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Draw image on canvas with a filter

I'm making a Zelda-like game on canvas + js and I don't how to do (or even if it can be done) to apply a filter to a sprite.
I have a scenario drawn, I have multiple characters (enemies, npcs and the player) and I'm trying to do an animation when the player kills an enemy. I'd like to make them "tilt" before I change the enemy sprite with an explosion animation. I'd like to change the whole color of the enemy sprite something like "enemy sprite -> red enemy sprite -> enemy sprite -> red enemy sprite -> explosion".
My question, can I apply a color filter to an image drawn with canvas drawImage? The code to draw the enemy into the canvas scenario is pretty simple:
ctx.drawImage(img, posX, posY, img.width, img.height);
And I have also tried this (as suggested in this other question):
ctx.globalAlpha = 0.62;
ctx.globalCompositeOperation = 'source-atop';
ctx.fillStyle = 'red';
ctx.fillRect(posX, posY, img.width, img.height);
But it applies a mask to a rectangle and not just the sprite (in this picture I applied the above code to every sprite just to watch if it worked):
Any suggestions or help? Thanks!!

smoother canvas noise transition - fade in of particles

Here is my canvas on jsfiddle:
https://jsfiddle.net/vzrandom/fkho6grf/8/
I'm using simplex-noise.js and dat.GUI to create movement of particles. There is simulated click on canvas every 5 seconds. On click first animation is coming in background and new animation starts.
My problem is that animation on click starts too abruptly. I would like to have some kind of fade in of particles.
It seems like a simple problem but somehow I can't get it how to make fade in of elements that are inside the canvas - not entire canvas himself.
Entire code is on jsfiddle, here is part that handles the click:
function onCanvasClick(e) {
context.save();
context.globalAlpha = 1;
context.fillStyle = Configs.backgroundColor;
context.fillRect(0, 0, screenWidth, screenHeight);
context.restore();
simplexNoise = new SimplexNoise();
}

You need to render to an offscreen canvas.
Just create a second canvas
var canvas2 = document.createElement("canvas");
canvas2.width = canvas.width;
canvas2.height = canvas.height;
var ctx2 = canvas.getContext("2d");
Then for all your drawing calls use the background canvas 2dContext
To do the fade just render that canvas onto the display canvas setting alpha to make it fade.
The function called by requestAnimationFrame is passed a hi resolution time as the first argument. The code below is for the update function. Note that if you are using a polyfill for requestAnimationFrame you should use one that matches the standard.
var fadeTime = 1; // one second
var fadeTimeStart = undefined; // when undefined then this indicates start of fade
function update(time){
// render your particles to offscreen canvas
if(fadeTimeStart === undefined){ // get the current time as start
fadeTimeStart = time;
}
// get amount of fade
var fTime = (time - fadeTimeStart) / fadeTime;
// is it fading
if(fTime < 1){ // yes
ctx.globalAlpha = fTime;
clearRect(0,0,canvas.width,canvas.height); // clear last rendered scene
ctx.drawImage(canvas2,0,0); // draw the offscreen canvas
}else{
// you may or may not have to clear the canvas
ctx.drawImage(canvas2,0,0); // if no fade and assuming canvas is opaque then just draw the canvas onto the display.
}
requestAnimationFrame(update);
}
Then in the click event to start a new fade in just set the fadeTimeStart = undefined and it will start a new fade in.

I don't know if this is the effect you want to achieve, but you might get away with clearing the canvas a little on each iteration by filling it with a semi-transparent color in update function, instead of clearing it completely on each click.
See my fork here: https://jsfiddle.net/640e32ua/
Main change is changing Configs.backgroundColor to something semi-transparent and adding these two lines to update:
context.fillStyle = Configs.backgroundColor;
context.fillRect(0, 0, screenWidth, screenHeight);

Javascript Scale(x,y) method doesn't work on Canvas with negative parameter

I am trying to flip an image using the scale() method.
I have this code:
ctx.save();
ctx.scale(-1, 1);
ctx.drawImage(Img, 0, 0, 100, 100);
ctx.restore();
When I put 1, 1 into the scale parameter, or any other real number, the scale function works fine. But once I put a negative parameter such as -1 (to flip image horizontally) the image doesn't draw at all. The code is in a function with a 12 fps setInterval. I've looked everywhere but can't seem to find a solution. Any help or suggestions would be appreciated. Thanks!

