Place objects on a defined shape - css

Question

I'm looking for a way, to allow a drag and drop, only on what's the area of an image.
So, if the image is the following:
I will have the option to add another layer on top, with some text, but that new layer, can't go anywhere, but inside the shape of that image.
So, after some research, I still wonder how can I achieve something like that?
I know I can use map and area to map existing elements on an image, but how can I add new elements, that fit only to that map? Any ideas?

Answer 1

To illustration your solution, let's assume you want to drop a raindrop in your cloud and be sure all raindrop pixels are fully inside the cloud...
Then the answer to your question requires asking this question:
Are all opaque raindrop pixels fully inside the cloud?
To answer this question, you must compare every pixel on the raindrop with every pixel underneath.
If the raindrop pixel is transparent, then ignore this pixel because this part of the raindrop is transparent anyway.
If the raindrop pixel is opaque and the pixel underneath is transparent, then this raindrop pixel is not contained in the cloud.
If both the raindrop pixel & underneath pixel are opaque, then this raindrop pixel is contained in the cloud.
You can get the required transparency information about the raindrop and cloud by drawing their images on a canvas and then requesting getImageData. 'getImageData' returns the red, green, blue & alpha information about every pixel on the canvas. To answer the question, we just need the alpha information.
Here's annotated code and a Demo:
// canvas related variables
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
var offsetX,offsetY;
// load the cloud and raindrop images
var cloudmap,rainmap;
var rain=new Image();
rain.crossOrigin='anonymous';
rain.onload=start;
rain.src='https://dl.dropboxusercontent.com/u/139992952/raindrop1.png';
var cloud=new Image();
cloud.crossOrigin='anonymous';
cloud.onload=start;
cloud.src="https://dl.dropboxusercontent.com/u/139992952/cloud.png";
var cloud1=new Image();
cloud1.crossOrigin='anonymous';
cloud1.onload=start;
cloud1.src="https://dl.dropboxusercontent.com/u/139992952/multple/cloud1.png";
var imageCount=3;
function start(){
if(--imageCount>0){return;}
// resize the canvas to the size of the cloud
// and draw the cloud on the canvas
cw=canvas.width=cloud.width;
ch=canvas.height=cloud.height;
draw();
// create a transparency map of the cloud
cloudmap={
width:cloud.width,
height:cloud.height,
map:transparencyMap(cloud),
};
// create a transparency map of the raindrop
rainmap={
width:rain.width,
height:rain.height,
map:transparencyMap(rain),
}
// listen for mousemove events
$("#canvas").mousemove(function(e){handleMouseMove(e);});
// listen for window scroll events
calcCanvasOffset();
$(window).scroll(function(){ calcCanvasOffset(); });
}
function transparencyMap(img){
// create a temp canvas sized to the img size
var c=document.createElement('canvas');
var cctx=c.getContext('2d');
c.width=img.width;
c.height=img.height;
// draw the img on the canvas
cctx.drawImage(img,0,0);
// get the pixel data for every pixel on the canvas
var data=cctx.getImageData(0,0,c.width,c.height).data;
// create an array that reports the status
// of every pixel on the canvas
// (status: true if opaque, false if transparent)
var map=[];
for(var i=0;i<data.length;i+=4){
map.push(data[i+3]>250);
}
return(map);
}
function draw(mouseX,mouseY,isContained){
// draw the cloud
ctx.clearRect(0,0,cw,ch);
if(isContained){
// draw the blue cloud indicating the raindrop is not fully contained
ctx.drawImage(cloud,0,0);
}else{
// draw the yellow cloud indicating the raindrop is fully contained
ctx.drawImage(cloud1,0,0);
}
// if the mouse position was supplied
if(mouseX){
ctx.drawImage(rain,mouseX-rain.width/2,mouseY-rain.height/2);
}
}
function AcontainsB(ax,ay,amap,bx,by,bmap){
// set a flag indicating of the raindrop is fully contained in the cloud
var isContained=true;
// calc the relative position of the raindrop vs cloud in the canvas
var deltaX=bx-ax;
var deltaY=by-ay;
// test every pixel of B against A
// if B is opaque and a is not opaque then B is not contained by A
var y=0;
while(isContained && y<bmap.height){
var x=0;
while(isContained && x<bmap.width){
// calc the map array indexes for the cloud(A) & raindrop(B)
var mapIndexA=(y+deltaY)*amap.width+(x+deltaX);
var mapIndexB=y*bmap.width+x;
// if the raindrop is opaque at this pixel
if(bmap.map[mapIndexB]){
// ...and if this pixel is off canvas
if(mapIndexA<0 || mapIndexA>=amap.map.length){
// ...then the raindrop is not in the cloud at this pixel
isContained=false;
// ...or if the pixel under the raindrop is transparent
}else if(!amap.map[mapIndexA]){
// ...then the raindrop is not in the cloud at this pixel
isContained=false;
}
}
x++;
}
y++;
}
return(isContained);
}
function handleMouseMove(e){
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
// get the current mouse position
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// calc the top-left corner of the raindrop image
var rainX=parseInt(mouseX-rain.width/2);
var rainY=parseInt(mouseY-rain.height/2);
// ask if the raindrop is fully contained in the cloud
var isContained=AcontainsB(0,0,cloudmap,rainX,rainY,rainmap);
// redraw the cloud & raindrop
draw(mouseX,mouseY,isContained);
}
// recalc the canvas offsetX & offsetY
function calcCanvasOffset(){
var BB=canvas.getBoundingClientRect();
offsetX=BB.left;
offsetY=BB.top;
}
body{ background-color: ivory; }
canvas{border:1px solid red;}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Use the mouse to drag the raindrop over the canvas<br>The cloud turns blue if the rain is fully inside the cloud</h4>
<canvas id="canvas" width=600 height=500></canvas>
You can use this "transparency mapping" to test anything you can draw into a canvas including images and text. Note that text is drawn with context.fillText.
If you're dropping a text element external to the canvas (maybe using jqueryUI or native "draggable"), you will have to:
Fetch the x,y position of the drop.
Fetch the text content of the dropped element.
Create a temporary canvas containing the text. Do it like this...
function textToCanvas(text,fontsize,fontface){
var c=document.createElement('canvas');
var cctx=c.getContext('2d');
cctx.font=fontsize+'px '+fontface;
var textWidth=cctx.measureText(text).width;
c.width=textWidth;
c.height=fontsize+4;
cctx.font=fontsize+'px '+fontface;
cctx.textBaseline='top';
cctx.fillText(text,0,0);
return(c);
}
Use the temporary canvas just like an image to create a transparency map. This is possible because the canvas will accept another canvas as its image source.
Good luck with your project.
[ Additional questions from comments ]
Additional Question:
"How will this react when you have a raindrop placed somewhere in the
cloud (saved it's position), and then you try to add something that's
on top of it?" Said another way: "How can I test if 2 objects are
overlapping?"
Answer: You can test if 2 objects overlap by using transparency maps again. Create another test (AintersectsB) which tests if any pixel in A is opaque while the associated pixel in B is also opaque. You can start with the AcontainsB and modify it to create the AintersectsB test.
Additional Question:
"How can I save and later restore dropped object positions?"
Answer: Since canvas doesn't remember what it draws, you must remember for it. This is usually done by creating a javascript object for each dropped item and saving all these objects in an array. That way if you need to save the positions on a server you can use JSON.stringify to turn the array of objects into a string and send that string to the server to be saved in a database (or file). To recreate your work, the server pulls the string from the database and sends it to the browser. The browser uses JSON.parse to turn the string back into an array of javascript objects. Then you can redraw the scene exactly as it was using the information in the objects.