I'm trying to use a canvas image as background here: http://www.cphrecmedia.dk/musikdk/stage/prelisten.php
The code is as below:
window.onload=function(){
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var imageObj = new Image();
stackBoxBlurImage('coverbgblur', 'canvas', 19, false, 2);
}
Beside the layout stuff that needs to be fixed, I need to darken the image a little bit, so the text will always be visible, also if I use a white album-cover. Can I somehow in the code above add a line or 2 that will darken the image? I know I can use CSS3, but its seems unsmart to create an extra layer of processing instead of doing it right the first time.
I'm quite new with canvas, so every kind of help is hugely appreciated!
You can make the canvas appear darker by drawing a semi-transparent dark rectangle over the image when after you draw the image
context.fillStyle = "rgba(0, 0, 0, 0.4)";
context.fillRect(0, 0, 700, 500);
Here is an example jsFiddle
You may also be able to use context.globalAlpha or context.globalCompositeOperation = "lighter"; as described in this SO post
Related
I want to create a Welcome Page with a responsive background like this fiddle
https://codepen.io/JulianLaval/pen/KpLXOO/.
I tried using
var ctx = canvasDiv.getContext("2d");
ctx.fillStyle = "RGBA(255, 255, 255, 0.8)";
ctx.fillText("Hi", canvasDiv.width / 2, canvasDiv.height / 2);
But it is not working.
Any help?
The problem is that you can not simply reuse the canvas of the particle library - at least not without modifications to the particle library itself.
This library has it's own update() function which we can see at this github link: ParticleNetwork.prototype.update
So initially it clears it's canvas, draws the particles and finally schedules a call to itself - so it constantly repaints the canvas with the displays refresh rate.
No here's the problem - if you simply draw something to it's canvas it will ultimately be overwritten by this update function.
If you want to draw text on it's canvas, you have to do it inside this function.
For example, find these lines of code:
if (this.options.velocity !== 0) {
requestAnimationFrame(this.update.bind(this));
}
if you add the following lines before:
this.ctx.fillStyle = "RGBA(255, 255, 255, 0.8)";
this.ctx.fillText("Hi", this.canvas.width / 2, this.canvas.height / 2);
You will have your desired text.
We are using a canvas to draw an image, and require that the image is smoothed.
so we use context.imageSmoothingEnabled in the 2d Canvas context.
We notice that if we use a jpg and smoothing is true, then the image fails to scale correctly on some machines (mainly windows 64bit).
In addition, if smoothingQuality is set to 'low' (rather than medium or high) then it works, but of course, who wants low.
If you modify the code to point to a PNG file, it seems to work ok.
Here is the JS Code
// Grab the Canvas and Drawing Context
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
// Create an image element
var img = document.createElement('IMG');
// When the image is loaded, draw it
img.onload = function () {
ctx.clearRect(0, 0, canvas.width, canvas.height);
ctx.imageSmoothingQuality = "medium";
// set to true and the image fails to scale
ctx.imageSmoothingEnabled = true;
ctx.drawImage(img, 0, 0, 50, 50);
}
// Specify the src to load the image
img.src = "http://i.imgur.com/gwlPu.jpg";
body {
background: #CEF;
}
<canvas id="c" width="800" height="600"></canvas>
Thanks in advance.
I have tried it on a Win10 64bit, Win10 32Bit (64bitCPU) and Win10 32bit on Chrome Beta, & Canary
Can not repeat problems you mention.
The image format (jpeg,png) will not effect the result of any of the smoothing or quality settings (apart from source image quality).
What can make a difference, especially at small scales is the ratio of the scaled down image to the original. Ratios like 1/2 1/4 1/8th will produce far better results than 1/3 2/9 3/19th. When comparing images of different sizes you should compare at same scale ratio rather than same size (if original have different sizes)
Below I used your code and the image you linked to to draw the image at the same size you had. Along the top from left to right is smoothing true from quality high on left to low on right.
The second row is same but smoothing turned off.
Below that is the same but zoomed via copy of canvas and no smoothing to show more details of the differences the setting make (if any).
If as your question outlines this was reproducible it would be a major issue for chrome and would be quickly fixed. I would say it is something very specific, maybe all the graphics cards are the same type and there is a driver issue, or images are cached and some old bad version is appearing.
If you want more help the best you could do is provide some screen shots of the problems, showing the problem and the desired result. That way we can at least narrow it down somewhat.
var ctx = c.getContext('2d');
var img = new Image;
img.src = "http://i.imgur.com/gwlPu.jpg";
img.onload = function () {
ctx.imageSmoothingQuality = "high";
ctx.imageSmoothingEnabled = true;
ctx.drawImage(img, 0, 0,50,50);
ctx.imageSmoothingQuality = "medium";
ctx.drawImage(img, 60, 0,50,50);
ctx.imageSmoothingQuality = "low";
ctx.drawImage(img, 120, 0,50,50);
ctx.imageSmoothingQuality = "high";
ctx.imageSmoothingEnabled = false;
ctx.drawImage(img, 0, 60,50,50);
ctx.imageSmoothingQuality = "medium";
ctx.drawImage(img, 60, 60,50,50);
ctx.imageSmoothingQuality = "low";
ctx.drawImage(img, 120, 60,50,50);
ctx.imageSmoothingQuality = "low"; // turn off all filtering
ctx.imageSmoothingEnabled = false;
ctx.drawImage(c,0,0,170,110,0,120,680,440)
}
body {
background: #CEF;
}
<canvas id="c" width="800" height="600"></canvas>
Following content edited in by OP user2739963
Here is a screen shot of this not working on a machine (We have many of these).