as being said by markE, whenever you scale, rotate or transform the canvas, the coordinates you are using will react to the change in the canvas transformation - in your case sending the image off the canvas. In order to flip the image in place, you should consider using translate with the width of the image as first parameter:
ctx.save();
//The scale will flip the whole canvas, and will move the image
//to the left (by the image width size)
ctx.scale(-1, 1);
//Using translate to move the image back to it's original origin
ctx.translate(Img.width, 0)
ctx.drawImage(Img, 0, 0, 100, 100);
ctx.restore();

Zooming on HTML5 <canvas> and no pixelation for text?

Let's put some text on a HTML5 <canvas> with
var canvas = document.getElementById('myCanvas'),
ctx = canvas.getContext('2d'),
ctx.textBaseline = 'top';
ctx.textAlign = 'left';
ctx.font = '14px sans-serif';
ctx.fillText('Bonjour', 10, 10);
When zooming the canvas on text, one can see pixelation.
Is there a way of zooming on a canvas without having pixelation on text ?

When you fillText on the canvas, it stops being letters and starts being a letter-shaped collection of pixels. When you zoom in on it, the pixels become bigger. That's how a canvas works.
When you want the text to scale as a vector-based font and not as pixels, don't draw them on the canvas. You could create <span> HTML elements instead and place them on top of the canvas using CSS positioning. That way the rendering engine will render the fonts in a higher resolution when you zoom in and they will stay sharp. But anything you draw on the canvas will zoom accordingly.
Alternatively, you could override the browsers zoom feature and create your own zooming algorithm, but this will be some work.
When the user zooms in or out of the window, the window.onresize event handler is triggered. You can use this trigger to adjust the width and the height of the canvas css styling accordingly (not the properties of the canvas. That's the internal rendering resolution. Change the width and height attributes of the style which is the resolution it is scaled to on the website).
Now you effectively disabled the users web browser from resizing the canvas, and also have a place where you can react on the scaling input events. You can use this to adjust the context.scale of your canvas to change the size of everything you draw, including fonts.
Here is an example:
<!DOCTYPE html>
<html>
<head>
<script type="application/javascript">
"use strict"
var canvas;
var context;
function redraw() {
// clears the canvas and draws a text label
context.clearRect(0, 0, context.canvas.width, context.canvas.height);
context.font = "60pt sans-serif";
context.fillText("Hello World!", 100, 100);
}
function adjustSize() {
var width = window.innerWidth;
var height = window.innerHeight;
// resize the canvas to fill the whole screen
var style = canvas.style;
style.width = width + "px";
style.height = height + "px";
// backup the old current scaling factor
context.save();
// change the scaling according to the new zoom factor
context.scale(1000 / width, 1000 / height);
// redraw the canvas
redraw();
// restore the original scaling (important because multiple calls to scale are relative to the current scale factor)
context.restore();
}
window.onload = function() {
canvas = document.getElementById("myCanvas");
context = canvas.getContext("2d");
adjustSize();
}
window.onresize = adjustSize;
</script>
</head>
<body>
<canvas id ="myCanvas" width = 1000 height = 1000 ></canvas>
</body>
</html>

