Is there a way to clear the canvas (html5)of only 1 element only ? I have a moving image on a canvas and when I erase the image the background color goes as well. Is there a way to just remove the image and not the whole background. My background is just a simple color but in the future it will be more complicated.
This is also tricky because there is no way to get image x,y pos from a property.
ClassLoadImages.prototype.m_move = function(){
this.x=++img1_x;
this.y=++img1_y;
//img1_x++;
//img1_y++;
// alert(img.x);
ctx.drawImage(img.imgElement, this.x, this.y);
// ctx.fillText("finished loading " ,10,40);
};
function doGameLoop() {
ctx.clearRect(0,0,600,400);
img.m_move();
if (img.x>30)
{
clearInterval(gameLoop);
}
}
var img= new ClassLoadImages('images/image4.jpg');
gameLoop = setInterval(doGameLoop, 100);
</script>
Simple answer is no. A canvas is a flat bitmap, not a layered collection of objects. Once you draw to it you lose the background behind the thing you draw.
You could try to implement the functionality yourself by recording the steps you used to create the canvas in the first place, and re-creating it with or without the relevant image.
Related
I have a very simple question : is it possible to use globalCompositeOperation for only a restricted number of images?
For example, I draw a lot of stuff into my canvas. And on top of everything, and completed unrelated, I want to do some operations on two images (and I just want the result to be displayed, not both images). How can that be done?
For now, doing such operations affects everything that's already drawn underneath.
So a solution I found is doing the operations in another canvas, which I display on top of my main, first canvas. But this looks bad. First, it hits performances. Then, it doesn't feel intuitive. And last, I loose control over the layers : whatever is in my second canvas will always be on top of the first canvas.
This looks like a pretty simple feature, I hope I'm just bad at googling!
Thanks a lot!
Create a offscreen canvas and use it as a work space. You can create many as long as you have the RAM to store them.
To create an offscreen canvas
function createCanvas(w,h){
var canvas = document.createElement("canvas");
canvas.width = w;
canvas.height = h;
canvas.ctx = ctx.getContext("2d");
return canvas;
}
Easy as that and then you just treat it as an image. To draw to another canvas
var offScreenCan = createCanvas(1024,1024);
offScreenCan.ctx.drawImage(myImage); // put something on the canvas
ctx.drawImage(offScreenCan,0,0); // draw that canvas to another
I attach the context to the canvas rather than wrap the canvas in a containing object. I used to worry that the canvas would affect performance now I convert all images to canvas as soon as I load them and it does not impact performance at all.
I need to clear a rectangle Drawn on Image in Canvas with out damage existing image. I can draw small rectangle points and clear that out. But the problem is when I clear rectangle it remains as white small patch on image.
Can someone tell me how to clear a rectangle on image without damage the existing image.
I have used following methods to clear rectangles but didn't work.
1) context.fillStyle ="white";
2) context.clearRect(xCoordinate, yCoordinate, 10, 08);
Thanks in advance!
Canvas doesn't work that way. It's a single layer, its also transparent by default. So with that in mind, you might be able to achieve what you want by simply giving the canvas element a CSS background. That way anything you draw on top of that background can easily be removed and the background will show through.
#backed-canvas{
background-image: url(http://www.placebear.com/300/200);
}
JSFiddle example: https://jsfiddle.net/yLf5erut/
There is one thing you can do.
When create a rectangle on the canvas just get the image data like:
var imgData = context.getImageData(xCoordinate, yCoordinate, 10, 8);
and draw the rectangle.
When clearing out the rectangle just place then image data back like this:
context.putImageData(imgData, xCoordinate, yCoordinate);
I suggest using 2 canvas elements one over another.
So you can have the original image drawn on the bottom canvas with low zIndex, and the top one with highter zIndex can be used to draw / clear whatever needed. This is a common practice, and for more complecated animations you will end up with better performance.
This may be off topic but I'm not sure where else to go with this question. I'm just getting started with HTML5 canvas element and all of the incredibly powerful things it can do. I was hoping someone could offer some advise. When working with custom paths and bezier curves, what is the easiest/best way to visualize where the points belong on the canvas to achieve a desired effect. Right now it feels like I'm just guessing plotting points in any place hoping to end up with the right angle/shape that I want.
To be more specific I want to create a shape that will act as an image mask, and will later need to animate this shape. Much like this fiddle http://jsfiddle.net/jimrhoskins/dDUC3/1/ (someone else's work) but since I can't see where the picture is on the canvas or where any of the points are, I'm really just guessing at the approximate shape I need to make. I'm just wondering if there's a better way, or some function in javascript that can map the location of an image and give me at least a better place to start.
