc.x *= scale;
rc.y *= scale;
rc.w *= scale;
rc.h *= scale;
return rc;
};
This handles scrolling and zooming, but the values I get are relative to the browser window, and not the screen.
How do I detect the offset of the client area of the actual rendering area of the browser?
I can even use native code (Win32) via js/ctypes so I tried to see if I could use FindWindow() / GetWindowRect() to get it, but the whole of firefox is a single HWND, the controls are all not native windows.
So one idea I have is, since the UI of firefox is an XUL document, I should be able to get the menubar, tab bar etc etc and find the browser areas absolute offset. But, I have no clue how to access the XUL tree that defines the browser UI.
Can someone give me a pointer?
[Edit]
Ignore rc.w and rc.h being undefined in the above code , it is irrelevant to the question.
You mostly got it already but I would recommend using getBoundingClientRect() instead of offsetLeft/offsetTop:
var rect = oElement.getBoundingClientRect();
var rc = {
x: rect.left,
y: rect.top,
w: rect.width,
h: rect.height
};
getBoundingClientRect() considers scrolling so that you no longer need to add it. You get the coordinates relative to screen using window.mozInnerScreenX and window.mozInnerScreenY:
rc.x += window.mozInnerScreenX;
rc.y += window.mozInnerScreenY;
And after that you multiply the values with screenPixelsPerCSSPixel. That should give you proper screen coordinates.
I will add one more "trick" that helped me with a similar problem.
Whenever my code receives a mousemove event, I capture the mouse-cursor positions in ALL the coordinates that event provides, which includes at least "screen", "client" and "pageXY" coordinates. Then for any OTHER purpose I can compute the difference between these coordinates simply by subtracting the appropriate two of those.
In your case, you'd probably compute an offset like this:
deltaX = event.screenX - event.clientX;
deltaY = event.screenY - event.clientY;
or
offsetX = event.screenX - event.pageX;
offsetY = event.screenY - event.pageY;
Then just add deltaX and deltaY or offsetX and offsetY to those coordinates to get screen coordinates.

jQuery image hover effect

I'm trying to achieve this effect with jQuery.
I wrote some of the code, but it's buggy (move to the bottom-right corder and you'll see).
check it out
Basically, if there's an already-built jQuery plugin that you know of that does this, I'd be very happy using it, if not, any help with my formula would be appreciated. This is what I get for not paying attention in Maths classes :)
Thanks in advance.
Maikel
Overall I think this is what you're looking for:
$.fn.sexyImageHover = function() {
var p = this, // parent
i = this.children('img'); // image
i.load(function(){
// get image and parent width/height info
var pw = p.width(),
ph = p.height(),
w = i.width(),
h = i.height();
// check if the image is actually larger than the parent
if (w > pw || h > ph) {
var w_offset = w - pw,
h_offset = h - ph;
// center the image in the view by default
i.css({ 'margin-top':(h_offset / 2) * -1, 'margin-left':(w_offset / 2) * -1 });
p.mousemove(function(e){
var new_x = 0 - w_offset * e.offsetX / w,
new_y = 0 - h_offset * e.offsetY / h;
i.css({ 'margin-top':new_y, 'margin-left':new_x });
});
}
});
}
You can test it here.
Notable changes:
new_x and new_y should be divided by the images height/width, not the container's height/width, which is wider.
this is already a jQuery object in a $.fn.plugin function, no need to wrap it.
i and p were also jQuery objects, no need to keep wrapping them
no need to bind mousemove on mouseenter (which rebinds) the mousemove will only occur when you're inside anyway.
Nick Craver beat me to an answer by about 10 minutes, but this is my code for this, using background-image to position the image instead of an actual image.
var img = $('#outer'),
imgWidth = 1600,
imgHeight = 1200,
eleWidth = img.width(),
eleHeight = img.height(),
offsetX = img.offset().left,
offsetY = img.offset().top,
moveRatioX = imgWidth / eleWidth - 1,
moveRatioY = imgHeight / eleHeight - 1;
img.mousemove(function(e){
var x = imgWidth - ((e.pageX - offsetX) * moveRatioX),
y = imgHeight - ((e.pageY - offsetY) * moveRatioY);
this.style.backgroundPosition = x + 'px ' + y + 'px';
});
The huge amount of variables are there because the mousemove event handler has to be as efficient as possible. It's slightly more restrictive, because you need to know the dimensions, but I think the code can be easily altered to work with imgs for which the size can be calculated easily.
A simple demo of this: http://www.jsfiddle.net/yijiang/fq2te/1/

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