If you only need to scale text you can simply scale the font size.
A couple of notes on that however: fonts, or typefaces, are not just straight forward to scale meaning you will not get a smooth progress. This is because fonts are often optimized for certain sizes so the sizes in between so to speak are a result of the previous and next size. This can make the font look like it's moving around a little when scaled up and is normal and expected.
The approach here uses a simply size scale. If you need an absolute smooth scale for animation purposes you will have to use a very different technique.
The simple way is:
ctx.font = (fontSize * scale).toFixed(0) + 'px sans-serif';
An online demo here.
For animation purposes you would need to do the following:
Render a bigger size to an off-screen canvas which is then used to draw the different sizes
When the difference is too big and you get problems with interpolation you will have to render several of these cached text images at key sizes so you can switch between them when scaling factor exceeds a certain threshold.
In this demo you can see that at small sizes the pixels gets a bit "clumpy" but otherwise is much smoother than a pure text approach.
This is because the browser uses bi-linear interpolation rather than bi-cubic with canvas (this may or may not change in the future) so it's not able to interpolate properly when the difference gets to big (see below for solution with this issue).
The opposite happens at big sizes as the text gets blurry also due to interpolation.
This is where we would have to switch to a smaller (or bigger) cached version which we then scale within a certain range before we again switch.
The demo is simplified to show only a single cached version. You can see halfway through that this works fine. The principle would be in a full solution (sizes being just examples):
(Update Here is a demo of a switched image during scale).
-- Cached image (100px)
-- Draw cached image above scaled based on zoom between 51-100 pixels
-- Cached image (50px) generated from 100px version / 2
-- Draw cached image above scaled based on zoom between 26-50 pixels
-- Cached image (25px) generated from 50px version / 2
-- Draw cached image above scaled based on zoom between 1-25 pixels
Then use a "sweet spot" (which you find by experiment a little) to toggle between the cached versions before drawing them to screen.
var ctx = canvas.getContext('2d'),
scale = 1, /// initial scale
initialFactor = 6, /// fixed reduction scale of cached image
sweetSpot = 1, /// threshold to switch the cached images
/// create two off-screen canvases
ocanvas = document.createElement('canvas'),
octx = ocanvas.getContext('2d'),
ocanvas2 = document.createElement('canvas'),
octx2 = ocanvas2.getContext('2d');
ocanvas.width = 800;
ocanvas.height = 150;
ocanvas2.width = 400; /// 50% here, but maybe 75% in your case
ocanvas2.height = 75; /// experiment to find ideal size..
/// draw a big version of text to first off-screen canvas
octx.textBaseline = 'top';
octx.font = '140px sans-serif';
octx.fillText('Cached text on canvas', 10, 10);
/// draw a reduced version of that to second (50%)
octx2.drawImage(ocanvas, 0, 0, 400, 75);
Now we only need to check the sweet spot value to find out when to switch between these versions:
function draw() {
/// calc dimensions
var w = ocanvas.width / initialFactor * scale,
h = ocanvas.height / initialFactor * scale;
ctx.clearRect(0, 0, canvas.width, canvas.height);
if (scale >= sweetSpot) {
ctx.drawImage(ocanvas, 10, 10, w, h); /// use cached image 1
} else {
ctx.drawImage(ocanvas2, 10, 10, w, h); /// use cached image 2
}
}
So why not just draw the second cached image with a font? You can do that but then you are back to the issue with fonts being optimized for certain sizes and it would generate a small jump when scaling. If you can live with that then use this as it will provide a little better quality (specially at small sizes). If you need smooth animation you will have to reduce a larger cached version in order to keep the size 100% proportional.
You can see this answer on how to get a large image resized without interpolation problems.
Hope this helps.

HTML5 canvas image caching/putImageData questions

I'm using the HTML5 canvas to load a single instance of an image which I then blit multiple times onto a single canvas. The image needs some slight pixel-based manipulation in order to customise it. My initial plan of attack had been to load the image, blit it to a backing canvas, draw my modifications on-top of it, and then grab the image data and cache it for future use.
Here's some code I've written to that effect:
context.drawImage(img, 0, 0);
context.fillStyle = '#ffffff';
context.fillRect(0, 0, 2, 2); // Draw a 2x2 rectangle of white pixels on the top left of the image
imageData = context.getImageData(0, 0, img.width, img.height);
cusomImage = imageData;
While this works, I've noticed that my image (which is a transparent PNG) does not maintain transparency. Instead, when using putImageData to place it onto my front-facing canvas, it is rendered with a black background. How do I maintain transparency?
Any suggestions are welcome!

putImageData() does not do what you might first expect:
http://dropshado.ws/post/1244700472/putimagedata-is-a-complete-jerk
putImageData() direct overrides the pixels of the canvas. So if you draw over something else on the same canvas it will not draw "over" it, it will instead replace the pixels of the canvas in the area with it's pixels.
I ran into this exact issue and finally found out why.
As for a solution, I haven't tried this yet but it seems promising:
Why is putImageData so slow?
[EDIT]: I tested this method and it works fine for me, my data is now displaying transparency correctly.

The canvas is black after being created. Make it transparent first with:
context.fillStyle = 'rgba(0,0,0,0)';
context.fillRect(0, 0, width, height);