I am drawing a full-window graph with the HTML5 canvas feature, and what I want to do is to define a small square of region inside this canvas, and snapshot that the part of region into certain format, like png, jpg or a base64 text.
First, create a new canvas in background with the width and height of the image you want to take:
var bgCanvas = document.createElement('canvas');
bgCanvas.width = 200;
bgCanvas.height = 150;
Then copy a part of the original canvas to the background canvas using context.drawImage. This example copies the 200x150 section starting at 400:500.
var bgContext = bgCanvas.getContext('2d');
bgContext.drawImage(mainCanvas, // source
400, 500, // source coordinates
200, 150, // source dimension
0, 0, // target coordinates
200, 150); // target dimensions
Now you just have to get the base64-encoded version of the bgCanvas as already explained in question "Getting binary (base64) data from HTML5 Canvas (readAsBinaryString)".
Hi i am building a windows store app with html5 and javascript in my app i am trying to implement an eraser tool but this is problematic because if the user moves an image or another layer to where they've previously erased, they see the white drawing where they erased.
i have been trying to do the eraser tool from different ways for example i have changed the default globalCompositeOperation to "destination-out" like this code
//Here is the error.
if (clickTool[j] == "eraser") {
ctx.globalCompositeOperation = 'destination-out';
ctx.fillStyle = 'rgba(255,0,0,0.5);';
ctx.strokeStyle = 'rgba(255,0,0,0.5);';
}
else {
ctx.globalCompositeOperation = "source-over";
ctx.strokeStyle = clickColor[j];
}
but unfortunately it doesn´t work for me. i have uploaded all my code to this link:
My code
Please i would like to somebody could help me.
Thanks and i'm sorry for my speech , i'm mexican.
Use multiple layers. Have one canvas for the background image and another for the drawing; that why you never erase any of the background image.
If you need to, you can have multiple layers as they don't generally impact performance.
And of course if you can combine layers, say the last drawn squiggle to the background layer, if you deem a drawing to be "permanent".
Maintain a array of mid points. Use the globalCompositeOperation as 'destination-out' first and 'source-over' later to make a transparent eraser trail .
Following is the code that you need to use with a mouse move function
var handleMouseMove = function (event) {
midPt = new createjs.Point(oldPt.x + stage.mouseX>>1, oldPt.y+stage.mouseY>>1);
if(curTool.type=="eraser"){
var tempcanvas = document.getElementById('drawcanvas');
var tempctx=tempcanvas.getContext("2d");
tempctx.beginPath();
tempctx.globalCompositeOperation = "destination-out";
tempctx.arc(midPt.x, midPt.y, 20, 0, Math.PI * 2, false);
tempctx.fill();
tempctx.closePath();
tempctx.globalCompositeOperation = "source-over";
drawingCanvas.graphics.clear();
// keep updating the array for points
arrMidPtx.push(midPt.x);
arrMidPty.push(midPt.y);
stage.addChild(drawingCanvas);
stage.update();
}
};
I use this code to make a eraser that behaves like pen and fills up transparent color instead of white
I have to draw a graph with 3 different lines. A line graph.
I tried doing this:
function draw()
{
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.lineWidth=10;
ctx.strokeStyle="teal";
ctx.moveTo(10,CompanyA[0]);
ctx.lineTo(110,CompanyA[1]);
ctx.lineTo(210,CompanyA[2]);
ctx.stroke();
ctx.strokeStyle="green";
ctx.moveTo(10,CompanyB[0]);
ctx.lineTo(110,CompanyB[1]);
ctx.lineTo(210,CompanyB[2]);
ctx.stroke();
ctx.strokeStyle="yellow";
ctx.moveTo(10,CompanyC[0]);
ctx.lineTo(110,CompanyC[1]);
ctx.lineTo(210,CompanyC[2]);
ctx.stroke();
}
But apparently, the last stroke draws for all the lines. So I get 3 yellow lines, instead of a Teal, a Green and a Yellow one.
I tried creating three different Context (ctx1, ctx2 and ctx3), but for some reason, all were drawn with the "ctx3.stroke()" call.
What would be the correct way to do this?
Add a ctx.beginPath() call before every line, and also a ctx.closePath() after every ctx.stroke()
If you don't, every time you call the stroke() method, not only the new line will be drawn but also all the previous lines will be drawn again (with the new strokeStyle), since it's the same line path that is still open.
Although there is a functional answer here, I'd just like to add this.
var ctx1 = canvas.getContext("2d");
var ctx2 = canvas.getContext("2d");
var ctx3 = canvas.getContext("2d");
They all refer to the same object. It does not create a new context, it uses the one that's already attached to the canvas element. Delta is totally right in saying that it strokes yellow over the entire path because you did not begin a new path. ctx.beginPath() will solve your troubles.