Here is what I know/have tried already
// Grab the Canvas and Drawing Context
var canvas = document.getElementById('c');
var context = canvas.getContext('2d');
// Create an image element
var img = document.createElement('IMG');
// When the image is loaded, draw it
img.onload = function () {
// Save the state, so we can undo the clipping
context.save();
// Create a shape, of some sort
context.beginPath();
context.moveTo(somex, somey);
context.bezierCurveTo(somexstart, someystart, somexcontrol, someycontro, somexend, someyend);
context.arcTo(somecoordinates);
context.closePath();
// Clip to the current path
context.clip();
context.drawImage(img, 0, 0);
// Undo the clipping
context.restore();
}
// Specify the src to load the image
img.src = "url";
How about opening the image in an SVG editor. Drawing a path on a layer above the image. Then open the SVG and copy the coordinates?
Try an SVG Editor. You can get the points there. You can add images too. SVG animation is used nowadays as well. If you have Adobe Illustrator, it will be easier to draw there and just save it as SVG.
For a Project I want to take the content of a canvas (Called SAVE_CV) and display it in another, smaller canvas.
Some things that I am aware of so far that could be causing me problems: resizing a canvas clears its content, the JS-size of a canvas is different from the CSS-size.
I want the smaller canvas to be 500px wide and appropriately high.
function restoreTaggingCV() {
var cv = document.getElementById( 'taggingCV' );
var ctx = cv.getContext( "2d" );
var styleHeight = SAVE_CV.height * 500 / SAVE_CV.width;
ctx.drawImage(SAVE_CV, 0, 0, cv.width, cv.height);
}
This is my Code so far. Whenever I try to resize the smaller canvas appropriately it only gives me a blank canvas with nothing in it. I tried to set the size with "cv.height = X" and "cv.style.height = styleHeight + 'px'" but neither worked. Also I would love to set the width of the canvas using CSS.
Appreciate any help.
EDIT
I want the image in a picture because later I want the user to mark areas in the smaller version which I then want to use to create individual imaged from the big version. I want to visualise thise area to the user. I probably could do all this by using an image and putting divs over it or something but I just fell more comfident using a canvas since I am pritty new to HTML and CSS.
Try using the CanvasRenderingContext2d.prototype.scale method. It sets the scale factor of the canvas and renders anything in the current state with it's dimensions multiplied by the factor.
So before you use the drawImage function, you scale the context appropriately (in this case, down). For example:
context.save();
context.scale(0.5, 0.5);
context.drawImage(canvas, 0, 0);
context.restore();
This would render the canvas on the context at 0.5 times it's current size. See in this fiddle how I used it to mirror a larger canvas onto a smaller, separate one.
Canvas objects don't like to be resised. After drawing Your image simply convert it toDataURL() and set as image source. They You may resize image as you want.
$img.attr('src',canvas.toDataURL());
Say I drew a rectangle on the canvas. Surely there is some sort of built in method to get the XY coordinates, and dimensions of that rectangle? But after some googling I came up with nothing. And just to clarify, I am not talking about the coordinates of the canvas element itself, but rather a shape/image that is drawn unto the canvas.
Any help is appreciated.
If you're talking about a 2D canvas drawing, then the drawing maps 1:1 with screen coordinates, so it is just location of <canvas> + location of the drawing.
To clarify, drawing on a <canvas> basically just changes the pixels of the canvas - after you draw to it, you can't reference the drawn object the same way you can reference an html element.
Canvas is 2D table (Array) of numbers (= pixels = colors). When drawing into canvas, you are just editing this table. When you draw into canvas (= change numbers in table), what should be the coordinates of your adjustment?
If you are drawing rectangles only and you can define the coordinates for your rectangle, you must know your coordinates inside a program, because you have just drawn it.
If you want your image to be separated into some "objects" (shapes), you should use SVG.
Basically, you should be using canvas as a means to output graphics to the screen and the rest of your logic goes straight into the JavaScript that powers your game/application. The best way to go about making something like this is to create objects and assign properties to them; in its simplest form that can look like this:
function Player(x, y)
{
this.x = x;
this.y = y;
}
var examplePlayerObject = new Player(20, 20);
By extending this object via prototyping you can create multiple copies of an object that has the exact same functions; such as draw. Drawing the player in this instance could just be a red square that is 20px*20px.
Player.prototype.draw = function()
{
context.clearRect(0, 0, canvas.width, canvas.height);
context.fillStyle = 'red';
context.fillRect(this.x, this.y, 20, 20);
}
Then, you should have an update step with some means of clearing what is on the screen and redrawing the parts which have changed.
function animationStep()
{
examplePlayerObject.x++;
examplePlayerObject.y++;
examplePlayerObject.draw();
}
This animation step should run each frame; look into requestAnimationFrame for smooth animation. Paul Irish has a good shim for older browsers. Add in requestAnimationFrame(animationStep) at the end of that function and you will have a red square moving slowly across the screen. Good luck!